Showing papers on "Aphid published in 1997"

Specificity of induced resistance in the tomato, Lycopersicon esculentum

Abstract: Specificity in the induced responses of tomato foliage to arthropod herbivores was investigated. We distinguished between two aspects of specificity: specificity of effect (the range of organisms affected by a given induced response), and specificity of elicitation (ability of the plant to generate distinct chemical responses to different damage types). Specificity of effect was investigated by examining the effect of restricted feeding by Helicoverpa zea on the resistance of tomato plants to an aphid species (Macrosiphum euphorbiae), a mite species (Tetranychus urticae), a noctuid species (Spodoptera exigua), and to a phytopathogen, Pseudomonas syringae pv. tomato. Prior H. zea feeding was found to increase the resistance of tomato plants to all four organisms. Specificity in elicitation was investigated by examining the effect of aphid feeding on the activities of four defense-related proteins and on the suitability of foliage for S. exigua. Aphid feeding was found to induce peroxidase and lipoxygenase activities but not polyphenol oxidase and proteinase inhibitor activities; this response is distinct from the response to H. zea feeding, which induces polyphenol oxidase and proteinase inhibitors but not peroxidase. Leaflets which had been fed upon by aphids were better sources of food for S. exigua than were leaflets which had not been fed upon by aphids. Studies of both these aspects of specificity are needed to understand the way in which plants coordinate and integrate induced responses against insects with other physiological processes.

Competition between gall aphids and natural plant sinks: plant architecture affects resistance to galling

Abstract: Gall forming herbivores induce sinks and act as phloem parasites within their host plants. Their performance on the host plant can depend on the sink-source relationship they establish with the plant. Because sink-source relationships within a plant are reflected in its architecture, we examined how architectural differences among cottonwoods might influence the success of the galling aphid, Pemphigus betae. Using cloned cottonwoods in common garden studies, we found three major patterns. First, there is a significant clonal or genetic component to tree architecture; cloned trees grown in a common garden maintain the architecture of parental trees. Second, resistant tree genotypes have more natural sinks (i.e., buds) relative to sources (i.e., stem volume) than susceptible trees. Third, these differences in architecture result in greater competition among sinks on aphid-resistant trees than on aphid-susceptible trees. Sink competition within a tree was estimated by the Gini coefficient which quantifies the size inequality of a shoot population (i.e., competition among sinks is low when shoots are nearly equal in size, and great when a few shoots are large and most are small). Aphid death through gall abortion increased significantly (r 2 = 0.65) on garden-grown trees as competition among sinks within a tree increased. Based on these observations we proposed the "sink competition hypothesis" to account for the performance of gall formers on their host plants. To test this hypothesis, we experimentally reduced sink densities (i.e., buds) on branches of resistant tree genotypes to resemble the bud densities of susceptible genotypes. By reducing the number of competing sinks, we predicted that aphid survival would increase. As predicted, aphid survival significantly increased. For example, in one removal experiment, aphid survival increased from 20% on control branches to 55% on branches with the highest level of bud removal. Similar bud removals on susceptible trees did not increase aphid survival, indicating that competition is relaxed on susceptible hosts. With the exception of the plant vigor hypothesis, most current hypotheses explaining herbivore distributions in nature focus on the importance to leaf-chewing herbivores of variation in chemistry. We believe that a sink competition model is needed to explain the distributions of the diverse group of herbivores that act as phloem parasites. The sink competition model is more mechanistic than the vigor hypothesis, and may account for apparent contradictions because it more clearly quantifies the resource base and the potential interactions that occur when sinks, either herbivore-induced or natural, compete for sources.

Occurrence and transmission of facultative endosymbionts in aphids.

Abstract: The occurrence of a secondary bacterial symbiont (PASS) of pea aphid, Acyrthosiphon pisum (Harris), was detected by polymerase chain reaction (PCR) with specific nucleotide primers based on PASS 16S rDNA nucleotide sequences from over 80% (50/57) of clones of pea aphid collected from widely separated locations in California. PASS was also detected by PCR in both red and green phenotypes of rose aphid, Macrosiphum rosae (L.), but not in six other species of aphids examined, including blue alfalfa aphid (A. kondoi Shinji). The nucleotide sequences of the PCR-amplified, partial 16S rDNAs (1060 bp) from pea aphid and rose aphid were identical and 99.9% similar to the published 16S rDNA of PASS. PASS and a recently described new rickettsia of pea aphid (PAR) were transmitted by needle injection of hemolymph from positive pea aphid clones into negative clones and into blue alfalfa aphids. Both PASS and PAR were maintained in the offspring of some of the injected mother aphids via high rate of maternal transmission.

