Showing papers in "Ecological Entomology in 2001"

Gut content analysis and a new feeding group classification of termites

Abstract: Summary 1. Gut content analysis of termites was undertaken using microscopical techniques. The 46 study species covered the entire range of taxonomic and feeding forms within the Order. 2. Inter-specific gut contents data were analysed using principal components analysis, placing species along a clear humification gradient based on variations in the amount of silica and plant tissue fragments in the gut. 3. Redundancy analysis was used to find morphological correlates of the observed variation in gut contents. A total of 22 morphological characters (out of 45 candidate characters) were correlated significantly with the gut contents. 4. Three of the 22 significantly correlated characters unambiguously defined feeding groups, which were designated groups I to IV in increasing order of humification of the feeding substrate. Group I contains lower termite dead wood and grass-feeders; group II contains Termitidae with a range of feeding habits including dead wood, grass, leaf litter, and micro-epiphytes; group III contains Termitidae feeding in the organic rich upper layers of the soil; group IV contains the true soil-feeders (again all Termitidae), ingesting apparently mineral soil. These groupings were generally supported statistically in a canonical covariance analysis, although group II apparently represents termite species with a rather wide range of feeding habits. 5. Using existing hypotheses of termite phylogenetic relationships, it seems probable that group I feeders are phylogenetically basal, and that the other groupings have arisen independently on a number of occasions. Soil-feeding (i.e. group III and group IV feeding) may have evolved due to the co-option of faecal material as a fungal substrate by Macrotermitinae-like ancestral forms. As a consequence, these forms would have been constrained to build nest structures from soil and would therefore have passed at least some soil through their guts.

The role of nutrition, crowding and interspecific interactions in the development of winged aphids

Abstract: 1. Winged morph production in aphids is a phenotypic trait that has traditionally been seen as a response to unfavourable environmental conditions. The evidence to support this theory is reviewed and the ecological and evolutionary significance of the findings is discussed. 2. The common assertion of poor host-plant nutritional quality leading to increased production of winged morphs does not always apply, particularly when the host-plant quality is exceptionally poor. The available data are skewed heavily towards Myzus persicae, and for this species the dynamical change in plant quality appears to be important with respect to wing induction. 3. Crowding may be a less influential stimulus for wing induction as study methods approach natural conditions experienced by aphids on their host plant. 4. The growing evidence that interactions with other organisms can induce the production of winged morphs by aphid colonies is reviewed. In the case of natural enemies, such a response by an aphid colony may be regarded as induced defence. Wing induction may also act as a means of transmission for a virus or fungal pathogen.

Jasmonate-mediated induced plant resistance affects a community of herbivores.

Abstract: Summary 1. The negative effect of induced plant resistance on the preference and performance of herbivores is a well-documented ecological phenomenon that is thought to be important for both plants and herbivores. This study links the well-developed mechanistic understanding of the biochemistry of induced plant resistance in the tomato system with an examination of how these mechanisms affect the community of herbivores in the field. 2. Several proteins that are induced in tomato foliage following herbivore damage have been linked causally to reductions in herbivore performance under laboratory conditions. Application of jasmonic acid, a natural elicitor of these defensive proteins, to tomato foliage stimulates induced responses to herbivory. 3. Jasmonic acid was sprayed on plants in three doses to generate plants with varying levels of induced responses, which were measured as increases in the activities of proteinase inhibitors and polyphenol oxidase. 4. Field experiments conducted over 3 years indicated that induction of these defensive proteins is associated with decreases in the abundance of all four naturally abundant herbivores, including insects in three feeding guilds, caterpillars, flea beetles, aphids, and thrips. Induced resistance killed early instars of noctuid caterpillars. Adult flea beetles strongly preferred control plants over induced plants, and this effect on host plant preference probably contributed to differences in the natural abundance of flea beetles. 5. The general nature of the effects observed in this study suggests that induced resistance will suppress many members of the herbivore community. By linking plant biochemistry, insect preference, performance, and abundance, tools can be developed to manipulate plant resistance sensibly and to predict its outcome under field conditions.

Detrimental effects of latex and cardiac glycosides on survival and growth of first‐instar monarch butterfly larvae Danaus plexippus feeding on the sandhill milkweed Asclepias humistrata

Abstract: 1. A novel experimental method was developed to study negative physical and chemical effects of latex and cardiac glycosides on first-instar monarch butterfly larvae in their natural environment in north central Florida. Forceps were used to nibble through the petioles of leaves of the sandhill milkweed Asclepias humistrata, mimicking the behaviour of mature monarch larvae. This notching cut off the supply-of latex to the leaves without significantly reducing either their cardiac glycoside concentration or water content. 2. The mean cardiac glycoside concentration in larvae that fed on intact leaves was nearly two: and a half times greater than in larvae that fed on notched leaves. This was probably because more latex is present in the gut of the larvae that fed on the intact leaves. Supporting this is the fact that the mean concentration of cardiac glycosides in the latex was 34-47 times that in the leaves. 3. Wet weights, dry weights, and growth rates of first-instar larvae that fed on intact leaves over a 72-h period were less than half those of larvae that fed on notched leaves. 4. Mortality due to miring in the latex was 27% on the intact leaves compared with 2% on the notched leaves. 5. Latex, cardiac glycosides, and other as yet undetermined plant factors all have a negative effect on:first-instar larval survival. 6. Video-analyses indicated that ingestion of latex caused the larvae to become cataleptic and increased their chances of being mired on the leaf by the setting latex glue. Dysfunction resulting from latex ingestion may lead to the larvae falling off the plant and being killed by invertebrate predators. 7. The difficulty of neonate monarch larvae surviving on A. humistrata - one of the principal milkweed species fed on each spring as monarchs remigrate from Mexico into the southern U.S.A. - is evidence that a co-evolutionary arms race is operating in this plant-herbivore system.

