Leaf surface wax and plant morphology of peas influence insect density

Insect predators and parasitoids adhere better, forage more effectively, and take more aphid prey on pea plants (Pisum sativum L.) (Leguminosae) with mutations that reduce the crystalline wax bloom on the plant surface. To assess the agronomic potential of this trait for pest management, abundance of pea aphids (Acyrthosiphon pisum L.) (Homoptera: Aphididae) and coccinellid predators, and percent parasitism of the aphids were evaluated on pea lines differing in wax bloom and plant architecture over two field seasons. Three pairs of pea lines were evaluated, each pair with a different architecture and differing within the pair in the amount of surface wax bloom (reduced or normal). The trials included plots treated with a narrow spectrum insecticide (pymetrozine) to reduce aphid populations and untreated controls. Reduced wax peas had significantly fewer aphids per plant in 2002 but not in 2003. Total natural enemy abundance was greater on reduced wax than on normal wax pea lines in both years of the study. Pymetrozine reduced aphid densities significantly in both years. Among the four pea lines evaluated for yield, seed yield per plant was affected by plant morphology and insecticide treatment. Yield was greatest on semileafless plants and on pymetrozine sprayed plots in both years. Yield of the reduced wax line in the semileafless background was similar to or exceeded yield in its normal wax sister line, suggesting that this morphological type was best for an agronomically viable reduced wax phenotype. Pea weevil (Bruchus pisorum L.) (Coleoptera: Bruchidae) damage to seed was overall more frequent on seeds from reduced wax varieties than from normal wax varieties. The results illustrate the trade‐offs associated with a reduced wax trait in peas but also show that certain combinations of reduced wax and gross morphology lead to reduced pea aphid populations and yields similar to those of normal wax peas.     

Delineating the effects of a plant trait on interactions among associated insects

Plant traits can affect ecological interactions between plants, herbivores, and predators. Our study tests whether reduced leaf wax in peas alters the interaction between the pea aphid ( Acyrthosiphon pisum), a foliar-foraging predator (a lady beetle, Hippodamia convergens) and a ground-foraging predator (a ground beetle, Poecilus scitulus). We performed a 2×2×2 factorial experiment in which wax level, presence of H. convergens, and presence of P. scitulus were manipulated. Experimental arenas consisted of a cage surrounding three pea plants. One plant in each cage was stocked with 15 pea aphids. In greenhouse and field cage experiments, we assessed the effect of each factor and their interactions on aphid density. As in previous studies, H. convergens foraged for aphids more effectively on reduced wax peas than on normal peas. Other interactions among H. convergens, P. scitulus , and A. pisum were the same on both types of peas. We consider how aphid movement, plant growth, and a high frequency of predation by P. scitulus on H. convergens influenced pea aphid density.     

Plant waxy bloom on peas affects infection of pea aphids by Pandora neoaphidis

This study examined the effects of the surface wax bloom of pea plants, Pisum sativum, on infection of pea aphids, Acyrthosiphon pisum, by the fungal pathogen Pandora neoaphidis. In prior field surveys, a higher proportion of P. neoaphidis-killed pea aphids (cadavers) had been observed on a pea line with reduced wax bloom, as compared with a sister line with normal surface wax bloom. Laboratory bioassays were conducted in order to examine the mechanisms. After plants of each line infested with aphids were exposed to similar densities of conidia, the rate of accumulation of cadavers on the reduced wax line was significantly greater than on the normal wax bloom line; at the end of the experiment (13d), the proportion of aphid cadavers on the reduced wax line was approximately four times that on the normal wax bloom line. When plants were exposed to conidia first and then infested with aphids, the rate of accumulation of cadavers was slightly but significantly greater on the reduced wax line, and infection at the end of the experiment (16d) did not differ between the lines. When aphids were exposed first and then released onto the plants, no differences in the proportion of aphid cadavers were observed between the pea lines. Greater infection of pea aphid on reduced wax peas appears to depend upon plants being exposed to inoculum while aphids are settled in typical feeding positions on the plant. Additional experiments demonstrated increased adhesion and germination by P. neoaphidis conidia to leaf surfaces of the reduced wax line as compared with normal wax line, and this could help explain the higher infection rate by P. neoaphidis on the reduced wax line. In bioassays using surface waxes extracted from the two lines, there was no effect of wax source on germination of P. neoaphidis conidia.     

