Showing papers in "Ecological Entomology in 2002"

Facultative bacterial endosymbionts benefit pea aphids Acyrthosiphon pisum under heat stress

Abstract: 1. Natural populations of pea aphids in California contain at least two facultative bacterial secondary symbionts (pea aphid secondary symbiont, PASS, or pea aphid rickettsia, PAR) in a range of frequencies throughout the state. 2. Two pea aphid clones without either of these facultative associates failed to reproduce in the first 8 days after the final moult if they had been heat-stressed for a period of about 4 h at 39 °C as 1-day-old larvae in the laboratory. 3. Aphids infected artificially with PASS, however, were able to produce up to 48% of the normal complement of offspring produced by PASS-positive aphids that had not been heat-stressed. Clones infected artificially with PAR did not have the same advantage as those with PASS after heat stress. 4. In aphids without PASS or PAR, heat stress reduced the number of bacteriocytes (in which the obligate primary symbiont, Buchnera, resides) to 7% of non-heat-stressed aphids, while aphids with only PASS retained 70% of their bacteriocytes. Bacteriocytes in aphids with PAR but not PASS were reduced to 42% of controls. 5. When larvae were heat-stressed as older instars (5 days old), a similar pattern emerged, though the effect of heat stress was less extreme. Clones containing PASS produced the most offspring, three to 14 times as many as aphids without PASS or PAR. Aphids with PAR only, or PASS and PAR together, had reduced or no advantage over aphids without facultative symbionts. 6. Aphids of all clones that had been heat-stressed as later instars gave birth to a variable number of stillborn offspring. Aphids without facultative symbionts produced the most stillborn larvae. 7. Field studies showed a higher incidence of PASS in aphids collected in California in summer compared with aphids from the same sites collected 2–4 months earlier. The difference was significant in two of three widely dispersed locations.

Uptake of Bt-toxin by herbivores feeding on transgenic maize and consequences for the predator Chrysoperla carnea

Abstract: 1. Chrysoperla carnea is an important predatory insect in maize. To assess the ecological effects of Bt-maize, expressing the Cry1Ab protein, on larvae of this predator, the following factors were examined: (1) the performance of three prey herbivores (Rhopalosiphum padi, Tetranychus urticae, and Spodoptera littoralis) on transgenic Bt and non-transgenic maize plants; (2) the intake of the Cry1Ab toxin by the three herbivores; and (3) the effects on C. carnea when fed each of the prey species. 2. The intrinsic rate of natural increase (rm) was used as a measure of performance for R. padi and T. urticae. No difference in this parameter was observed between herbivores reared on Bt or non-transgenic plants. In contrast, a higher mortality rate and a delay in development were observed in S. littoralis larvae when fed Bt-maize compared with those fed the control maize plants. 3. The ingestion of Cry1Ab toxin by the different herbivores was measured using an immunological assay (ELISA). Highest amounts of Cry1Ab toxin were detected in T. urticae, followed by S. littoralis, and only trace amounts detected in R. padi. 4. Feeding C. carnea with T. urticae, which were shown to contain the Cry1Ab toxin, or with R. padi, which do not ingest the toxin, did not affect survival, development, or weight of C. carnea. In contrast, a significant increase in mortality and a delay in development were observed when predators were fed S. littoralis larvae reared on Bt-maize. 5. A combined interaction of poor prey quality and Cry1Ab toxin may account for the negative effects observed on C. carnea when fed S. littoralis. The relevance of these findings to the ecological risks of Bt-maize on C. carnea is discussed.

Dung beetles in a Central Amazonian rainforest and their ecological role as secondary seed dispersers

Abstract: 1. The role of several factors that affect the composition of the dung beetle assemblages in an Amazonian rainforest was quantified, together with the effect of these factors on the role that dung beetles play as secondary seed dispersers. 2. A total of 61 dung beetle species was captured during 3360 h of trapping. During nocturnal trapping periods, more dung beetles, of larger mean size, and more species were captured per trap than during diurnal trapping periods. 3. During the rainy season, more dung beetle species were captured per trap than during the dry season, but the number of individuals and their mean size did not vary between seasons. 4. Bait size had a significant effect on the mean number of beetles and mean number of species but not on mean beetle size. As bait size increased from 5, 10, 25, to 50 g, more beetles and more species were captured per trap. 5. Between 6 and 73% of plastic beads, used as seed mimics, were buried by dung beetles at depths that ranged from 0.5 to 7 cm. Both the proportion of beads buried and burial depth decreased with increasing bead size, and increased with increasing amounts of dung surrounding each bead (5, 10, and 25 g). 6. The proportion of buried seeds for three species varying in size between 5 and 27 mm, increased with increasing dung beetle size, using beetles of seven sizes, varying between 10 and 25 mm. 7. Seeds surrounded by dung were buried more often and more deeply when placed on the forest floor during the late afternoon than when placed during the early morning. Seeds were also buried more often when placed on the forest floor during the rainy season than when placed during the dry season, but season had no effect on burial depth. 8. Forests in Central Amazonia hold a rich dung beetle community that plays an active role in secondary seed dispersal, and consequently in plant regeneration. The interaction between seeds and beetles is complex because it is affected by many factors.