The occurrence and effectiveness of hypersensitive reaction against galling herbivores across host taxa

Abstract: 1. Hypersensitive reaction is an important type of induced defence by which the plant elicits a defence response to pathogens and insects. Hypersensitive reaction has been argued to be the most common plant resistance mechanism against insect herbivores that have intimate associations with their host plants. 2. The work reported here attempted to establish how important and widespread hypersensitive reaction might be against gall-forming species across host taxa. 3. Hypersensitive reaction was the most important mortality factor against gall formation across host plant taxa in seven out of eight cases. 4. The number of insect galls correlated with the size of the leaves but module (leaf) size was a weak factor influencing the incidence of plant hypersensitive reaction to galling. 5. Insect galls and hypersensitive reactions occurred in genetically distant as well as geographically widespread host plant taxa.

Fitness costs and stability of resistance to Bacillus thuringiensis in a field population of the diamondback moth Plutella xylostella L.

Abstract: Summary 1. The stability of resistance to Bacillus thuringiensis Crystal (Cry) toxins in highly and moderately resistant sub-populations of a Plutella xylostella field population (SERD4) was compared under laboratory conditions. The relative rate of decrease in resistance was greater in a highly resistant Cry1Ac-selected population than in moderately resistant Cry1Ab- and Cry1Ca-selected populations. 2. The intrinsic rate of population increase (rm) was similar in all populations tested. 3. These results suggest that there are no obvious overall fitness benefits as the frequency of the resistance alleles is reduced.

Response of Cydia pomonella to selection on mobility: laboratory evaluation and field verification

Abstract: Summary 1. The codling moth Cydia pomonella (L.), largely regarded as a sedentary species, shows great variation in flight capacity among individuals in the laboratory, as has been reported for tethered flight. The occurrence of individuals with the ability to fly over long distances is considered ecologically significant for the colonisation of new habitats in response to deteriorating environmental conditions. The work reported here was designed to investigate the importance of the genetic component in regulating dispersal in C. pomonella. 2. Bi-directional selection on mobility measured by actograph was carried out in the laboratory. Both male and female codling moths responded significantly to the upward and downward selections. Divergence between the two selected strains occurred after one generation of selection and increased as further selections continued. The realised heritabilities for mobility were estimated as 0.43 in males and 0.29 in females when averaged from the two selection directions. 3. The dispersal capacity of the selected mobile and sedentary strains was verified by mark–release–recapture experiments in the field, though only the released male moths were captured effectively. Flight distances by males differed significantly between the two strains, with the mobile strain showing a greater dispersal capacity than the sedentary strain. 4. This study demonstrated a positive correlation between mobility measured by actograph and field dispersal capacity in the codling moth. Furthermore, it provides the first experimental data to show field performances of different genotypes in insect dispersal.

Vigour of a dioecious shrub and attack by a galling herbivore

Abstract: Summary 1. The pattern of attack by the leaf-galling insect Neopelma baccharidis (Homoptera: Psyllidae) was studied in three populations of the dioecious shrub Baccharis dracunculifolia (Asteraceae) in south-eastern Brazil. The plant vigour hypothesis, which predicts higher rates of attack and increased herbivore performance on the longest plant shoots, was tested. This work also provides further information for the study of differential herbivory in dioecious plants. 2. In total, 9200 shoots were collected randomly from 46 male and 47 female plants belonging to the three populations. Shoot length, number of leaves per shoot, rate of galling, and survival of psyllids did not differ between male and female plants. Another population on the Campus of the Federal University of Minas Gerais was used only to determine the pattern of shoot growth. 3. The hypothesis of sex-mediated herbivory was not corroborated in this study. 4. The frequency of galling increased with increasing shoot length, as predicted by the plant vigour hypothesis. Nevertheless, the number of oviposition sites (leaf buds) increased with shoot length. 5. The performance of the galling herbivore was not related to shoot length in the plant populations studied. 6. In conclusion, Neopelma baccharidis did not select shoots based on length only.

Spiders and harvestmen as gastropod predators

Abstract: Summary 1. Reports are reviewed of gastropod feeding (malacophagy) by spiders and harvestmen. Although the standard textbooks on arachnids recognise the importance of gastropods as prey of harvestmen, none apparently refers to malacophagy by spiders. A review of several hundred papers on spider feeding habits revealed that species from several families kill and devour slugs and snails in the laboratory and/or field. 2. Malacophagy has been reported most frequently among mygalomorph spiders, and can make up a substantial proportion of the diets of some species, however gastropods make up an insignificant percentage of the prey of most araneomorph spiders. The spiders that eat gastropods are species with broad diets composed predominantly of arthropod prey. No species of spider appears to feed exclusively on gastropod prey. 3. Harvestmen from several families have broad diets that often include gastropods. Several species of the family Trogulidae and at least one species of the family Ischyropsalididae [Ischyropsalis hellwigi (Panzer)] are specialised gastropod predators. The trogulids are slender animals that attack the snail through the shell aperture (shell intruders). Ischyropsalis hellwigi, on the other hand, can crush snail shells with its powerful chelicerae (shell breakers). 4. The review highlights apparent convergent evolution by harvestmen and Carabidae of two mutually exclusive morphologies found among gastropod predators. It also suggests that there is an urgent need for systematic studies to be conducted to establish the extent and ecological importance of malacophagy in natural and anthropogenically altered habitats.