The relative importance of glucosinolates and amino acids to the development of two aphid pests Brevicoryne brassicae and Myzus persicae on wild and cultivated brassica species

Abstract: This paper describes an experiment to investigate the relative importance of glucosinolates and the availability of free amino acids in the phloem to the feeding behaviour and development of the specialist brassica aphid, Brevicoryne brassicae (L) (cabbage aphid) and the generalist, Myzus persicae, (Sultzer) (peach potato aphid). Aphid development was determined on brassica species and cultivated brassica varieties. Analysis of individual glucosinolates in the wild brassica species identified significant differences in their profiles and in their concentrations present in freeze dried leaves. Multiple regression analysis showed a significant correlation (r = 0.83) between the intrinsic rate of increase of B. brassicae and glucosinolate concentrations; four glucosinolates, 2-OH-3-butenyl, and 2-propenylglucosinolate, 3-methoxyindolyl and 4-pentenylglucosinolates accounted for 79% of the variation. The intrinsic rate of increase of M. persicae was less correlated, though still significant (r = 0.5); 3-indolyglucosinolate and 3-butenylglucosinolate accounted for 47% of the variation. Regression analysis also showed a correlation between phloem amino acid concentrations and the intrinsic rate of increase of B. brassicae (r = 0.48) but not of M. persicae. The concentration of the important amino acids tyrosine, alanine, leucine and glutamic acid, accounted for 43% of the variation in intrinsic rate of increase. Leaf water potential of the Brassica species showed no relationship with the intrinsic rate of increase of either B. brassicae or M. persicae.

Biology and ecology of Aphis gossypii glover (Homoptera: Aphididae)

Abstract: Aphis gossypii Glover is a destructive pest of over two dozen crops world wide Damage to a few of these crops is due to direct feeding, but for most of these crops its impact is through its role as a virus vector As expected, this has resulted in many articles dealing with methods of controlling this insect The aphid has the ability to become resistant to many pesticides and there is growing concern over environmental impacts of pesticide use As a result, manipulation of the agroecosystem will play an ever increasing role in the management of this insect Most aspects of the biology of this aphid are covered in this review The recurrent theme centers around the importance of host plant influences on the biology of the aphid In addition to examining the literature on the biology of the aphid, a large section is devoted to organisms which influence mortality in the aphid, and to aphid borne viruses transmitted by this aphid This review covers the literature from 1912 to 1995, but articles on aphid management (eg pesticide efficacy, planting date) were omitted except where they deal with the aphid's biology or ecology Research on the biology of this aphid has been heavily skewed into several disparate categories First, research has focused on the influence of host plant and temperature on the reproductive rate of this aphid Second, research has focused on the cause for alate production such as nutritional stress, other nutritional factors, crowding, and temperature Third, research has focused on the viral borne pathogens transmitted by this aphid Three specific cases are highlighted: citrus tristeza, cucumber mosaic virus, and the potyviruses Last, research has focused on the role of organisms which feed on this aphid Due to the complexity of research possible in this area, very little research has explored the effect of these "beneficial organisms" on life history traits of this aphid

Apparent Competition between Two Aphid Species

Abstract: 1. The population dynamics of colonies of nettle aphids (Microlophium carnosum Buckton) were studied on potted nettle plants placed beside plots of grass on some of which outbreaks of grass aphids (Rhopalosiphum padi L.) had been induced by fertilizer application. 2. Nettle aphid colonies adjacent to grass aphid concentration suffered an earlier population decline, and produced fewer alate dispersers, than control colonies. 3. The reduced performance of nettle aphids in the vicinity of grass aphid was due to increased predation by Coccinellidae, attracted into the area by the large concentrations of grass aphids. The indirect interaction between the two species of aphids is an example of apparent competition. 4. Because predators pre-emptively exploited nettle aphids on plants in the grass aphid treatment, the numbers of nettle aphids attacked by parasitoid wasps was greatly reduced in these sites compared to the controls.

Comparison of two population-level ecotoxicological endpoints: The intrinsic (rm) and instantaneous (ri) rates of increase

Abstract: The instantaneous rate of increase (ri) was compared to the intrinsic rate of increase (rm) as an alternative population-level ecotoxicological endpoint. The terrestrial arthropod Acyrthosiphon pisum Harris, the pea aphid, was used as the model species and the new nicotinergic insecticide, imidacloprid, as the xenobiotic. In the first experiment, rm was generated each day from a life table for A. pisum neonates exposed to nontreated (control) potted broad bean, Vicia faba L. (variety Banner), and compared to determine the earliest rm which was not significantly different from the final rm. The rm for neonates introduced to nontreated plants was significantly different on days 6 and 7 but not significantly different between 8 to 40 d postintroduction. A second experiment examined the change in ri over time for neonates introduced at birth to nontreated broad bean and censused at 8, 11, 13, 15, 17, 19, and 21 d postintroduction. The ri for neonate populations introduced to nontreated broad bean peaked on days 11 and 19. The ri on nontreated beans was significantly different (p < 0.05) from rm on days 8 and 15 but not significantly different at 11, 13, 17, 19, and 21 d. A third experiment was conducted comparing rm and ri following 11 d exposure to a range of imidacloprid concentrations applied to broad bean. There was a high correlation (r = 0.91, n = 21, p < 0.01) between ri and rm 11 d after the start of the study, suggesting that ri can be used as a predictive measure in place of rm. There was a negative relationship, y = 0.32 – 0.43x (r2 = 0.88, n = 21), between ri and exposure to imidacloprid-treated broad bean, suggesting that simple regressions may be utilized as tools in the assessment of xenobiotics on population growth rates.