Waxy bloom in peas influences the performance and behavior of Aphidius ervi, a parasitoid of the pea aphid

Leaf surface waxy bloom can influence the predator–prey interactions that take place on peas. We tested whether the interaction between the pea aphid Acyrthosiphon pisum Harris (Homoptera: Aphidae) and a parasitoid, Aphidius ervi Haliday (Hymenoptera: Aphidiidae), is affected by reduced wax. We performed greenhouse experiments comparing aphid parasitism by individual A. ervi on two varieties of reduced wax peas to two normal wax sister varieties. We also observed the behavior of individual A. ervi in the greenhouse and measured field parasitism in small plots of reduced wax and normal wax peas. In the greenhouse, individual A. ervi parasitized more aphids on the reduced wax varieties than on their normal wax counterparts. Wasps spent more time actively foraging on reduced wax pea plants, which may contribute to the higher parasitism observed on those varieties. The greenhouse results suggested that the improved performance of individual A. ervi on reduced wax peas might contribute to a higher parasitism on reduced wax peas in the field. Field parasitism was significantly higher in reduced wax pea plots during 2000. Overall parasitism was higher in 2000 than in 2001 and 2002. In the latter years, parasitism was higher on reduced wax plants, but not significantly different from normal wax plants. Improved foraging by individual A. ervi resembles improved foraging by other carnivorous insects on reduced wax peas. The advantages of reduced wax for biological control of the pea aphid may hold when any of several different natural enemies is abundant.     

EVOLUTION OF AN APHID-PARASITOID INTERACTION: VARIATION IN RESISTANCE TO PARASITISM AMONG APHID POPULATIONS SPECIALIZED ON DIFFERENT PLANTS

The evolution of associations between herbivorous insects and their parasitoids is likely to be influenced by the relationship between the herbivore and its host plants. If populations of specialized herbivorous insects are structured by their host plants such that populations on different hosts are genetically differentiated, then the traits affecting insect-parasitoid interactions may exhibit an associated structure. The pea aphid (Acyrthosiphon pisum) is a herbivorous insect species comprised of genetically distinct groups that are specialized on different host plants (Via 1991a, 1994). Here, we examine how the genetic differentiation of pea aphid populations on different host plants affects their interaction with a parasitoid wasp, Aphidius ervi. We performed four experiments. (1) By exposing pea aphids from both alfalfa and clover to parasitoids from both crops, we demonstrate that pea aphid populations that are specialized on alfalfa are successfully parasitized less often than are populations specialized on clover. This difference in parasitism rate does not depend upon whether the wasps were collected from alfalfa or clover fields. (2) When we controlled for potential differences in aphid and parasitoid behavior between the two host plants and ensured that aphids were attacked, we found that pea aphids from alfalfa were still parasitized less often than pea aphids from clover. Thus, the difference in parasitism rates is not due to behavior of either aphids or wasps, but appears to be a physiologically based difference in resistance to parasitism. (3) Replicates of pea aphid clones reared on their own host plant and on a common host plant, fava bean, exhibited the same pattern of resistance as above. Thus, there do not appear to be nutritional or secondary chemical effects on the level of physiological resistance in the aphids due to feeding on clover or alfalfa, and therefore the difference in resistance on the two crops appears to be genetically based. (4) We assayed for genetic variation in resistance among individual pea aphid clones collected from clover fields and found no detectable genetic variation for resistance to parasitism within two populations sampled from clover. This is in contrast to Henter and Via's (1995) report of abundant genetic variation in resistance to this parasitoid within a pea aphid population on alfalfa. Low levels of genetic variation may be one factor that constrains the evolution of resistance to parasitism in the populations of pea aphids from clover, leading them to remain more susceptible than populations of the same species from alfalfa.     