Oviposition preferences of the Hessian fly and their consequences for the survival and reproductive potential of offspring.

Abstract: Summary 1. The relationship between oviposition preference and offspring performance was investigated experimentally for the Hessian fly Mayetiola destructor (Say) using 10 grass genotypes that represented five different genera and six different species. Oviposition preferences were quantified in a choice test using arrays of 200–400 plants. Offspring performance was estimated by recording survival during three phases of the insect–plant association and by measuring wing length, a correlate of adult reproductive potential. Density effects were examined for all offspring variables, and were taken into account when offspring performance was compared across the 10 grasses. 2. Egg counts from the oviposition choice test revealed a consistent ranking of plants: 18ITSN triticale > Otane bread wheat = Caldwell bread wheat = Stacy bread wheat > 3424 bread wheat = PND durum wheat = Fleet barley = Valetta barley = Matua brome grass > Awapuni oat. 3. Survival and adult wing length varied significantly on the 10 plant types. Survival was ranked: Otane bread wheat > 3424 bread wheat = Fleet barley = Matua brome grass > Caldwell bread wheat = Valetta barley > 18ITSN triticale = PND durum wheat = Stacy bread wheat > Awapuni oat. Adult male and female wing lengths were greatest on Otane, the bread wheat that also provided the best survival. 4. For Hessian flies on each of the 10 plant types, data on survival, wing length, and wing length–fecundity relationships were combined into a single fitness measure. When these fitness measures were compared with egg counts, no overall pattern emerged. On seven of the 10 plant types, there was a positive linear relationship between egg counts and offspring fitness. On the other three plant types, egg counts were high while fitness was low. Possible reasons why Hessian fly females lay eggs on plants that are poor hosts for their offspring are discussed.

Structure of insect communities inhabiting old‐growth forest specialist bracket fungi

Abstract: Summary 1. This paper describes the structure of the insect communities inhabiting two old-growth forest specialist bracket fungi, Amylocystis lapponica and Fomitopsis rosea. To study the consequences of old-growth forest fragmentation on community structure, non-fragmented and fragmented forest areas in eastern and southern Finland were compared. 2. Both fungal species are inhabited by more than 50 insect species. The most abundant species in the A. lapponica community is a melandryid beetle Hallomenus sp., and in the F. rosea community a tineid moth Agnathosia mendicella. The occurrence of several nationally or globally rare species underscores the importance of specific insect microhabitats, such as the specialist fungi A. lapponica and F. rosea, for the persistence of specialised insects in old-growth forests. 3. The species composition in both fungal species differs greatly from each other. The food webs are characterised by one numerically dominant host-specific primary fungivore, which implies that fungivorous species inhabiting bracket fungi may be more specialised on a particular host than is generally thought. 4. The community structure was the same in the highly fragmented and in the non-fragmented study areas, probably because fragmentation has occurred relatively recently.

Instar‐specific sensitivity of specialist Manduca sexta larvae to induced defences in their host plant Nicotiana attenuata

Abstract: 1. The time delay associated with the activation of induced defences is thought to be a liability for this type of defence because it allows herbivores to remove biomass before the defence is fully induced. When defences are costly and plants grow with competitors, however, it may be more advantageous not to induce defences too fast and motivate the herbivore to move to the neighbour when it is most voracious. 2. Such a strategy can only work when the costs for the herbivore of moving to a neighbouring plant are smaller than the costs of staying on a fully induced plant. For lepidopteran herbivores, both the sensitivity to induced defences and the costs of moving may vary considerably between instars and this variation may constrain the plant's defensive opportunities. 3. This study was designed to examine whether the cost of moving, mimicked by a starvation period of 8 h. was larger than the cost of staying on a fully induced plant for each larval instar of the specialist Manduca sexta feeding on induced and control tissues of Nicotiana attenuata. 4. For first- and second-instar larvae, the costs of moving were larger than the costs of staying on a fully induced plant. In contrast, feeding on induced plant material retarded development in third-instar larvae more than did starvation, indicating that in this instar the costs of leaving are smaller than the costs of staying on an induced plant. More than 98% of the lifetime leaf mass consumed by a M. sexta larva is consumed during the fourth and fifth instars, and during these instars larval development was not affected by either induced defences or starvation. Thus the third instar, the stage just before larvae cause the majority of damage, represents a window of sensitivity to induced defences during which larvae can be motivated to change plants. 5. These results suggest that N. attenuata plants, which commonly compete with conspecifics in nature. have the opportunity to manipulate the behaviour of the specialist herbivore M. sexta to minimise the fitness effects of inducing defences when these defences are most costly, i.e, when plants grow under intraspecific competition. [KEYWORDS: compensatory feeding; effects of starvation; induced defence; intraspecific competition; larval development; lepidopteran herbivores; nicotine; optimal defence theory; protease inhibitors; time delay Jasmonate-induced responses; induced resistance; gene-expression; herbivore; growth; costs; sylvestris; induction; nitrogen; defenses]