High-altitude migration of the diamondback moth Plutella xylostella to the U.K.: a study using radar, aerial netting, and ground trapping.

Abstract: 1. The high-altitude wind-borne migration of the diamondback moth Plutella xylostella in the U.K. in 2000 was investigated (a) by direct monitoring of insect flight by vertical-looking radar and by aerial netting, and (b) through evidence of temporal variation in P. xylostella abundance deduced from a network of light traps. 2. Migrating P. xylostella were identified by a unique combination of size and shape data derived from the continuously operating vertical-looking radar. 3. Radar-detected migratory overflights correlated significantly with associated peaks in abundance of P. xylostella estimated by catches in a U.K.-wide light trap network; however the correlation was stronger when light trap catches were lagged by 1 day. 4. The first notable catches of P. xylostella in the U.K. occurred in early May, and were accompanied by migrations over the radar from the east. 5. Radar data and back-tracking indicated that a major wind-borne migration of P. xylostella from The Netherlands to southern England took place in early May, and that this was responsible for the establishment of the U.K. population. 6. The origin of early-season P. xylostella occurring in Britain is discussed.

Landscape context affects trap‐nesting bees, wasps, and their natural enemies

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A minimalist approach to the effects of density-dependent competition on insect life-history traits.

Abstract: 1. Due to its effects on the phenotypic and genotypic expression of life-history traits, density-dependent competition is an important factor regulating the growth of populations. Specifically for insects, density-dependent competition among juveniles is often associated with increased juvenile mortality, delayed maturity, and reduced adult size. 2. The aim of the work reported here was to test whether the established phenotypic effects of density-dependent competition on life-history traits could be reproduced in an experimental design requiring a minimal number of individuals. Larvae of the mosquito Aedes aegypti were reared at densities of one, two, or three individuals per standard Drosophila vial and in six different conditions of larval food availability. This design required relatively few individuals per independent replicate and included a control treatment where individuals reared at a density of one larva per vial experienced no density-dependent interactions with other larvae. 3. Increased larval densities or reduced food availability led to increased larval mortality, delayed pupation, and the emergence of smaller adults that starved to death in a shorter time (indicating emergence with fewer nutritional reserves). 4. Female mosquitoes were relatively larger than males (as measured by wing length) but males tended to survive for longer. These differences increased as larval food availability increased, indicating the relative importance of these two traits for the fitness of each sex. The role of nutritional reserves for the reproductive success of males was highlighted in particular. 5. This minimalist approach may provide a useful model for investigating the effects of density-dependent competition on insect life-history traits.

Allee effect in larval resource exploitation in Drosophila: an interaction among density of adults, larvae, and micro-organisms

Abstract: 1. Aggregation pheromones can evolve when individuals benefit from clustering. Such a situation can arise with an Allee effect, i.e. a positive relationship between individual fitness and density of conspecifics. Aggregation pheromone in Drosophila induces aggregated oviposition. The aim of the work reported here was to identify an Allee effect in the larval resource exploitation by Drosophila melanogaster , which could explain the evolution of aggregation pheromone in this species. 2. It is hypothesised that an Allee effect in D. melanogaster larvae arises from an increased efficiency of a group of larvae to temper fungal growth on their feeding substrate. To test this hypothesis, standard apple substrates were infested with specified numbers of larvae, and their survival and development were monitored. A potential beneficial effect of the presence of adult flies was also investigated by incubating a varying number of adults on the substrate before introducing the larvae. Adults inoculate substrates with yeast, on which the larvae feed. 3. Fungal growth was related negatively to larval survival and the size of the emerging flies. Although the fungal growth on the substrate was largely reduced at increased larval densities, the measurements of fitness components indicated no Allee effect between larval densities and larval fitness, but rather indicated larval competition. 4. In contrast, increased adult densities on the substrates prior to larval development yielded higher survival of the larvae, larger emerging flies, and also reduced fungal growth on the substrates. Hence, adults enhanced the quality of the larval substrate and significant benefits of aggregated oviposition in fruit flies were shown. Experiments with synthetic pheromone indicated that the aggregation pheromone itself did not contribute directly to the quality of the larval resource. 5. The interaction among adults, micro-organisms, and larval growth is discussed in relation to the consequences for total fitness.