Non‐consumptive effects of a natural enemy on a non‐prey herbivore population

1. Predator–prey interactions have traditionally focused on the consumptive effects that predators have on prey. However, predators can also reduce the abundance of prey through behaviourally‐mediated non‐consumptive effects. For example, pea aphids (Acyrthosiphon pisum Harris) drop from their host plants in response to the risk of attack, reducing population sizes as a consequence of lost feeding opportunities. 2. The objective of the present study was to determine whether the non‐consumptive effects of predators could extend to non‐prey herbivore populations as a result of non‐lethal incidental interactions between herbivores and foraging natural enemies. 3. Polyculture habitats consisting of green peach aphids (Myzus persicae Sulzer) feeding on collards and pea aphids feeding on fava beans were established in greenhouse cages. Aphidius colemani Viereck, a generalist parasitoid that attacks green peach aphids but not pea aphids, was released into half of the cages and the abundance of the non‐host pea aphid was assessed. 4. Parasitoids reduced the population growth of the non‐host pea aphid by increasing the frequency of defensive drops; but this effect was dependent on the presence of green peach aphids. 5. Parasitoids probably elicited the pea aphid dropping behaviour through physical contact with pea aphids while foraging for green peach aphids. It is unlikely that pea aphids were responding to volatile alarm chemicals emitted by green peach aphids in the presence of the parasitoid. 6. In conclusion, the escape response of the pea aphid provided the opportunity for a parasitoid to have non‐target effects on an herbivore with which it did not engage in a trophic interaction. The implication is that natural enemies with narrow diet breadths have the potential to influence the abundance of a broad range of prey and non‐prey species via non‐consumptive effects.     

Aphid population dynamics: does resistance to parasitism influence population size?

1. Insect population size is regulated by both intrinsic traits of organisms and extrinsic factors. The impacts of natural enemies are typically considered to be extrinsic factors, however insects have traits that affect their vulnerability to attack by natural enemies, and thus intrinsic and extrinsic factors can interact in their effects on population size. 2. Pea aphids Acyrthosiphon pisum Harris (Hemiptera: Aphididae) in New York and Maryland that are specialised on alfalfa are approximately two times more physiologically resistant to parasitism by Aphidius ervi Haliday (Hymenoptera: Braconidae) than pea aphids specialised on clover. To assess the potential influence of this genetically based difference in resistance to parasitism on pea aphid population dynamics, pea aphids, A. ervi, and other natural enemies of aphids in clover and alfalfa fields were sampled. 3. Rates of successful parasitism by A. ervi were higher and pea aphid population sizes were lower in clover, where the aphids are less resistant to parasitism. In contrast, mortality due to a fungal pathogen of pea aphids was higher in alfalfa. Generalist aphid predators did not differ significantly in density between the crops. 4. To explore whether intrinsic resistance to parasitism influences field dynamics, the relationship between resistance and successful field parasitism in 12 populations was analysed. The average level of resistance of a population strongly predicts rates of successful parasitism in the field. The ability of the parasitoid to regulate the aphid may vary among pea aphid populations of different levels of resistance.     

Effects of hydrophilic and hydrophobic kaolin-based particle films on pea aphid (Homoptera: Aphididae) and its entomopathogen Pandora neoaphidis (Entomophthorales: Entomophthoraceae)