The reproductive flight phenology of a neotropical ant assemblage

Abstract: Summary 1. Alate flights reflect an ant colony's investment in sexual reproduction and dispersal yet little is known about community-wide patterns of alate phenology. Two Malaise traps (for 2 years) and two light traps (for 1 year) were used to explore the flight phenologies of 22 common neotropical species from Barro Colorado Island, Panama. 2. The traps caught 23 182 individuals and 286 species/morphospecies. The two trap methods shared only 18 species. Samples also differed in sexual composition: light trap samples were 80% female, Malaise trap samples were 2.6% female. 3. Of 22 common species, all but one flew over half the year, with about half flying every month of the year. These data, combined with a literature review, suggest a latitudinal gradient in alate flight season: one north temperate assemblage (42°N) averaged 1.6 lunar months per species. The ever-warm tropical year provides a larger flight window that allows a diversity of phenologies, from continuous to strongly pulsed. 4. Rainfall was correlated with alate flights in one-third of the species. Quantile regression suggested that high weekly rainfall was necessary but not sufficient to produce alate flights in about a quarter of the species. 5. By decreasing the number of nests releasing alates on a given day, long flight seasons may lower the probability of finding a mate. At the same time, long flight seasons may increase the opportunity of finding vacant nest sites. High population densities and high incidence of nest disturbance in this community may ameliorate the first cost while enhancing the second benefit.

Macrogeographic clines in fecundity, reproductive allocation, and offspring size of the forest tent caterpillar Malacosoma disstria

Abstract: Summary 1. The fecundity of the forest tent caterpillar varies considerably across its geographic range. Field data indicate that populations in the southern United States (Gulf States) produce nearly twice as many eggs as females from Canada or the Lake States, with little or no difference in the size of adult females. 2. In controlled rearing experiments, female forest tent caterpillar from the southern United States (Louisiana) had much larger clutch sizes than same sized females from northern populations in Michigan or Manitoba, Canada. Increased fecundity in Louisiana females was achieved through a significant reduction in egg size and a concomitant increase in the allocation of resources to egg production. 3. Comparison of 10 forest tent caterpillar populations spanning a 27° latitudinal gradient, validated the results of detailed comparisons among the three populations above by confirming the strong negative correlation between latitude and clutch size. 4. Neonate forest tent caterpillars from Manitoba were significantly larger than larvae from either Michigan or Louisiana. Michigan larvae were intermediate in size. It is postulated that large neonates are advantageous in thermally limiting environments. More than three times as many degree-days are available to Louisiana neonates during the first 2 weeks after hatching. A consistently favourable climate during the vulnerable post-hatching period may have allowed the evolution of larger clutches at the expense of neonate size in southern populations.

Females of the European beewolf preserve their honeybee prey against competing fungi

Abstract: Summary 1 Females of the European beewolf Philanthus triangulum (Hymenoptera, Sphecidae) provision brood cells with paralysed honeybees as larval food Because brood cells are located in warm, humid locations there is a high risk of microbial decomposition of the provisions Low incidence of fungus infestation (Aspergillus sp) in nests in the field suggested the presence of an anti-fungal adaptation 2 To test whether the paralysis caused the protection from fungus infestation, the timing of fungus growth on bees that were freeze-killed, paralysed but not provisioned, and provisioned regularly by beewolf females was determined Fungus growth was first detected on freeze-killed bees, followed by paralysed but not provisioned bees By contrast, fungus growth on provisioned bees was delayed greatly or even absent Thus, paralysis alone is much less efficient in delaying fungus growth than is regular provisioning 3 Observations of beewolves in their nests revealed that females lick the body surface of their prey very thoroughly during the period of excavation of the brood cell 4 To separate the effect of a possible anti-fungal property of the brood cell and the licking of the bees, a second experiment was conducted Timing of fungus growth on paralysed bees did not differ between artificial and original brood cells By contrast, fungus growth on bees that had been provisioned by a female but were transferred to artificial brood cells was delayed significantly Thus, the treatment of the bees by the female wasp but not the brood cell caused the delay in fungus growth 5 Beewolf females most probably apply anti-fungal chemicals to the cuticle of their prey This is the first demonstration of the mechanism involved in the preservation of provisions in a hunting wasp Some kind of preservation of prey as a component of parental care is probably widespread among hunting wasps and might have been a prerequisite for the evolution of mass provisioning

Effects of gender and mating status on self‐directed dispersal by the whitefly parasitoid Eretmocerus eremicus