Early-season predation impacts the establishment of aphids and spread of beet yellows virus in sugar beet

Abstract: The potential of predators to impact the establishment of aphid vectors and the spread of beet yellows virus in sugar beet was examined. Myzus persicae carrying beet yellows virus (BYV) were released on six interior sites and six edge sites in each of four fields at the end of May. Aphids established at low densities and BYV was spread in circular patches around the infested plants at all sites. The number of diseased plants per patch at the end of September ranged from a field-average of 130 to 210 in the four fields. There was a weak tendency towards better aphid establishment and greater virus spread in fields in less complex landscapes. Edge sites had less virus spread than interior sites in one field, more virus spread in two other fields, and there was no statistically significant difference in the fourth field. In the field where virus spread was lowest at edge sites, we used predator exclosure and direct observation to manipulate and quantify the effects of early season predation. On a warm day in early June, 81% ofAphis fabae exposed to predators on young beet plants disappeared during a 24 h period, compared to 10% of aphids protected by clipcages. Intermediate levels of predator exclusion, allowing aphids to walk away but restricting predator access, showed that predation was responsible for aphid disappearance.Cantharis lateralis L. (Coleoptera: Cantharidae) was the most frequently observed foliar predator (>90%). It was found eating aphids on several occasions. The incidence of predators was 1.8 per plant per h in the field interior and 3.8 per plant per h. near the edge. In the same field, aphids and virus were released in six edge and six interior sites, that were surrounded by 0.5 m high plastic open-top barriers (‘exclosures’). Pitfall trapping inside the barriers reduced potential soil predator densities to ca. one-tenth of the open field level and arrivals of flying predators were reduced. Inside the exclosures, aphid establishment was enhanced, and virus spread at exclosure sites was increased by about 50% compared to open sites. Foliar and pitfall sampling yielded the following predators:Cantharis lateralis, C. rufa L. (Coleoptera: Cantharidae),Coccinella septempunctata L.,C. undecimpunctata L. (Coleoptera: Coccinellidae),Pterostichus cupreus (L.),Harpalus rufipes (de Geer),Patrobus atrorufus (Strom),Trechus quadristriatus (Schrk.),Bembidion lampros (Herbst) (Coleoptera: Carabidae). In a laboratory no-choice trial (with 10M. persicae /day offered), each of these species ate aphids with consumption rates varying from 1.7 to 9.2 aphids/day. The results show that early predation substantially impacted aphid establishment in one field, and resulted in reduced virus spread. Results in the other fields show that these results cannot be easily generalized.

Prospects for biopesticides for aphid control

Abstract: Diseases form an important component of the natural enemy complex of aphids. The most common and obvious of these diseases are entomophthoran fungi such asErynia neoaphidis Remaudiere & Herbert,Entomophthora planchoniana Cornu,Zoophthora radicans (Brefeld) Batko andConidiobolus obscurus (Hall & Dunn) Remaudiere & Keller. The pest status of some aphids such as the pea aphid,Acyrthosiphon pisum (Harris), is considerably reduced by natural epizootics of fungal disease. However, disease may contribute little to practical control as it is mainly effective in high density populations when weather conditions are suitable. Introduction of exotic diseases for classical biological control is only rarely possible since most diseases, like their aphid hosts, are distributed world-wide. One exception was the successful introduction into Australia of a strain ofZ. radicans for control of spotted alfalfa aphid,Therioaphis trifolii (Monell) f.maculata in 1979. Attempts to manipulate entomophthoran fungi have had limited success because of problems with mass production, the fragility of the conidia and the need for suitably moist conditions. Hyphomycete fungi such asVerticillium lecanii (Zimm.) Viegas,Metarhizium anisopliae (Metsch.) Sorokin,Beauveria bassiana (Bals.) Vuill. andPaecilomyces spp. are more suitable for development as mycoinsecticides as they are cheap to mass produce and form stable conidia. “Vertalec™”, a formulation ofV. lecanii, has been sold in small quantities commercially in Britain and parts of Europe for many years and used mainly in glasshouses. Recently promising results have been obtained with use ofM. anisopliae for control of lettuce root aphid,Pemphigus bursarius (L.) in the UK. Laboratory studies on selected isolates ofB. bassiana andPaecilomyces spp. show a promising level of activity. Problems may occur with these species as they can kill aphid predators such as coccinelids. In addition, more research is needed on developing improved formulations which enable control to be achieved under low humidity conditions.

Honeydew sugars and osmoregulation in the pea aphid Acyrthosiphon pisum

Abstract: Pea aphids, Acyrthosiphon pisum, containing their symbiotic bacteria (untreated aphids) and experimentally deprived of their bacteria by treatment with the antibiotic rifampicin (antibiotic-treated aphids) were reared on the plant Vicia faba. The sugars in the honeydew produced by untreated aphids comprised predominantly the monosaccharides glucose and fructose, while the honeydew of antibiotic-treated aphids contained considerable amounts of oligosaccharides of up to 16 hexose units. The honeydew and haemolymph of the aphids were iso-osmotic, and their osmotic pressure was significantly lower in untreated aphids (0.91­0.95 MPa) than in antibiotic-treated aphids (1.01­1.05 MPa) (P<0.05). For insects reared on chemically defined diets containing 0.15­1.0 mol l-1 sucrose (osmotic pressure 1.1­4.0 MPa), the osmotic pressure of the aphid haemolymph did not vary with dietary osmotic pressure, but was regulated to approximately 1.0 MPa in untreated and 1.3 MPa in antibiotic-treated aphids. The sugars in the aphid honeydew varied with dietary sucrose concentration; with monosaccharides dominant at low concentrations and oligosaccharides dominant at high concentrations of dietary sucrose. The lowest dietary sucrose concentration at which honeydew oligosaccharides were detected was 0.2 mol l-1 for the antibiotic-treated aphids and 0.3 mol l-1 for untreated aphids. These data indicate that the aphid, and not its associated microbiota, mediates the synthesis of oligosaccharides when the osmotic pressure of the ingesta is high.