Hydrophobic and hydrophilic kaolin-based particle films are effective for control of insect pests in certain agricultural crops. How these products interact with potential biological control agents is not well documented. This study was conducted to evaluate the effects of the hydrophobic (M96-018) and hydrophilic (Surround WP) kaolin-based particle films (Engelhard Corporation, Iselin, NJ) on pea aphid, Acyrthosiphon pisum (Harris), on peas (Pisum spp.), and on the fungal aphid pathogen Pandora neoaphidis (Remaudière and Hennebert) Humber. Over two field seasons (2001 and 2002) in northern Idaho, applications of M96-018 significantly reduced the rate of pea aphid increase on pea, but Surround WP, tested only in 2001, did not reduce aphid population growth rate. Neither particle film treatment was as effective as a standard application of esfenvalerate (DuPont Asana). In the laboratory, particle films suppressed pea aphid populations by up to 30%. M96-018 seemed to have some repellent activity based on aphid distributions after treating plants. When applied along with P. neoaphidis conidia, M96-018 but not Surround WP caused higher percentage of infection mortality of pea aphids by P. neoaphidis than occurred on controls treated only with P. neoaphidis conidia. P. neoaphidis conidia deposited on glass slides coated with M96-018, produced more germ tubes and secondary conidia than those deposited on untreated glass slides or slides treated with Surround WP. This result suggests that greater infection of pea aphids on plants treated with M96-018 is in part a result of a direct enhancement of fungal germination by the particle film.     

Aphid clonal resistance to a parasitoid fails under heat stress

Parasitoid virulence and host resistance are complex interactions depending on metabolic rate and cellular activity, which in aphids additionally implicate heritable secondary symbionts among the Enterobacteriaceae. As performance of the parasitoid, the aphid host and its symbionts may differentially respond to temperature, the success or failure of aphid parasitism is difficult to predict when temperature varies. We tested the hypothesis that resistance of the pea aphid Acyrthosiphon pisum to the parasitoid Aphidius ervi, which is linked to aphid secondary symbionts, may depend on temperature in several resistant and non-resistant aphid clonal lineages of different geographic origin and of known bacterial symbiosis, using experiments in controlled environments. Complete immunity to A. ervi at 20 degrees C in three different aphid clones whose symbiosis is characterized by the possession of Hamiltonella defensa reversed to high susceptibility at 25 degrees C and especially 30 degrees C, suggesting that the aphid's immune responses to the establishment and early development of the parasitoid is strongly reduced at moderately high temperatures. There was no evidence that a pea aphid control genotype that was susceptible to A. ervi at 20 degrees C could become more resistant as temperature increases, as has been suggested for insect fungal pathogens. By contrast, our results suggest that aphid clonal resistance to A. ervi and related parasitoids is characteristic of cool temperature conditions, similar to various other fitness attributes of aphids. Based on evidence that H. defensa symbionts characterized all three A. ervi resistant pea aphid clones studied, but was absent in control aphids that remained susceptible at all temperatures, we suggest that secondary symbiosis plays a key role in the heat sensitivity of aphid clonal resistance. Our study may also indicate that aphid natural control of variably susceptible host populations by aphid parasitoids is more likely at moderate to high temperatures.     

Causes of regional and yearly variation in pea aphid numbers in eastern England

Regional variation in the number of pea aphids caught in the suction traps of the Rothamsted Insect Survey (RIS) was associated with the proportion of each region under pea crops. The degree of infestation of crops was similar in areas of high and low pea production as the mean annual abundance of aphids per hectare of crop remained constant. Yearly variation in abundance was loosely associated with temperature from January to July. Cold weather in January and February resulted in large numbers of aphids. Warm weather in February led to early colonization, and emigration from, pea crops as well as making early sowing of the crop more likely. A forecast of the time of first appearance of Acyrthosiphon pisum in the aerial plankton can be made, based on February temperature. Populations of A. pisum on peas appear to be regulated by alata production. High densities of aphids resulted in almost all the nymphs developing into alatae which, on reaching maturity, emigrated, causing populations on the crop to decline. This explains population crashes of the pea aphid observed at early growth stages of the crop, on vining and combining peas. Late sowing of peas, a probable effect of cold winters, results in higher aphid densities at flowering. The probable explanation for this is that late-sown crops are colonized at an earlier growth stage, so that the aphid population has a longer period of time in which to develop.     