Effects of photoperiod, temperature, and host plant age on induction of reproductive diapause and development time in Danaus plexippus

Abstract: 1. Diapause induction in monarch butterflies Danaus plexippus was studied using adults captured from the wild in Minnesota and Wisconsin, and individuals reared under outdoor and controlled conditions. 2. Oocyte presence in females and ejaculatory duct mass in males were used to indicate reproductive status. 3. Some wild individuals were in diapause in mid-August, and all males and females were in diapause by late August and early September respectively. 4. Individuals reared under decreasing day lengths and fluctuating temperatures were more likely to be in diapause than were individuals reared under long or short day lengths or constant temperatures. 5. Individuals fed potted old Asclepias curassavica plants were more likely to be in diapause than were those fed potted young host plants; when cuttings of Asclepias syriaca plants from the field or greenhouse pots were used, there was no effect of host plant age. 6. Extremely high temperatures increased the number of day-degrees required for development from egg to adult, while decreasing day lengths and older host plants tended to decrease the number of day-degrees required for development. 7. There appears to be a continuum of reproductive development in monarchs, with gradual declines in mean ejaculatory duct mass and oocyte production during the late summer. 8. None of the experimental treatments led to 100% diapause, and diapause was more likely to occur in monarchs subjected to more than one diapause-inducing cue.

A trade‐off between diapause duration and fitness in female parasitoids

Abstract: 1. Diapause allows insects to cope with adverse weather conditions but also poses substantial fitness costs, e.g. through reduced survival of the diapausing stage or sublethal effects in the following season. 2. The energetic costs of diapause in females of the parasitoid Asobara tabida Nees (Hymenoptera: Braconidae) were assessed using experimental manipulation of diapause duration. 3. An increase in diapause length not only led to higher mortality among diapausing pupae but also caused a significant decrease in egg load, fat reserves, and dry weight of the emerging adult females. 4. Only larvae with sufficient resources were able to survive the entire diapause period and there was a trade-off between the metabolic costs of diapause and adult fitness components. 5. In contrast, the size (tibia length) of emerging females increased with increasing diapause duration. It is proposed that this may be due to a genetic correlation between larval dry weight and adult size, so that selection for heavier larvae would result in larger adults even when resources are limited. 6. Ultimately, the evolution of diapause duration and its consequences for adult performance will depend on the costs of overwintering and the ecological conditions in the habitat.

Body size as an estimator of production costs in a solitary bee

Abstract: 1 Body weight is often used as an estimator of production costs in aculeate Hymenoptera; however, due to differences between sexes in metabolic rates and water content, conversion of provision weight to body weight may differ between males and females As a result, the cost of producing female progeny may often have been overestimated 2 Provision weight and body weight loss throughout development were measured in a solitary bee, Osmia cornuta (Latreille), to detect potential differences between sexes in food weight/body weight conversion 3 Male O cornuta invest a larger proportion of larval weight in cocoon spinning, and presumably have higher metabolic rates than females during the larval period; however, this is compensated by a slightly longer larval period in females 4 Overall, body weight loss throughout the life cycle does not differ significantly between sexes As a result, cost production ratios calculated from provision weights and from adult body weights are almost identical 5 The validity of other weight (cocoon, faeces) and linear (head width, intertegular span, wing length, cocoon length, and cell length) measures as estimators of production costs is also discussed 6 Valid estimators of production costs vary across species due to differences in sex weight ratio, cocoon shape, provision size in reference to cell size, and adult body size

Senescence of immune defence in Bombus workers

Abstract: 1. Senescence in workers of social insects is a particularly intriguing life-history trait as the future fitness of workers relies primarily on age-dependent survival rate. The pattern of senescence of immune defence traits was investigated under laboratory conditions in workers of two bumble bees: Bombus terrestris and B. lucorum. 2. In both species, there was a significant decrease with age in the ability to encapsulate a foreign object (a global measure of the efficiency of immune systems). This pattern of senescence was observed in all colonies in B. terrestris (seven) and B. lucorum (eight) assayed, even though, for the latter, there was some heterogeneity among colonies. 3. In B. terrestris, two other measures of immune defence were taken: the relative percentage of fat body in the abdomen and the concentration of haemocytes (the immune defence cells). The quantity of fat body increased only slightly with age and there was no effect for the concentration of haemocytes. Interestingly, the concentration of haemocytes decreased strongly after an encapsulation response, regardless of the age of workers. 4. The importance of the senescence pattern observed for the immune defence traits is discussed in the context of the social biology of workers.