Semiochemicals mediating spacing behavior of bird cherry-oat aphid, Rhopalosiphum padi feeding on cereals

Abstract: Olfactometry using an apterous individual of Rhopalosiphum padi (L.) showed an arresting effect by volatiles from a wheat seedling and a repellent effect by volatiles from a wheat seedling infested with aphids at a high population density (ca. 9 aphids/cm2). Four compounds, 6-methyl-5-hepten-2-one, (−)- and (+)-6-methyl-5-hepten-2-ol, and 2-tridecanone, were identified by GC-MS in air entrainments from the wheat seedlings with high aphid density but not from the wheat seedlings alone. The mixture of the four compounds in the natural proportion counteracted the attractivity of the volatiles from the intact uninfested wheat seedling. The likely role of these compounds in the spacing behavior of this aphid species, when present in high densities on wheat, is discussed.

Host plant effects on the performance of the aphid Aulacorthum solani (Kalt.) (Homoptera: Aphididae) at ambient and elevated CO2

Abstract: In future elevated CO2 environments, chewing insects are likely to perform less well than at present because of the effects of increased carbon fixation on their host plants. When the aphid, Aulacorthum solani was reared on bean (Vicia faba) and tansy (Tanacetum vulgare) plants under ambient and elevated CO2, performance was enhanced on both hosts at elevated CO2. The nature of the response was different on each plant species suggesting that feeding strategy may influence an insect’s response to elevated CO2. On bean, the daily rate of production of nymphs was increased by 16% but there was no difference in development time, whereas on tansy, development time was 10% shorter at elevated CO2 but the rate of production of nymphs was not affected. The same aphid clone therefore responded differently to elevated CO2 on different host plants. This increase in aphid performance could lead to larger populations of aphids in a future elevated CO2 environment.

The evolution and genetics of aphid endosymbionts

Abstract: O ne of the striking attribute.s of prokaryotes (boch Bacteria and Archaea) is the dlversity of thei! catabolic pathways and ehe ir biosynthetic capabilities (Brock er al. 1994). Many prnkaryores are able co L1se unusual substrates for growth and synthesize a11 of the constituenes of cells from relative!y simple compounds. In contrast, many eukaryotcs lack such capa bi litics and ha ve developed elose associations in which they take advantage of thc metabolie versatility of prokaryotes. In some of these associations, the organisms live in elose contace bm remain separate. In orher cases, called endosymbioses, the prokaryote i5 scqucstered \vithin the eukaryotic eell. The c1assicaJ eompilation of endosymbiotic assoClations is that of suchner (1965). Inseets are particularly prone ta endosymbiotic associatiom .. Such associ~tions are widespread among members of the orders Homoptera (a phids, whiteflies, mealybugs, psylIid!>, and cicadas), Blattaria (cock-

The importance of adverse weather conditions for behaviour and population ecology of an aphid parasitoid

Abstract: 1. Weather conditions are known to influence insect behaviour and population dynamics. We investigated how wind and rain affect reproductive success and parasitization patterns of the aphid parasitoid Aphidius rosae in rose bushes. 2. A model was developed to simulate the life history of females foraging in the field under different environmental conditions. The model was parameterized using field and laboratory data on foraging behaviour of A. rosae. Field data on the daily occurrence of wind and rain were used for the simulation of different weather conditions. 3. When 1 day in the life of a female was modelled, both wind and rain reduced the number of aphid colonies and rose bushes visited. During rain, the parasitoid ceased all foraging activities, resulting in a negative correlation between the number of eggs being laid in visited aphid colonies, and the duration of rain during a day. During wind, the parasitoid continued to forage at a reduced rate, which led to increasing rates of parasitism in encountered aphid colonies during long wind intervals. 4. In a field study, we linked patterns of parasitism in aphid colonies to the weather conditions at the time when the eggs were laid by the parasitoids. The results confirmed the predictions of the model and showed that wind and rain change the distribution of parasitism among host colonies. 5. When the lifetime reproductive success of foraging females was simulated, both wind and rain affected parasitoid reproductive success. Even if weather conditions were benign, the reproductive success of females was greatly reduced compared to the maximum achievable oviposition numbers in the absence of any bad weather periods. Weather conditions had a less pronounced effect on parasitoid life history if travel mortalities between patches were taken into consideration. 6. Both bad weather conditions and travel mortality led to a strong skew in the frequency distribution of female reproductive success. A small proportion of females was able to realize oviposition numbers close to the maximum lifetime fecundity, as measured in the laboratory.