The effects of plant epicuticular waxy blooms on attachment and effectiveness of predatory insects

Phytophagous insects and their natural enemies frequently must cling to plants in order to forage. Typically, this involves attachment to the layer of lipophilic materials, or 'epicuticular waxes' (EW) that covers all primary plant surfaces. EW occurring as crystalline waxy 'blooms' can provide a defense against herbivory by interfering with attachment to plants by phytophagous insects. On the other hand, EW blooms can reduce attachment by predators and parasitoids, potentially releasing populations of phytophagous insects from regulation by their natural enemies. The net effect of EW blooms on herbivory should therefore vary from system to system. When it has been measured, EW bloom typically reduces the attachment forces insects can generate on plants. Some herbivores and predators preferentially forage on plants with EW blooms. Among these, some appear to have physiological or behavioral adaptations that either improve attachment to EW blooms or cope with reduced attachment to plants surfaces bearing EW blooms. How EW blooms disrupt insect attachment and how insects potentially overcome this challenge has not been determined. Some hypotheses are offered in this review. Their elucidation may help understand the mechanics of attachment to plants, with applications including helping to improve biological control of some insect pests.     

Predator avoidance behavior in the pea aphid: costs,frequency, and population consequences

Induced prey defenses can be costly. These costs have the potential to reduce prey survival or reproduction and, therefore, prey population growth. I estimated the potential for predators to suppress populations of pea aphids (Acyrthosiphon pisum) in alfalfa fields through the induction of pea aphid predator avoidance behavior. I quantified (1) the period of non-feeding activity that follows a disturbance event, (2) the effect of frequent disturbance on aphid reproduction, and (3) the frequency at which aphids are disturbed by predators. In combination, these three values predict that the disturbances induced by predators can substantially reduce aphid population growth. This result stems from the high frequency of predator-induced disturbance, and the observation that even brief disturbances reduce aphid reproduction. The potential for predators to suppress prey populations through induction of prey defenses may be strongest in systems where (1) predators frequently induce prey defensive responses, and (2) prey defenses incur acute survival or reproductive costs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

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

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.Deceased, formerly at the Shafter Research Station, 17053 Shafter Ave., Shafter, CA 93263, USA     

Examination of the success rate of secondary symbiont manipulation by microinjection methods in the pea aphid system

Insects harbor a wide range of microbial symbionts, but their influence on host phenotypes is described in a limited number of biological models. One experimental approach to gain knowledge on the effects of symbionts to their hosts is to create insect lines with and without symbionts and examine their phenotypes. However, the success rate of symbiont elimination and introduction methods is dependent on several parameters that are scarcely tested or described. The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), is a model insect of symbiosis studies. It harbors a primary symbiont that supplies the host with essential amino acids, and an array of secondary symbionts whose effects have been assessed by manipulating their presence/absence in the insect. Here, we describe the influence of key parameters on the success rate of symbiont manipulation using the pea aphid–secondary symbiont system. We compared two elimination methods differing in antibiotic treatment using several aphid–symbiont combinations. We also created new aphid host–symbiont combinations by secondary symbiont introduction and examined the effects of larval stage of recipient aphids on introduction success. Our study revealed that the aphid–symbiont combination has strong influence on both symbiont introduction and elimination success rates, and that the type of antibiotics and the larval stage of recipient aphids influence the elimination and introduction success rate, respectively.     

Interspecific symbiont transfection confers a novel ecological trait to the recipient insect