Patterns of pollen feeding and habitat preference among Heliconius species

Abstract: 1. The ecological circumstances that precipitate speciation remain poorly understood. Here, a community of Heliconius butterflies in lowland Panama was studied to investigate patterns of pollen use, and more specifically the ecological changes associated with the recent divergence of Heliconius melpomene (Linnaeus) and H. cydno (Doubleday). 2. Considering the seven commonest Heliconius species in the community, 32 types of pollen or spore were encountered in pollen loads but only five pollen species were common. Systematic exploitation of pollen was therefore confined to a small proportion of the flowers visited. 3. Most of the variation in pollen load composition between individuals was explained by differences in collecting locality. The exception was Psiguria, which was used in all habitats by the melpomene/hecale clade far more than by the erato/sapho clade. This may suggest an ancestral switch within Heliconius towards increased reliance on Psiguria pollen. 4. Heliconius cydno and H. melpomene differed significantly in pollen load composition for three of the five most commonly collected pollen species. This is most probably explained by differences in habitat preference; H. melpomene and its co-mimic H. erato are found in open habitat while H. cydno and its co-mimic H. sapho are found in closed-canopy forest. 5. As melpomene and cydno are known to hybridise occasionally, such differences in adult microhabitat contribute to pre-mating isolation. Habitat divergence between H. cydno and H. melpomene, which is associated with changes in mimicry, must have played a role in their recent speciation.

An experimental examination of the effects of habitat quality on the dispersal and local abundance of the butterfly Parnassius smintheus

Abstract: 1. Nectar flower abundance was manipulated through flower removal, and sex ratio was manipulated by moving individual butterflies within a series of nine alpine meadows. The movement and abundance of the butterfly Parnassius smintheus in the meadows were monitored using mark-release-recapture methods. 2. A total of 937 butterflies, 698 males and 239 females, was captured. There were 223 observed between-meadow movements. Fifty-two per cent of males and 35% of females moved among meadows. 3. The immigration of male butterflies was related positively to nectar flowers, host plant abundance, and female butterflies. Male emigration was not affected by any of the treatments. The number of males captured was related positively to nectar flowers and host plants but not affected by sex ratio. The number of resident male butterflies was greater in meadows containing flowers and was related positively to host plant abundance, but unaffected by sex ratio. 4. Flower removal, sex ratio, and abundance of Sedum had no significant effect on the abundance, movement, or residence time for female butterflies, in part due to small sample size. 5. The fact that males immigrate to higher quality meadows suggests that male butterflies are assessing meadow quality, either by sampling meadows or potentially from a distance using olfactory cues.

A comparison of the host‐searching efficiency of two larval parasitoids of Plutella xylostella

Abstract: Summary 1. A host specialist parasitoid is thought to have greater efficiency in locating hosts or greater ability to overcome host defence than a generalist species. This leads to the prediction that a specialist should locate and parasitise more hosts than a generalist in a given arena. The work reported here tested these predictions by comparing the host-searching behaviour of Diadegma semiclausum (a specialist) and Cotesia plutellae (an oligophagous species), two parasitoids of larval Plutella xylostella. 2. Both parasitoids employed antennal search and ovipositor search when seeking hosts but D. semiclausum also seemed to use visual perception in the immediate vicinity of hosts. 3. Larvae of P. xylostella avoided detection by parasitoids by moving away from damaged plant parts after short feeding bouts. When they encountered parasitoids, the larvae wriggled vigorously as they retreated and often hung from silk threads after dropping from a plant. 4. These two parasitoids differed in their responses to host defences. Diadegma semiclausum displayed a wide-area search around feeding damage and waited near the silk thread for a suspended host to climb up to the leaf, then attacked it again. Cotesia plutellae displayed an area-restricted search and usually pursued the host down the silk thread onto the ground. 5. Diadegma semiclausum showed a relatively fixed behavioural pattern leading to oviposition but C. plutellae exhibited a more plastic behavioural pattern. 6. The time spent by the two parasitoids on different plants increased with increasing host density, but the time spent either on all plants or a single plant by D. semiclausum was longer than that of C. plutellae. Diadegma semiclausum visited individual plants more frequently than C. plutellae before it left the patch, and stung hosts at more than twice the rate of C. plutellae. 7. The results indicated that the host-location strategies employed by D. semiclausum were adapted better to the host's defensive behaviour, and thus it was more effective at detecting and parasitising the host than was C. plutellae.

Host utilisation by moth and larval survival of pine processionary caterpillar Thaumetopoea pityocampa in relation to food quality in three Pinus species

Abstract: 1. The work reported here analysed host utilisation by the pine processionary moth Thaumetopoea pityocampa (Lepidoptera: Thaumetopoeidae), the relationship between moth oviposition patterns and larval performance, the chemical characteristics of the plant in relation to the performance of different larval instars, and the role of these factors in the outbreak capacity of the species. In order to do this, a combination of field and laboratory techniques was used to study three pine species differing in nutritional characteristics. 2. Moths oviposited in the three pine species analysed, although cluster pine received a lower number of batches. Late-instar larvae were able to feed on all three pine species, however first-instar larvae developed on Scots and black pine but died on cluster pine. Consequently, oviposition in cluster pine can be considered an oviposition mistake, and indicates that moths are rather unselective when ovipositing on different pine species. 3. Chemical analysis of needles suggested that the quantity of nitrogen was the main factor responsible for the difference in survival of larvae. 4. The oviposition pattern of the moth and the larval susceptibility to food quality in Thaumetopoea pityocampa agree with the theory that unselective oviposition of the moth is a precondition for eruptive dynamics in phytophagous insects. 5. The success of the larvae depends mainly on the probability of the moth finding a suitable host. This probability is determined strongly by the changes of structure and coverage in Mediterranean pine woodlands, due to human management.