Chemically Mediated Associative Learning in the Host Foraging Behavior of the Aphid Parasitoid Aphidius ervi (Hymenoptera: Braconidae)

Abstract: The effect of experience on the responsiveness of the braconid parasitoidAphidius ervi to host(Acyrthosiphon pisum)-associated cues was investigated on bean plants(Vicia faba) using a wind tunnel bioassay. Oviposition experience on the plant-host complex significantly increased the oriented flight and landing responses ofA. ervi females to an undamaged plant and to a plant-host(A. pisum) complex. However, oviposition experience onA. pisum aphids when isolated from the plant did not change their responses to the intact plant and the complex. Searching on an unwashed plant which had been previously damaged byA. pisum also increased their response to an undamaged plant and a host-damaged plant, whereas the experience of searching on an undamaged plant did not significantly change their responses to undamaged plants. However, when parasitoids were allowed to search on an undamaged plant which had been sprayed withA. pisum honeydew, this significantly increased their response to an undamaged plant. Oviposition experience on the plant-host complex and foraging experience on a host-damaged plant or an undamaged plant sprayed with honeydew also significantly reduced the mean time taken by the parasitoids to respond in the wind tunnel. The behavioral changes associated with such experience were acquired within 30 min and persisted for at least 3 days. The results demonstrate the capacity ofA. ervi to learn associatively olfactory cues from plants, host-damaged plants, and plant-host complexes and confirm the role of aphid honeydew as a host recognition kairomone forA. ervi.

Evaluating the role of predation in population fluctuations of the soybean aphid Aphis glycines in farmers' fields in Indonesia

Abstract: The soybean aphid Aphis glycines can build up to high densities in young soybean, causing farmers to apply insecticides early in the season. The seasonal phenology of A. glycines was studied at 15 sites on Sumatra. Aphis glycines densities peaked in the vegetative plant stage and declined rapidly afterwards or towards the flowering stage of soybean. A clear statistical dependency of coccinellid predator densities on aphid densities was found. Age-specific survival and reproduction of A. glycines were determined in clip-cages on plants 3, 5 and 7 weeks of age. The growth rate was highest on 3-week-old plants and declined linearly with plant age. Predation rates were measured through direct observations of individual coccinellid and staphylinid predators in the field at different times during daylight hours. Predator observations were made at various field densities of A. glycines, and thus a functional response of predation rate to prey density was obtained for each predator type. A simple deterministic model was developed to calculate population changes of A. glycines based on field-measured diurnal predation functions and aphid growth rates. The model was adopted to evaluate whether aphid population declines observed in the field could be ascribed to predation. In young soybean crops ( 40 days), the observed population declines to low levels were attributable to predation. This pattern was observed on subsequent occasions in the same crop, and was observed concurrently in crops of three different ages planted in randomized blocks. The influence of other mortality factors (competition and migration of A. glycines, rainfall, plant age) on A. glycines dynamics is discussed. Coccinellid predators play a role in suppressing A. glycines in soybean. Therefore it is important that predators are conserved early in the season by avoiding unnecessary insecticide applications. Exercises developed from this study have been added to integrated pest management training curricula in Indonesia, in order that farmers learn to make more appropriate pest management decisions.

Seasonal Abundance of Aphid Vectors of Potato Virus Y in the Red River Valley of Minnesota and North Dakota

Abstract: An epidemic of aphid-transmitted potato virus Y (PVY) in seed potato grown in the Red River Valley of Minnesota and North Dakota was the reason we began a study to determine which aphid species were found in traps placed near potato fields. Knowledge of which aphid species are present and which of these are known vectors of PVY is needed to develop management strategies that minimize virus spread. The spread of PVY to healthy indicator plants and captures of alate aphids in tile traps were monitored weekly for 3 yr consecutively (1992-1994) throughout the Red River Valley. Thirty-four aphid species were identified from green and yellow tile traps in 1992, 25 in 1993, and 26 in 1994. Yellow tile traps caught significantly more aphids overall than green tile traps and were significantly preferred by Aphis helianthi Monell, Capitophorus elaeagni (del Guercio), and Rhopalosiphummaidis (Fitch). Intervals of greatest aphid capture in green traps were generally between mid-July and mid-August at all sites in all 3 yr. Aphid captures at all sites were 3-25 times greater in 1992 and 1994 than in 1993. PVY infection of indicator plants exposed at the trapping sites also was greater in 1992 (25 plants) and 1994 (18 plants) than in 1993 (2 plants). Eighty-nine percent of PVY spread to indicator plants occurred between 8 July and 19 August. Eight species comprised 89.9% of the aphids collected in green traps during intervals of PVY transmission to indicator plants: Acyrthosiphon pisum (Harris), pea aphid; A. helianthi; C. elaeagni; Lipaphis erysimi (Kaltenbach), turnip aphid; R. maidis , com leaf aphid; R. padi (L.), bird cherry-oat aphid; Schizaphis graminum (Rondani), greenbug; and Sitobion avenae (F.), English grain aphid. Seven of these species were previously known PVY vectors. We found that A. helianthi transmitted PVY with low efficiency under greenhouse conditions. The 8 most common aphid species were associated with crops and weeds common in the Red River Valley. Our data suggest that although the relative importance of individual PVY vectors varies from year to year and location by location, total aphid captures may be the best indicator of the risk of PVY spread.