In Japan, pea aphids Acyrthosiphon pisum mainly feed on vetch and clover, and many aphid clones produce more progeny on vetch than on clover. In this context, particular genotypes of the facultative symbiont Regiella insecticola enhance reproduction of infected pea aphids specifically on clover, thereby broadening the suitable food plant range of the insect. A species that is sympatric to A. pisum, vetch aphids Megoura crassicauda, are commonly found on vetch but not on clover. Laboratory rearing of M. crassicauda strains revealed active reproduction on vetch but substantially no reproduction on clover. Experimental transfection of Regiella from A. pisum to M. crassicauda by haemolymph injection established stable and heritable infection in the recipients, although no Regiella infection has been detected in natural populations of M. crassicauda. Different strains of Regiella-transfected M. crassicauda grew and reproduced on vetch, but exhibited lower fitness in comparison with corresponding uninfected aphid strains. Strikingly, the Regiella-transfected M. crassicauda exhibited improved survival and some reproduction on clover. These results suggest that Regiella has the potential to confer an ecological trait, adaptation to clover, on novel insect hosts, and also account for why Regiella is able to infect M. crassicauda but is scarcely found in these aphid populations.     

Impact of abiotic factors on predator-prey interactions: DNA-based gut content analysis in a microcosm experiment

The effects of predators on prey populations can be modified by a number of abiotic factors. Here, we investigated the combined and separate effects of rain and ground-dwelling predators on aphid populations in a microcosm experiment lasting for 21 days, using PCR to analyse the gut content of the predators. Rain significantly dislodged aphids from shoots and ears by 57% and 25%, respectively. The gut content analysis showed that more predators consumed aphids in the rain treatment than without rain, indicating higher availability of aphids to ground-dwelling predators after rain. However, no synergistic effects of rain and ground-dwelling predators on aphid population development could be demonstrated. Rain alone significantly decreased aphid populations by 27%, suggesting that this is a significant mortality factor. Predators alone had no significant effect on aphid numbers, but the gut content analyses showed aphid consumption also in the no-rain treatments, indicating that aphids were available to the predators on the soil surface even without rain. Our results suggest that weather conditions such as rain can modify predator-prey interactions in the field. Employing PCR-based predator gut content analyses proved to be useful as trophic links could be directly verified.     

Fitness effects of two facultative endosymbiotic bacteria on the pea aphid,Acyrthosiphon pisum,and the blue alfalfa aphid,A. kondoi

The effects of two bacterial endosymbionts, designated PASS and PAR, were evaluated on the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera:Aphididae), in which they occur facultatively, and on the blue alfalfa aphid, A. kondoi Shinji, in which these bacteria have not been found in natural populations. Subclones of pea aphids and blue alfalfa aphids, derived from parent aphid clones that did not contain PASS or PAR, were infected with one or both bacteria, generating PASS- and/or PAR-positive subclones with minimal genetic differences from the parent clones. Under laboratory conditions at 20 °C, PAR consistently reduced the fecundity (by between 19 and 60%) of subclones derived from three different parent pea aphid clones on bur clover, Medicago hispida Gaertn. PAR had intermediate effects on pea aphids reared on sweet pea, Lathyrus odoratus L., and had no significant effect on pea aphids on alfalfa, Medicago sativa L. The effect of PASS was either neutral or negative, depending on parent clone as well as host plant. Also at 20 °C, PASS reduced fecundity (70–77%) and longevity (40–48%), and increased the age of first reproduction (by up to 1.5 days) of blue alfalfa aphid reared on alfalfa and clover. PAR had a less dramatic effect (e.g., 30–39% reduction in fecundity) on these traits of blue alfalfa aphid. In contrast, PAR and PASS increased the fitness of pea aphid subclones of one parent clone reared for three generations at 25 °C on each of the three test plants. Without facultative bacteria, fecundity of the parent clone was reduced to a mean total of < 6 offspring per adult at this elevated temperature, but with PASS or PAR, mean total fecundity of its subclones was > 35. However, this ameliorative effect of facultative bacteria at 25 °C was not found for two other sets of parent clones and their derived subclones. Alate production in pea aphids was significantly increased in large populations of two PASS- and PAR-positive subclones relative to their parent clones. Attempts to transmit PASS or PAR horizontally, i.e., from aphid to aphid via feeding on host plants (bur clover), were unsuccessful.     