Flexibility in the composition and concentration of amino acids in honeydew of the drepanosiphid aphid Tuberculatus quercicola

Abstract: 1. Mutualistic interactions between aphids and ants are mediated by honeydew that aphids produce. Previous work showed that when attended by the ant Formica yessensis Forel (Hymenoptera: Formicidae), nymphs of the aphid Tuberculatus quercicola (Matsumura) (Homoptera: Aphididae) developed into significantly smaller adults with lower fecundity than did nymphs that were not ant attended. 2. This study tested the hypothesis that this cost of ant attendance arises through changes in the quality and quantity of honeydew. Ant-attended and ant-excluded aphid colonies were prepared in the field. The composition and concentration of amino acids were compared between the honeydew produced by ant-attended colonies and that produced by ant-excluded colonies. 3. The aphids excreted smaller droplets of honeydew, but also excreted them more frequently, in ant-attended colonies than in ant-excluded colonies. The honeydew of ant-attended aphids contained more types of amino acid, and a significantly higher total concentration of amino acids, than did the honeydew of ant-excluded aphids. 4. These results suggest that the increase in the concentration of amino acids in honeydew leads to a shortage of nitrogen available for aphid growth and reproduction, resulting in lower performance under ant attendance. 5. With the advance of seasons, a significant reduction was found in both the total free amino acid concentration in phloem sap and the frequency of honeydew excretion; however the total concentration of amino acids in the honeydew did not vary significantly during the seasons, suggesting that aphids keep the quality of honeydew constant in order to maintain ant visitation.

Oviposition patterns in a predatory mite reduce the risk of egg predation caused by prey

Abstract: 1. Predatory arthropods lay their eggs such that their offspring have sufficient prey at their disposal and run a low risk of being eaten by conspecific and heterospecific predators, but what happens if the prey attacks eggs of the predator? 2. The egg distribution and time allocation of adult female predatory mites Iphiseius degenerans as affected by predation of their eggs by prey, the western flower thrips Frankliniella occidentalis, were studied on sweet pepper plants. The predatory mites attack the first instar of thrips but all active stages of thrips are capable of killing the eggs of the predator; however the predatory mite is used for biological control of thrips. 3. The majority of predatory mite eggs was laid on the underside of leaves in hair tufts (domatia). During the experiment, females spent increasing amounts of time in flowers where they fed on pollen and thrips larvae. The risk of predation on predator eggs by thrips was lower on leaves than in flowers where the majority of thrips resides. Moreover, predation risk was higher outside leaf domatia than inside. 4. This suggests that predators avoid ovipositing in places with abundant prey to prevent their eggs from being eaten by thrips.

Cascading effects of variation in plant vigour on the relative performance of insect herbivores and their parasitoids

Abstract: 1. Consequences of variation in food plant quality were estimated for a system consisting of two monophagous noctuid herbivores and three ichneumonid parasitoids. 2. In a natural population, pupal weights of the herbivores in this system, Nonagria typhae and Archanara sparganii, were found to be highly variable. Pupal weights increased strongly and consistently with the increase in the vigour of the host plant, Typha latifolia, providing support for the plant vigour hypothesis. Correspondingly, as the moths do not feed as adults, a strong, positive correlation between host vigour and fecundity of the herbivores would be expected. 3. There were strong and positive relationships between adult body sizes of the parasitoids and the sizes of their lepidopteran hosts. Moreover, a direct, positive link between plant quality and parasitoid size was documented. 4. For all three parasitoids, cascading effects of plant quality on body size were weaker than for the herbivores. Differences in the importance of adult feeding and oviposition behaviour suggest that dependence of fitness on body size is also weaker in the parasitoids than in the moths. It is therefore concluded that the numerical response of the herbivore population to a change in plant quality should exceed the corresponding response in the parasitoids. 5. The results of this work imply that variation in plant variables may affect performance of different trophic levels to a different extent. It is suggested that the importance of adult feeding for the reproductive success (capital vs. income breeding strategies) in both herbivores and parasitoids is an essential aspect to consider when predicting responses of such a system to changes in plant quality.