Ontogenetic change of prey preference in the generalist predator Zelus renardii and its influence on predator–predator interactions

Abstract: 1. Two field experiments were conducted to test the hypothesis that the intensity of predation by a generalist predator on two species of prey changes with the developmental stage of the predator. The generalist predator studied was Zelus renardii Kolenati (Hemiptera: Reduviidae) and the prey were the lacewing larva, Chrysoperla carnea Stephens, and the cotton aphid, Aphis gossypii Glover. 2. Zelus renardii and lacewings feed on aphids, thereby acting as potential competitors. In addition, Z. renardii feeds on lacewings. Thus, Z. renardii is an intraguild predator of lacewings. 3. Zelus renardii exhibited changes in prey preferences across developmental stages. The older stages of Z. renardii exerted greater mortality on lacewings and fed on larger lacewing larvae than did the younger stages. 4. Lacewings suppressed aphid population growth strongly. In contrast, none of the stages of Z. renardii was an effective control agent of the cotton aphid. 5. The addition of Z. renardii frequently disrupted the effective control of aphids generated by lacewings. In one of the two replicates of the experiment, the disruption increased with the developmental stage of Z. renardii, paralleling the increase in lacewing mortality. 6. Although the developmental stage of Z. renardii can influence the prevalence of intraguild predation and the intensity of the disruption of the aphid biological control, these experiments have demonstrated that even the youngest instars of Z. renardii can cause substantial lacewing mortality and release aphid populations from regulation.

Quantifying the impact of polyphagous invertebrate predators in controlling cereal aphids and in preventing wheat yield and quality reductions

Abstract: Summary Exclusion barriers were used to manipulate numbers of polyphagous invertebrate predators so that their impact on cereal aphids and consequently wheat yield and quality could be examined. Experiments were conducted within the framework of the LINK Integrated Farming Systems Project which allowed comparisons to be made between the integrated and conventional farming systems under examination on a study farm in Hampshire, UK. Only in 1995 were the numbers of aphids per tiller, the aphid peak and rate of increase to the peak significantly greater in the exclusion areas where the density of polyphagous predators had been reduced. The maximum increase in aphids as a result of excluding polyphagous predators was 31%, which was equivalent to 130 aphid days. However, the polyphagous predators did not reduce the number of tillers infested. The relatively low impact of polyphagous predators was attributed to the aphid population phenology and greater effects may have been found had aphids infested the crops earlier in their development. Sowing date was shown to govern the time over which a crop may be susceptible to yield loss from aphids, with later-drilled crops being more susceptible to late-summer aphid infestations. Aphid numbers rarely affected grain yield but were found to be related to some grain quality parameters, but reducing polyphagous predators had no direct impact on grain yield or quality even where the aphid burden increased. The peak period of activity and density differed between the species of Carabidae, Staphylinidae and Araneae consequently influencing their relationship with the aphids. Some negative correlations were found between these groups of polyphagous predators and aphids. Species composition and abundance differed between fields thereby influencing the level of aphid predation. The exclusion barriers were most effective at reducing numbers of Carabidae although numbers of Staphylinidae and Araneae were also reduced. The consequences for Integrated Crop Management are discussed.

Plant compensation, natural biological control, and herbivory by Aphis gossypii on pre‐reproductive cotton: the anatomy of a non‐pest

Abstract: Crop plant compensation for herbivory and the population dynamics of herbivores are two key elements in defining an herbivore's pest status. We studied the dynamics of natural, unmanipulated populations of the aphid Aphis gossypii on seedling plantings of cotton, Gossypium hirsutum and Gossypium barbadense, over a 4-year period in California's Central Valley. Aphid populations colonized all plantings, but reached densities in excess of 0.5 aphids/leaf during only one year (1991), when outbreaks occurred. Outbreak populations were, however, ephemeral; predation and parasitism suppressed aphid populations prior to the initiation of flower bud production, when cotton plant growth may become photosynthate-limited. Effective natural biological control was observed despite the action of hyperparasitoids and the heavy mortality of immature parasitoids that occurred when predators consumed parasitized aphids. We conducted manipulative experiments during 1991 and 1992 to quantify the ability of pre-reproductive G. hirsutum to compensate for aphid herbivory. In 1991 aphid populations in the high-damage treatment reached densities as high as any observed naturally during the past 37 years. Damage symptoms were severe: leaf area was reduced by up to 58% and total above-ground plant biomass was reduced by 45%. By the time of crop harvest, however, plants had compensated fully for the early damage in each of the three traits that define cotton's economic value: the timing of crop maturation, the yield of cotton fiber, and the quality of cotton fiber. Aphid feeding damage did, however, produce some changes in plant architecture that persisted throughout the growing season, including a decrease in the number of vegetative branches. In 1992 aphid populations and associated damage were much lighter, but the qualitative responses to herbivory were consistent with those observed in 1991. Plant compensation for early damage was complete for economically significant measures, and vegetative branch production was again suppressed in mature cotton plants. There was no evidence for a change in the suitability of G. hirsutum as a host plant for A. gossypii as a result of early damage (‘induced resistance’). We conclude that pre-reproductive G. hirsutum, which has not yet begun strong allocations to reproductive structures or established architectural complexity, has retained effective means of compensating for herbivory. In contrast to other systems exhibiting strong compensation, G. hirsutum appears to compensate in part by enhancing apical dominance. The recognition of early-season A. gossypii as non-pests is critical to the sustainability of cotton production, because it will allow growers to forego pesticide applications that accelerate the evolution of pesticide-resistance and disrupt natural communities of predators and parasitoids.