Abstract: Summary 1. The impacts of gender and mating on short-range (< 10 km) dispersal by the whitefly parasitoid Eretmocerus eremicus Rose and Zolnerowich (Hymenoptera: Aphelinidae) were examined. A fluorescent dust-marking technique was also tested. 2. In a vertical flight chamber, female flight duration was significantly longer than that of males, and unmated parasitoids flew for longer than mated parasitoids. The mean flight durations were: unmated females 34 min, mated females 10 min, unmated males 7 min, mated males < 1 min. 3. The dispersal behaviour of E. eremicus was investigated in the field using fluorescent dust. Before doing so, it was determined in the laboratory that this dust did not affect flight behaviour, was retained over the length of the experiments, and allowed large samples to be processed quickly and inexpensively. 4. In the field, traps were placed along annuli at 3, 5, 7, and 10 m from release points. Eighty-seven per cent of the 4153 parasitoids captured were males. Sex ratios were near parity on release. 5. The difference in dispersal characteristics between males and females may be resource based, suggesting that certain requirements were met within the field plots for males that were not met for females. 6. Locally, males dispersed in a manner consistent with a simple diffusion model while females engaged in wind-directed flight soon after leaving release sites. The fact that the genders exhibited dissimilar dispersal characteristics, supports the claim that insect flight, even by small species, can be self-directed.

The nature of migration in the red admiral butterfly Vanessa atalanta: evidence from the population ecology in its southern range

Abstract: Summary 1. The migrant Vanessa atalanta (L.) occurs throughout Europe and North Africa. In autumn, populations emigrate from northern and central Europe to the Mediterranean region to overwinter. In the spring, the northern range is recolonised by migrants from the south. The dynamics of the species in the winter range is poorly known. 2. From 1994 to 1999, adults and immatures of V. atalanta were monitored all year round in Mediterranean habitats in north-east Spain. 3. Data showed that the Catalonia lowlands is an area to which V. atalanta migrates to breed during the winter. Migrants arrive in October and early November and initiate a period of intensive breeding. Larval development occurs throughout the winter until a first annual generation of adults appears in early spring. 4. Most of the butterflies emerging in the spring emigrate and leave the area without breeding. The data suggest strongly that recolonisation of the northern range is by these butterflies not by wintering adults. Altitudinal migration also seems to be a common phenomenon, allowing a further summer generation of adults to occur at high elevations within the Mediterranean region. 5. The complex phenology of V. atalanta in its southern range has evolved as a strategy to track larval resources through space and time. Autumn migration coincides with the greatest availability of the main food plant, Urtica dioica L. Late spring migration occurs by the time food quality is decreasing.

The role of resources and natural enemies in determining the distribution of an insect herbivore population

Abstract: Summary 1. Both resources and natural enemies can influence the distribution of a herbivore. The ideal free distribution predicts that herbivores distribute themselves to optimise utilisation of resources. There is also evidence of herbivores seeking out refuges that reduce natural enemy attack (enemy-free space). Which of these theories predominates in a thistle–tephritid Terellia ruficauda (Diptera: Tephritidae)–parasitoid interaction is examined. 2. The plant, Cirsium palustre, had a contagious distribution approximated by the negative binomial distribution. Terellia ruficauda foraged preferentially and oviposited on isolated plants although its larvae gained neither nutritional benefit nor reduced natural enemy pressure from such behaviour. 3. Parasitoids of T. ruficauda foraged and oviposited more frequently on isolated than on crowded T. ruficauda, resulting in inverse density-dependent parasitoid attack at all spatial scales examined. Neither the herbivore nor natural enemies distributed themselves according to the predictions of the ideal free distribution and the herbivore did not oviposit to reduce natural enemy attack. 4. Extrapolating from the theoretical predictions of the ideal free distribution and enemy-free space to the field requires considerable caution. Terellia ruficauda and its parasitoids appear to select their oviposition sites to spread the risk of losses through factors (e.g. mammal herbivory) that may damage dense clusters of C. palustre.

Egg deposition patterns of fig pollinating wasps: implications for studies on the stability of the mutualism

Abstract: Summary 1. Fig pollinating wasps (Agaonidae) enter Ficus inflorescences (figs), oviposit in some of the flowers, and pollinate in the process. Each larva completes its development within a single flower. In most cases, an inflorescence entered by a wasp will represent its only egg-laying site. The mechanisms that prevent pollinating wasps from exploiting all the flowers inside a fig are not understood. In this study, hypotheses about flower use by pollinating fig wasps were tested by investigating egg deposition patterns in three species. 2. Either one or three wasps were introduced into figs. The figs were then harvested. Serial sections allowed assessment of the presence or absence of a wasp egg in a sample of flowers in each fig. The overall proportion of flowers with eggs and the spatial distribution of eggs were then compared in single wasp figs and three foundress figs. 3. In all species, the proportion of flowers with a wasp egg increased with foundress number but less than three-fold. 4. In all species, at least in single foundress figs, flowers near the fig cavity were more likely to receive a wasp egg than were flowers near the fig wall. 5. In two species, when the number of foundresses was multiplied by three, there was an increase in the use of flowers near the fig wall, while in the third species, the increase was spread evenly among flowers. 6. Factors affecting wasp egg deposition patterns and the potential of investigating such patterns for studying the stability of the mutualism are discussed.

The spatial distribution of beetles within the canopies of oak trees in Richmond Park, U.K.