Triangular fecundity function and ageing in ladybird beetles

Abstract: 1. In insects, the age schedules of fecundity tend to be triangular and this has been attributed more to temporal patterns of mortality than to fecundity. The objective of the work reported here was to test the assumption that senescence shapes the fecundity function in ladybird beetles, and in particular that the production function declines with age. 2. The results of a laboratory study on three species of predatory ladybird beetle indicated that the efficiency with which these insects acquire and process food declined with age. Although supplied with the same amount of food each day, after the onset of reproduction, these beetles ate less and less with increasing age. Egg production mirrored the decline in aphid consumption. Associated with this was a decline in fertility, assimilation, and speed of locomotion with age. 3. This study indicates that production declined with age and that this shaped the fecundity schedules in these ladybird beetles. In addition, the results indicated that ladybirds are income breeders and, as predicted, the reproductive effort of the small species was greater than that of the large species used in this study.

Geographical variation in host-ant specificity of the parasitic butterfly Maculinea alcon in Denmark

Abstract: 1. Maculinea alcon uses three different species of Myrmica host ants along a north–south gradient in Europe. Based on this geographical variation in host ant use, Elmes et al. (1994) suggested that M. alcon might consist of three or more cryptic species or host races, each using a single and different host-ant species. 2. Population-specific differences in allozyme genotypes of M. alcon in Denmark (Gadeberg & Boomsma, 1997) have suggested that genetically differentiated forms may occur in a gradient across Denmark, possibly in relation to the use of different host ants. 3. It was found that two host-ant species are indeed used as hosts in Denmark, but not in a clear-cut north–south gradient. Furthermore, specificity was not complete for many M. alcon populations. Of five populations investigated in detail, one used primarily M. rubra as a host, another exclusively used M. ruginodis, while the other three populations used both ant species. No population in Denmark used M. scabrinodis as a host, although this species was present in the habitat and is known to be a host in central and southern Europe. 4. In terms of number of parasites per nest and number of nests parasitised, M. rubra seems to be a more suitable host in populations where two host species are used simultaneously. Host-ant species has an influence on caterpillar size but this varies geographically. Analyses of pupae did not, however, show size differences between M. alcon raised in M. rubra and M. ruginodis nests. 5. The geographical mosaic of host specificity and demography of M. alcon in Denmark probably reflects the co-evolution of M. alcon with two alternative host species. This system therefore provides an interesting opportunity for studying details of the evolution of parasite specificity and the dynamics of host-race formation.

Prior flea beetle herbivory affects oviposition preference and larval performance of a potato beetle on their shared host plant

Abstract: 1. The herbaceous plant Solanum carolinense (L.) (Solanaceae) is host to a number of specialist insects, including the leaf-feeding beetles Epitrix fuscula (Crotch) and Leptinotarsa juncta (Germar) (Coleoptera: Chrysomelidae). Potted individuals of S. carolinense were subjected to one of two treatments: exposure to herbivory by E. fuscula or exclusion of all herbivores. The effects of E. fuscula herbivory on larval performance and oviposition preference of L. juncta were investigated. 2. Although the masses of the L. juncta pupae did not differ between the two treatments, larvae feeding on damaged plants developed more slowly than those feeding on undamaged plants. 3. In both paired leaf choice trials and whole plant choice trials, larvae of L. juncta showed no preference for undamaged versus damaged hosts. 4. In a field transplant experiment, adult L. juncta females showed slight feeding preferences and strong oviposition preferences for undamaged plants versus plants that had been fed on by E. fuscula. 5. The results are discussed with reference to their implications for plant-mediated competition among herbivores and constraints on the evolution of plant resistance.

Plant volatiles in the sexual communication of Melolontha hippocastani: Response towards time-dependent bouquets and novel function of (Z)-3-hexen-1-ol as a sexual kairomone

Abstract: Summary 1. Swarming males of Melolontha hippocastani are known to locate females that stay feeding within the host trees by orienting towards damage-induced plant volatiles (green leaf volatiles) and a sex pheromone. Thus, volatiles emitted by freshly damaged leaves might indicate to a male the presence of currently feeding females. 2. The hypothesis was studied that volatiles from freshly damaged leaves are more attractive to males than volatiles from old damaged leaves. The odour bouquets of damaged leaves from three plant species that have been shown to attract male M. hippocastani in the field were analysed 10 min (fresh damage) and 1.5 h (old damage) after damaging, using coupled gas chromatography–mass spectrometry. The results showed clear differences between the bouquets: the bouquet of freshly damaged leaves of all species consisted of typical leaf aldehydes, i.e. hexanal, (Z)-3-hexenal, (Z)-2-hexenal, (E)-2-hexenal, the leaf alcohol (Z)-3-hexen-1-ol, and the corresponding acetate. One and a half hours after damage, aldehydes had almost vanished and (Z)-3-hexen-1-ol and (Z)-3-hexenyl acetate predominated; however males of M. hippocastani were equally attracted to traps baited with volatiles from old and freshly damaged leaves in field experiments. When traps were baited with synthetic volatile mixtures mimicking the bouquets of old and freshly damaged leaves, M. hippocastani males even preferred the old damage mixture. 3. Experiments addressing the role of individual green leaf volatiles revealed that only (Z)-3-hexen-1-ol was highly attractive while the other compounds tested individually were behaviourally inactive, however all tested compounds elicited comparable electrophysiological responses on male M. hippocastani antennae. 4. In analogy to the term aggregation kairomone used for feeding-induced plant volatiles that attract both sexes of an insect, the term sexual kairomone is suggested to describe the novel function of (Z)-3-hexen-1-ol in the sexual communication of M. hippocastani.