Effects of aphid-tending Argentine ants, nitrogen enrichment and early-season herbivory on insects hosted by a coastal shrub

The presence of the exotic Argentine ant, Linepithema humile Mayr (Hymenoptera: Dolichoderinae), nitrogen enrichment, and early-season herbivory by the specialist beetle Trirhabda bacharidis (Coleoptera: Chrysomelidae) have been shown, through separate experiments, to affect the densities of insect herbivores of the coastal shrub Baccharis halimifolia (Asteraceae), in Florida. Using a fully-factorial field experiment, we examined the relative importance of all three of these factors to the six most common insect herbivore species utilizing this host plant in a West Central Florida coastal habitat. The presence of ants affected more herbivore species than either early-season herbivory by larval T. bacharidis or nitrogen enrichment. Experimental reductions of L. humile resulted in reductions of an aphid, its coccinellid predators, and adult T. bacharidis, and increases of two species of leafminers and one species of stemborer. Due to the strong negative effects of stemborer herbivory on host plant survival, the increase in stemborer abundance led to increased host plant mortality. Early-season herbivory by larval T. bacharidis only affected the abundance of aphids and their predators, both of which were more abundant on trees with reduced early-season herbivory. Nitrogen fertilization had the most limited effects and only T. bacharidis larvae achieved higher densities on fertilized trees. Our results indicate that aphid tending by the exotic L. humile affects other insects on B. halimifolia more so than herbivory by the exploitative competitor T. bacharidis or nitrogen as a limiting nutrient.     

Water stress and insect herbivory interactively reduce crop yield while the insect pollination benefit is conserved

Climate change is predicted to hamper crop production due to precipitation deficits and warmer temperatures inducing both water stress and increasing herbivory due to more abundant insect pests. Consequently, crop yields will be impacted simultaneously by abiotic and biotic stressors. Extensive yield losses due to such climate change stressors might, however, be mitigated by ecosystem services such as insect pollination. We examined the single and combined effects of water stress, insect herbivory and insect pollination on faba bean yield components and above‐ and belowground plant biomass under realistic field conditions. We used rainout shelters to simulate a scenario in line with climate change projections, with adequate water supply at sowing followed by a long period without precipitation. This induced a gradually increasing water stress, culminating around crop flowering and yield formation. We found that gradually increasing water stress combined with insect herbivory by aphids interactively shaped yield in faba beans. Individually, aphid herbivory reduced yield by 79% and water stress reduced yield by 52%. However, the combined effect of water stress and aphid herbivory reduced yield less (84%) than the sum of the individual stressor effects. In contrast, insect pollination increased yield by 68% independently of water availability and insect herbivory. Our results suggest that yield losses can be greatly reduced when both water stress and insect herbivory are reduced simultaneously. In contrast, reducing only one stressor has negligible benefits on yield as long as the crop is suffering from the other stressor. We call for further exploration of interactions among ecosystem services and biotic and abiotic stressors that simulate realistic conditions under climate change.     

Combined effects of host-plant resistance and intraguild predation on the soybean aphid parasitoid Binodoxys communis in the field

Soybean varieties that exhibit resistance to the soybean aphid Aphis glycines have been developed for use in North America. In principle, host-plant resistance to soybean aphid can influence the interactions between the soybean aphid and its natural enemies. Resistance could change the quality of soybean aphids as a food source, the availability of soybean aphids, or resistance traits could directly affect aphid predators and parasitoids. Here, we focus on the effect of soybean aphid resistance on the interactions between soybean aphids, the parasitoid Binodoxys communis (Hymenoptera: Braconidae), and predators of these two species. We determined whether host-plant resistance affected within-season persistence of B. communis by releasing parasitoids into resistant and susceptible soybean plots. We observed higher B. communis densities in susceptible soybean plots than in resistant plots. There were also higher overall levels of intraguild predation of B. communis in susceptible plots, although the per-capita risk of intraguild predation of B. communis was affected neither by plant genotype nor by aphid density. We discuss these effects and whether they were caused by direct effects of the resistant plants on B. communis or indirect effects through soybean aphid or predators.