Abstract: Summary 1. 5054 adult beetles of 144 species were collected in a total of 696 1-m2 collecting trays by knockdown insecticide fogging of 36 different oak trees in closed canopy woodland at Richmond Park, U.K., with three of the trees sampled on each of 12 dates, at 2- to 3-week intervals, between April and October 1984. 2. In late spring (April/May), more individuals and species of beetles were collected in trays close to the trunks of trees than in trays more distant from the trunk. The reverse was the case in late September/October. Neither pattern prevailed in the intervening months. 3. Individual species exhibited a variety of patterns, with some species more abundant near the trunk, e.g. Leiopus nebulosus (L.), Strophosoma melanogrammum (Forster), Cylindronotus laevioctostriatus (Goeze), and Dromius agilis (Fabricius), and some less abundant near the trunk, e.g. Curculio pyrrhoceras (Marsham) and Rhynchaenus signifer (Creutzer). For Adalia decempunctata (L.), this preference changed with season. The observed species preferences for parts of a tree crown near or distant from the main trunk are discussed with reference to their known biologies. 4. No pronounced pattern of preference for north- or south-facing aspects of trees in closed canopy woodland was observed, however populations of some species exhibited patterns of within-tree distribution that correlate with compass angle; for one species, the ladybird Adalia decempunctata, this distribution changed with season and between colour morphs.

The effect of plant drought‐stress on populations of the pea aphid Acyrthosiphon pisum

Abstract: Most studies on the relationship between drought-stress in host plants and aphids have been carried out in the laboratory. These experiments have usually shown that drought-stress results in a decrease in the abundance, survival, and fecundity of aphids (Kennedy et al., 1958; Wearing & van Emden, 1967; Wearing, 1972; Miles et al., 1982; Sumner et al., 1983, 1986a,b; Behle & Michels, 1988; Fereres et al., 1988; Pons & Tatchell, 1995), together with an increase in the proportion of aphids that are winged (alatae) and therefore capable of long distance migration (Stadler, 1990; Baugh & Phillips, 1991; Pons & Tatchell, 1995), although in some cases little effect on the aphids was observed (McMurtry, 1962; Lorenz De Vries & Manglitz, 1982; Salas & Corcuera, 1991). A few contradictory studies have reported increased abundance, fecundity, or survival of aphids on drought-stressed plants (Wearing, 1967, 1972; Miles et al., 1982; Dorschner et al., 1986), which may have been due to reduced transpirational cooling leading to raised leaf temperatures (Mattson & Haack, 1987). The few manipulative studies conducted outside the laboratory have mostly shown that host plant drought-stress decreases aphid abundance (Kennedy & Booth, 1959; Michels & Undersander, 1986; Oswald & Brewer, 1997). The pea aphid Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae) causes extensive damage to pea crops in Europe (Bournoville, 1973; Lane & Walters, 1991; Bommarco & Ekbom, 1995) and North America (Maiteki et al., 1986). The crop growth stage at which aphid numbers begin to decline is variable but always occurs after ̄owering (Lane & Walters, 1991; Walters et al., 1994; McVean & Dixon, 1999) when peas become particularly susceptible to drought-stress (Salter & Drew, 1965; Pate, 1975; Biddle et al., 1988). The work reported here was designed to examine how drought-stress in host plants affects populations of A. pisum outside the laboratory by comparing the growth of aphid populations on watered and experimentally drought-stressed pea plants. The hypotheses tested were that drought-stress reduces the rate at which aphid numbers increase and that aphid numbers decline at a younger plant growth stage on drought-stressed plants than on non-stressed plants. The importance of aphid population density (natural rather than manipulated) to this process was also considered. There is no universally accepted de®nition of plant droughtstress (Larsson, 1989). Speight (1986) suggested that host plant drought-stress should be described as a decrease in plant growth and reproduction. Larsson (1989) proposed that host plant drought-stress should be based solely on insect performance. In the study reported here, when plants are described as drought-stressed, both these criteria of plant stress are met. Changes in both insect performance and in plant productivity followed when the amount of water available to the plant was reduced.

Can imperfect host discrimination explain partial patch exploitation in parasitoids

Abstract: 1. Host discrimination by Aphidius rhopalosiphi (De Stefani Perez) (Hymenoptera: Braconidae) was first studied on the grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae). Females tended to avoid oviposition in hosts parasitised 3h earlier. No evidence of host discrimination ability on freshly parasitised hosts was suggested, however, and ovipositional experience had no effect on host discrimination. 2. The effects of host discrimination ability on the exploitation strategy of patches containing different proportions of unparasitised hosts and hosts parasitised for 3 h were studied. Females spent less time on patches with a higher proportion of parasitised hosts, reflecting the females' ability to perceive the potential profitability of the patch. This ability may be based on the nature of the hosts encountered (unparasitised or parasitised). 3. Incomplete exploitation of unparasitised hosts was also observed. It seems that this partial exploitation is related to the inability of A. rhopalosiphi to recognise freshly parasitised hosts. As a female may experience a risk of self-superparasitism during patch depletion, this could promote early departure from incompletely exploited patches. 4. The effect of previous experience on the patch exploitation strategy was also assessed. Females were tested twice on two patches of the same quality. Results suggested that the experience acquired during a previous visit led the females to leave the patch sooner and to lay fewer eggs in parasitised hosts. 5. Patch exploitation strategy may therefore be the result of different factors such as host discrimination and experience. The evolutionary consequences of the results are discussed.