Flowers at the front line of invasion

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Conspicuous colouration attracts prey to a stationary predator

Abstract: 1. Conspicuous body colouration is counter-intuitive in stationary predators because sit-and-wait tactics frequently rely on concealed traps to capture prey. Consequently, bright colours and contrasting patterns should be rare in predators using traps as they may alert potential prey. Yet, some orb-weaving spiders are brightly coloured and contrastingly patterned. How can conspicuousness of trap-building sit-and-wait predators be favoured by natural selection? 2. Observations of spiny spiders Gasteracantha fornicata in north-eastern Australia showed that the size of spiders relative to their orb webs correlated positively with relative prey numbers already captured in their webs. A possible explanation is that the relatively larger appearance of the yellow-black striped dorsal surface of this spider attracts more visually oriented prey items. Prey attracted to webs may get trapped, thereby increasing the spiders' foraging success. 3. To test this hypothesis for the function of conspicuous body colouration, a field experiment was conducted that documented the prey capture rates of spiny spiders after manipulating or sham-manipulating their appearance. 4. As predicted, spiders that were dyed black on their striped dorsal surface caught relatively fewer prey items than did control spiders. Thus, conspicuous dorsal body colouration may be adaptive in spiny spiders because it increases foraging success and, presumably, survival rates and reproductive outputs. Overall, these data support the colour-as-prey-attractant hypothesis in a stationary, trap-building predator.

Immune expression in a damselfly is related to time of season, not to fluctuating asymmetry or host size

Abstract: Summary 1. Variation in immune responsiveness within and among species is the subject of the emerging field of ecological immunology. The work reported here showed that individuals of Lestes forcipatus Rambur differ in their likelihood of mounting immune responses, and in the magnitude of those responses, against a generalist ectoparasite, the water mite Arrenurus planus Marshall. 2. Immune responses took the form of melanotic encapsulation of mite feeding tubes, occurred in the few days after host emergence, and resulted in mites dying without engorging. Such immune responses were more probable and stronger for hosts sampled later rather than earlier in the season. Such responses may act as selection affecting seasonal patterns of egg hatching and larval abundance of mites. 3. Contrary to expectation, metrics of host size (wing length) and wing cell fluctuating asymmetry were not related to the likelihood of immune responses. 4. The importance of season on immune expression of insects has not been explored in detail. These results suggest possible trade-offs in allocation of melanin (or its precursors) to maturation versus immunity, and indicate the need for studies on the synergistic effects of weather and parasitism on host species that use melanotic encapsulation to combat parasites and pathogens.

Dung feeding in adult scarabaeines (tunnellers and endocoprids): even large dung beetles eat small particles

Abstract: 1. The maximum size of ingested particles was determined in 15 species of adult dung beetle (Scarabaeidae: Scarabaeinae) by mixing small latex or glass balls of known diameter into the dung used as food. Twelve species (tribes Coprini, Onitini, Oniticellini, and Onthophagini) were tunnellers (making dung stores for feeding and breeding in the soil below the pat) and three species (tribe Oniticellini) were endocoprids (feeding and breeding in the dung pat itself). 2. The test species, covering a wide range of body size (fresh weights 0.05–7.4 g), ingested minute particles only (maximum diameter 8–50 µm), and there was a statistically significant but numerically small increase in particle size with body weight. 3. When the effect of body size was taken into account, taxon (tribe), ecological group (tunneller/endocoprid), and dung preference (coarse/fine) had no significant effect on the size of ingested particles. 4. Tests using two tunnelling species did not indicate that beetles use their mandibles to grind dung particles prior to ingestion. 5. The results suggest essentially the same feeding mechanism in all adult tunnelling or endocoprid scarabaeines that eat fresh dung. Larger, indigestible plant fragments are avoided by filtration, and ingestion is confined to very small particles of higher nutritional value.