Dropping of pea aphids from feeding site: a consequence of parasitism by the wasp, Monoctonus paulensis

Abstract: We examined the dropping behaviour of the pea aphid, Acyrthosiphon pisum (Harris), feeding on broad-bean plants in the laboratory. Aphid responses to foraging and oviposition by females of Monoctonus paulensis (Ashmead) were instar-specific and included kicking with the hind legs, walking away and dropping from the feeding site. Fourth nymphal instars were most likely to drop, followed by second, third, and first instars, in that order. Compared with first instars, the odds that a fourth-instar aphid will drop were 6-times higher independent of the stimulus, and 16-times higher after parasitoid attack. Dropping from the feeding site increases an aphid's mortality risk. If parasitoid offspring are more likely to survive in small pea aphids, it may be adaptive for M. paulensis to choose smaller hosts, regardless of possible fitness gains due to increased body size from development in larger aphids.

Relay intercropping to enhance abundance of insect predators of cotton aphid (Aphis gossypii Glover) in Texas cotton

Abstract: Relay intercropping winter and spring strip crops with cotton was used to conserve and enhance the numbers of predators of the cotton aphid ( Aphis gossypii Glover) in 1992, 1993 and 1994 in northern Texas, USA. Numbers of aphids and aphid predators in cotton isolated from other crops were compared with numbers where canola, wheat, sorghum (strip intercrops) and cotton were planted adjacent to each other and overlapped temporally. Aphid and predator populations were also monitored in the intercrop. The intercrops acted as a reservoir for predators during the non-cotton season; these intercrops 'relayed' aphid predators from canola and wheat in the winter to sorghum in the spring and finally to cotton in the summer. Averaged over the three years ofthe study, predator numbers in the intercrops were highest in sorghum and lowest in wheat. In all years, canola had higher predator numbers than wheat, suggesting that it would be a better winter intercrop than wheat for enhancing predator numbers. Average aphid ...

The impact of Meu1-mediated resistance in tomato on longevity, fecundity and behavior of the potato aphid, Macrosiphum euphorbiae

Abstract: The effect of the tomato resistance gene, Meu1, on feeding, longevity, fecundity and developmental rate of the pink biotype of the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera, Aphididae), was determined using nearly isogenic tomato (Lycopersicon esculentum Mill, Solanaceae) lines. Aphid mortality was significantly higher on resistant plants, with 60% of the aphids dying by the 4th day of exposure. By the 10th day, all the aphids on the resistant plants were dead whereas 100% of the aphids on susceptible plants were alive. Meu1-mediated resistance resulted in significantly decreased fecundity with a ten-fold decrease in the net fertility rate (4.5 and 45.7 progeny per aphid on resistant and susceptible tomato, respectively). A qualitative analysis showed that honeydew was produced by aphids on resistant and susceptible plants, suggesting that aphids initiate feeding on both plant types. However, significantly lower quantities of honeydew were present when aphids were caged on resistant plants. There were also significant differences in aphid location on resistant and susceptible leaves. Experiments evaluating behavior in less than 24 h showed that aphids left resistant leaves after relatively short exposure (3–6 h). Aphids transferred from resistant to susceptible tomato at intervals between 3 h and 24 h resumed feeding as evidenced by presence of honeydew. Although the mechanism by which Meu1-mediated resistance operates is not yet known, our data suggest that resistance factors act rapidly after initiation of feeding and that lower fecundity and longevity are related to reduction in aphid feeding.

Effects of pea aphid (Homoptera: Aphididae) honeydew on longevity and fecundity of the alfalfa weevil (Coleoptera: Curculionidae) parasitoid Bathyplectes curculionis (Hymenoptera: Ichneumonidae)

Abstract: The ability of parasitoids to attack their hosts may be influenced by the availability of adult food sources such as homopteran honeydew. To test this hypothesis for the wasp Bathyplectes curculionis (Thomson), a parasitoid of the alfalfa weevil, Hypera postica (Gyllenhal), we performed laboratory experiments to determine whether availability of pea aphid, Acyrthosiphon pisum (Harris), honeydew influenced adult longevity and fecundity of the wasp. Adult wasps caged with alfalfa, Medicago sativa L., and pea aphids fed on the aphid honeydew and lived a50% longer than wasps caged with alfalfa alone. In a related experiment, newly emerged, unmated B. curculionis females were allowed to feed for 2 d on pea aphid honeydew, sucrose dissolved in water, or water alone, and were then dissected to determine the number of eggs in a lateral oviduct. Egg production was slightly (11–15%) but significantly greater in females with access to pea aphid honeydew or sugar water than in females with access to water only. These results suggest that availability of pea aphid honeydew in alfalfa fields may enhance the realized fecundity of B. curculionis . Hence, the presence of pea aphids in moderate numbers may be beneficial for biological control of the alfalfa weevil.