Assessment of the relative impact of different natural enemies on population dynamics of the grain aphid Sitobion avenae in the field

Abstract: Summary 1. A detailed population dynamics model was devised to provide a tool for integrated pest management against the cereal aphid Sitobion avenae on winter wheat. 2. This model allowed investigation of the relative impact of different natural enemies on aphid population dynamics. 3. The output of the model was compared with a set of data collected in the western part of France from 1976 to 1986. 4. Fungal diseases accounted for 75% of the reduction in peak aphid density and were the key factor acting on aphid dynamics in this region. 5. This study highlights the importance of detailed population dynamics modelling of keystone species, like aphids, for elucidation of the relations between the keystone species and other species associated in the ecosystem.

Microgeographic genetic structure and intraspecific parasitism in the ant Leptothorax nylanderi

Abstract: 1. Genetic colony structure of the small central European ant Leptothorax nylanderi is affected strongly by ecological constraints such as nest site availability and intraspecific social parasitism. 2. Although L. nylanderi is generally monogynous and monandrous, more than a quarter of all nests collected in a dense population near Wurzburg, Germany, contained several matrilines. As shown by microsatellite analysis, the average nest-mate relatedness in these nests was 0.20. Genetically heterogeneous nests arise from nest take-over by alien colonies or founding queens, a result of severe competition for nest sites. 3. In summer, more than one-third of all colonies inhabited several nest sites at a time. Polydomy appears to be rather limited, with two or three nests belonging to a single polydomous colony. 4. Queens appear to dominate male production; only a small fraction (8%) of males was definitively not progeny of the queen present but might have been worker progeny or offspring of another queen. 5. Strong evidence for heterozygote deficiency was found and a total of nine diploid males was discovered in two colonies. These findings suggest deviation from random mating through small, localised nuptial flights.

Short‐range clinal variation in the prevalence of a sexually transmitted fungus associated with urbanisation

Abstract: Sexually transmitted pathogens and parasites have attracted much recent attention by virtue of differences in epidemiology and biology from classical pathogens and parasites (Getz & Pickering, 1983; Anderson & May, 1991; Lockhart et al., 1996). Epidemiologically, the rate of transmission of a sexually transmitted parasite is likely to be less dependent on host density than for ordinary infectious diseases, copulation frequency being generally less affected by density than by gross contact rate (Anderson & May, 1991). While work on the dynamics of sexually transmitted diseases in mammals, especially humans, has been intense (Anderson & May, 1991), study of these diseases in insects is rare. Where studies have been carried out, they have gone little beyond the observation that certain parasites and pathogens of insects are sexually transmitted, with some note of their virulence but little or no information about the dynamics of infection (e.g. Hurst et al., 1995). This is unfortunate, as the dynamics of these diseases in insects are likely to differ from those of their better understood mammalian analogues because of the short lifespan of most adult insects and the voltinism of many insect populations. Study of the factors affecting the dynamics, and therefore prevalence and incidence, of sexually transmitted disease in insects is therefore timely. One interaction that bears inspection is that between Laboulbeniales fungi and their hosts. The Laboulbeniales is a group of Ascomycete fungi where species grow on the cuticle of their host, are generally specialised on one or two host species, and cause little pathology beyond increased melanisation of the cuticle around the point of infection (Weir & Beakes, 1995). They are frequently, but not exclusively, transmitted during sexual contact (Benjamin & Shalor, 1952; Whisler, 1968; Strandberg & Tucker, 1974; De Kesel, 1995). The interaction between a coccinellid beetle, Adalia bipunctata, and the laboubenialian fungus Hesperomyces virescens presents a potentially useful case study for aiding understanding of disease dynamics, where incidence appears to vary over fairly short distances. The fungus was originally described on A. bipunctata in the London area (Weir & Beakes, 1996) but earlier examination of the host outside London failed to reveal the presence of infection (G. D. D. Hurst, pers. obs.), suggesting an effect of urbanisation on disease incidence and dynamics. The work reported here had two objectives: (1) To examine whether transmission during host copulation is important for this parasite. This was attained by testing whether males were more likely to be infected on their ventral surface and females on their dorsum, paralleling the position of the sexes during copulation, and echoing observations of other sexually transmitted fungi (Benjamin & Shalor, 1952; Whisler, 1968; De Kesel, 1995). (2) To quantify systematically the extent of variation in prevalence over space, the extent to which this variation altered with time, and the potential role of variation in host phenology in producing the observed changes. Extreme variation in prevalence over a remarkably short distance was observed, with high prevalence in urban areas of London but absence outside these areas. The possible causes of this shortrange variation in prevalence in terms of the effect of urbanisation on host phenology are discussed.

Multiple mating reduces longevity of females of the windmill butterfly Atrophaneura alcinous

Abstract: 1. The effects of successive matings by females on their performance were tested in laboratory experiments and field censuses for the windmill butterfly Atrophaneura alcinous. 2. In laboratory experiments, newly eclosed females were hand-paired either once or twice successively. The number of eggs laid and the sucrose solution intake of these females were examined daily. Fecundity and sucrose solution intake did not differ significantly between once- and twice-mated females, however the longevity of twice-mated females was significantly shorter than that of once-mated females, implying that successive matings involve a physiological cost. 3. The results of a mark-release-recapture census in a natural population also indicated that multiply mated females could not survive for as long as once-mated females. 4. The results did not support the general consideration that multiple mating is beneficial to females in Lepidoptera. The importance of the timing of remating by females on the consequences of multiple mating is discussed.