Spatial patterns, temporal variability, and the role of multi-nest colonies in a monogynous Spanish desert ant

Abstract: 1. The colonies of the Spanish desert ant Cataglyphis iberica are polydomous. This study describes the temporal and spatial patterns of the polydomy in this species at two different sites, and presents analyses of its role in reducing the attacks of the queen over sexual brood, and in allowing better habitat exploitation. 2. The spatial distribution of nests was clumped while colonies were distributed randomly. Mean nearest neighbour distance ranged from 3.4 to 7.0 m for nests and from 12.3 to 14.1 m for colonies. Distance of foragers searching for food varied among nests: mean values were between 6.1 and 12.6 m. 3. At both sites, the maximum number of nests per colony occurred in summer, during the maximum activity period of the species. Colonies regrouped at the end of this period but overwintered in several nests. 4. Nest renewal in C. iberica colonies was high and showed great temporal variability: nests changed (open, close, re-open) continuously through the activity season and/or among years. The lifetime of up to 55% of nests was only 1–3 months. 5. Polydomy in C. iberica might decrease the interactions between the queen and the sexual brood. In all colonies excavated just before the mating period, the nest containing the queen did not contain any virgin female. Females were in the queenless nests of the colony. 6. The results also suggest that polydomous C. iberica colonies may enhance habitat exploitation because foraging activity per colony increases with nest number. The relationship between total prey input and foraging efficiency and number of nests per colony attains a plateau or even decreases after a certain colony size (four to six nests). This value agrees with the observed mean number of nests per colony in C. iberica.

Dynamics of springtail and mite populations: the role of density dependence, predation, and weather

Abstract: 1. Ecological theory suggests that density-dependent regulation of organism abundance will vary from exogenous to endogenous factors depending on trophic structure. Changes in abundance of soil arthropods were investigated at three trophic levels, springtails (Collembola), predaceous mites (Acari), and macro-arthropods (spider, adult and larval beetles, centipedes). Predictions were that springtails are predator regulated and mites are food limited according to the Hairston et al. (1960) model, which predicts alternating regulation by competition and predation from fungi to springtails to mites to macro-arthropods. The alternate hypothesis was based on the bottom-up model of trophic dynamics, which predicts that each trophic level is regulated by competition for resources. 2. The relative contributions to springtail and mite population dynamics of endogenous (i.e. density-dependent population growth related to food availability) and exogenous (i.e. predation and weather) factors were tested using time-series analysis and experimental manipulation of water conditions. Box patterns were distributed within an aspen forest habitat located in the Canadian prairies and surveyed weekly from May to September 1997–1999. Each box depressed the leaf litter, creating a microhabitat island for soil arthropods that provided counts of invertebrates located immediately beneath the boxes. 3. Strong evidence was found for endogenous control of springtail and mite numbers, indicated by a reduction in population growth related to density in the previous week. Contrary to predictions, no evidence was found for regulation of springtail numbers by mites, or for regulation of mite numbers by macro-arthropods. Springtail population growth rate was related positively to current springtail density (8 and 23% variation explained) and related negatively to 1-week lagged density (85 and 58%), and related negatively to temperature (5 and 5%) for time-series data and for experimental addition of water respectively. Mite population growth rate was related positively to current mite density (54%) and temperature (4%), and negatively to 1-week lagged mite density (20%) and precipitation (6%) for time-series analysis. For experimental addition of water, mite growth rate was related positively to current mite density (44%) and temperature (5%), and negatively to 1-week lagged density (11%). Results differed from the Hairston et al. (1960) model predictions but were consistent with a bottom-up view that springtail and mite populations were regulated intrinsically by competition for food and secondarily by temperature as a function of reproduction.

Preference and performance are correlated in the spittlebug Aphrophora pectoralis on four species of willow

Abstract: 1. There was a positive correlation between oviposition and feeding preferences and offspring performance in the spittlebug Aphrophora pectoralis Matsumura (Homoptera: Cercopoidea, Aphrophoridae) on four species of willow Salix sp. (Salicaceae) growing near Sapporo, Japan. Spittlebugs preferred rapidly growing shoots where performance was highest. 2. When the effects of shoot length were removed, egg densities on willow species were associated with offspring performance on three of four species. Egg densities and survival rates were low on Salix integra and Salix miyabeana. Survival rates were high on Salix sachalinensis, which had high egg densities, and Salix hultenii, which had low egg densities. 3. Aphrophora pectoralis formed mating aggregations almost exclusively on S. sachalinensis but then dispersed to other willow species to oviposit. 4. Nymphs dispersed from the oviposition site to feed on nearby shoots within the same plant but they did not disperse to other willow plants. Nymphs had the same preference for rapidly growing shoots as ovipositing females, so they were able to refine the maternal choice by moving to larger shoots near the shoot on which they had eclosed. 5. The spittlebugs were highly aggregated at all life stages so that even at high densities only a small proportion of the most vigorously growing shoots was utilised.