Small-Grain Aphid Parasitoids (Hymenoptera: Aphelinidae and Aphidiidae) of Washington: Distribution, Relative Abundance, Seasonal Occurrence, and Key to Known North American Species

Abstract: New information is provided on the small-grain aphid parasitoids of eastern Washington—species population diversity, relative abundance, hosts, and spatial and temporal distribution across differing precipitation and cropping zones. In total 13 species of primary parasitoids were discovered attacking grain aphids: Aphelinus albipodus Hayat & Fatima, A. asychis Walker, A. varipes (Foerster), Aphidius avenaphis (Fitch), A. ervi Haliday, A. matricariae Haliday, Diaeretiella rapae (M'Intosh), Ephedrus sp., Lysiphlebus testaceipes (Cresson), Monoctonus washingtonensis Pike & Star´y, Praon unicum Smith, P. occidentale Baker, and P. yakimanum Pike & Starý. The predominant parasitoids were D. rapae on Russian wheat aphid, Diuraphis noxia (Kurdjumov); L. testaceipes on bird cherry-oat aphid, Rhopalosiphum padi (L.) and corn leaf aphid, R. maidis (Fitch); and A. avenaphis, A. ervi , and D. rapae on English grain aphid, Sitobion avenae (F.). This study provides baseline information essential for assessing future changes in aphid parasitoid species dynamics for eastern Washington. A comprehensive key to the known primary and secondary parasitoids associated with grain aphids in North America also is provided.

Determining the relative roles of different aphid species as vectors of cucumber mosaic and bean yellow mosaic viruses in lupins

Abstract: Summary Glasshouse and field studies were done to determine the relative roles of different colonising and non-colonising aphid species as vectors of two non-persistently transmitted viruses, cucumber mosaic cucumovirus (CMV) and bean yellow mosaic potyvirus (BYMV) in narrow-leafed lupin (Lupinus angustifolius) crops in Australia. The abilities of nine different aphid species in transmitting CMV from infected to healthy lupins and BYMV from infected subterranean clover to healthy lupins were compared in the glasshouse using 5–10 min acquisition access feeds. The percentage transmission efficiencies found with lupin-colonising aphid species were (CMV/BYMV): Acyrthosiphon kondoi (6/15), Aphis craccivora (10/14) and Myzus persicae (11/77). With non-colonising species the respective efficiencies were: Brachycaudus rumexicolens (0.9/0), Lipaphis erysimi (4/8), Rhopalosiphum maidis (9/6), R. padi (5/5), Sitobion miscanthi (2/11) and Therioaphis trifolii (4/5). When flying aphids were trapped in the field in four successive years (1993–1996) on vertical nets downwind of virus-infected lupins, 13 different species were caught at a “wheatbelt” site and 18 at an urban irrigated site. Of 2833 aphids caught at the “wheatbelt” site, 64 transmitted CMV to lupin test plants. At the irrigated site, numbers of aphids transmitting CMV/numbers caught were 12/186 while the corresponding numbers for BYMV were 11/727. M. persicae, A. kondoi and R. padi transmitted both viruses, while additional vectors of CMV found were A. craccivora, Acyrthosiphon pisum, B. rumexicolens, L erysimi, R. insertum, T. trifolii and Toxoptera citricidus. Averaged over four years, A. kondoi accounted for 50% of CMV transmissions at the “wheatbelt” site, M. persicae for 16% and R. padi for 22%, and these three species were caught in the greatest numbers, comprising 28%, 13% and 37% respectively of the total catch. At the irrigated site R. padi accounted for half the CMV transmissions, while R. padi and A. kondoi together accounted for most of the BYMV transmissions. R. padi, A. kondoi, M. persicae and T. citridus were the most common aphid species at this site. These findings suggest that M. persicae, A. kondoi and R. padi are the aphid species likely to be most important as vectors of CMV and BYMV in narrow-leafed lupins grown in mediterranean-type climatic zones of southern Australia.

Phenotypic Plasticity in Australian Cotton Aphid (Homoptera: Aphididae): Host Plant Effects on Morphological Variation

Abstract: Fourteen samples of cotton aphid, Aphis gossypii Glover, were obtained from different localities in Australia. Wingless parthenogenetic females from each sample were transferred to seedlings of various host plants in the laboratory and reproduced to form colonies. Winged aphids from 120 colonies were mounted on microscope slides. Twenty-five characters were measured on each aphid and analyzed for differences in origin and laboratory host plant. Aphids reared on cucumber and rockmelon were significantly larger than on cotton, broad bean, or eggplant. Large and highly significant differences among the means of different colonies on the same host plant species in many characters were probably caused by different quality of the individual seedlings. Significant morphological differences were detected among source-line means of some characters, but their magnitude was small compared with variation within the same source line. The inescapable conclusion from this study is that A. gossypii in Australia is phenotypically plastic and its morphology is affected by the host plant far more strongly than by genetic differences (if any) among means of local populations. Moreover, only 13% of A. gossypii host records in Australia are from native Australian plants, implying that it was introduced into Australia, perhaps by the Europeans in the 18th century. Characters of 36 colonies on cucumber and 40 colonies on cotton were analyzed by analysis of covariance (ANCOVA) using wing length (a measure of aphid size) as a covariate. No significant differences in character means remained among source lines on cucumber, when the variation caused by aphid size was thus removed. On cotton, means of some characters still differed among source lines, after size adjustment by ANCOVA. Some evidence indicates that geographical samples of A. gossypii in Australia may differ (genetically?) in shape rather than size. When the 40 colonies reared on laboratory cotton were grouped by the host plant on which the source samples were collected, colonies originating in cotton fields were larger, in most characters, than colonies collected on other host plants. This pattern was not observed in the 36 colonies (of the same sources) reared on laboratory cucumber. If aphid size is a measure of fitness, then cotton-collected aphids had higher fitness than other lines when reared on cotton. The cotton aphids may then show traces of adaptation to their field host.