An adaptive benefit of facultative coprophagy in the German cockroach Blattella germanica

Abstract: A possible adaptive benefit of coprophagy was investigated in nymphs of the German cockroach Blattella germanica (L.). 2. Newly ecdysed first instars, given no source of food other than conspecific faeces, survived significantly longer than first instars deprived of faeces. The faeces of adult males and females may be of different quality, however, because nymphs given female faeces were more likely to moult into the second stadium than nymphs given male faeces. 3. In contrast to first instars, second instars provided adult faeces survived only slightly longer than starved counterparts. Faecal feeding is therefore stage-specific, as is the benefit derived from it. 4. The relationship between the nutrient composition of faeces and the survival of nymphs was also examined. First instars fed the faeces of adults that had been maintained on a high (50%) protein diet, died more slowly than first instars fed the faeces of adults that had been maintained on medium (22.5%) and low (5%) protein diets. Chemical analysis of faeces showed that the concentration of protein in adult faeces increased with the level of protein in the diet. Moreover, food choice assays showed that first instars, unlike adults, ingested more of the high-protein diets. 5. These data support the idea that coprophagy is a stage-specific adaptive behaviour that permits first instars to moult into the second stadium with minimal foraging.

What causes male‐biased sex ratios in mature damselfly populations?

Abstract: Summary 1. Several hypotheses to account for biased sex ratios in mature insect populations were tested by monitoring two field populations of the damselfly Lestes sponsa and by performing experiments in field cages. The population sex ratios are heavily male biased in this species. 2. The observed sex ratio at emergence was even and both sexes emerged synchronously. Females had longer maturation times but these were insufficient to explain the observed sex ratio shift. 3. Mass increases during maturation were consistently larger in females. In agreement with this, immature females made more flights per unit of time, which should make them more vulnerable to predation, however maturation probabilities were lower in females only in one field cage experiment. This inconsistency may be due to long bad weather conditions. Interestingly, predators reduced mass increase and this reduction was larger in females than in males. 4. Calculations based on the sex specific maturation times show that only slightly lower daily survival probabilities during maturation in females are enough to generate the observed sex ratio shift. 5. Mature survival was higher in males than in females in one field population but not in another, indicating that this cannot be a general mechanism causing the sex ratio. A higher maturation probability in males is therefore the most plausible mechanism causing the sex ratio shift in damselfly populations.

Male‐biased sex ratios in mature damselfly populations: real or artefact?

Abstract: Summary 1. There is ongoing controversy about whether biased sex ratios in diploid insect populations are real or an artefact caused by different behaviours and/or different catchability of the sexes. This was tested by monitoring two field and three semi-natural populations of the damselfly Lestes sponsa. 2. Capture–mark–recapture data showed that population size estimates were about twice as large for males as for females at both field sites. Independent estimates of the sex ratios based on total numbers of males and females captured supported the male bias. 3. Males had higher recapture probabilities than females due to longer times between successive visits in females. Because the same pattern was found in the semi-natural populations, the longer intervals in females are no artefact due to their lower detectability. 4. Theoretical models show that the strong temporary emigration of females tends, if anything, to overestimate female population sizes and that the heterogeneity of recapture probabilities observed in males tends to underestimate male population sizes. Hence, behavioural differences between the sexes do not cause an artificially male-biased sex ratio. 5. Spatial data show that operational sex ratios are male biased at the pond but become female biased in the plots further away from the shoreline; however because of the decrease in densities away from the shoreline, this does not result in a global even sex ratio. 6. Spatial data, temporary emigration patterns, and independent estimates suggest strongly that the male-biased sex ratios in mature damselfly populations are real.

Do parasitoids diversify in response to host‐plant shifts by herbivorous insects?

Abstract: Summary 1. For herbivorous insects, the incorporation of a novel host into the diet, and subsequent formation of distinct host associations (races), is thought to be a significant early step in the speciation process. While many studies have addressed this issue, virtually nothing is known about the evolutionary response of natural enemies to herbivore host-race formation. 2. The hypothesis that the parasitoid wasp Eurytoma gigantea (Hymenoptera: Eurytomidae) has formed host races in direct response to the host shift and subsequent host-race formation by its host, the gallmaker Eurosta solidaginis (Diptera: Tephritidae) was tested. Emergence time, mating preference, and female oviposition preference were determined for parasitoids derived from galls of each Eurosta host race. 3. Male and female E. gigantea overlap broadly in their emergence times from each Eurosta host race, suggesting that there is no phenological barrier to gene flow. 4. In choice experiments, female parasitoids did not mate assortatively: females that emerged from one Eurosta host race were equally likely to mate with males from either Eurosta host race. 5. Oviposition behaviour experiments revealed that female parasitoids do not prefer to oviposit on their host race of origin and that there is no overall preference for one host race, even though fitness is higher when parasitoids are reared from Eurosta galls of the Solidago gigantea host race than when reared from Eurosta galls of the Solidago altissima host race. 6. These results suggest that E. gigantea has not diverged in parallel with its host in response to the herbivore host-plant shift. Further studies are needed before the ubiquity of this diversification mechanism can be evaluated fully.