High Aphid Density Research Articles

Biotic and Abiotic Influences on Within-Plant Distribution of Soybean Aphid (Hemiptera: Aphididae: Aphis glycines)

The within-plant distribution of soybean aphid, Aphis glycines Matsumura has been attributed to various causes, yet it is unclear which factor or combination of factors most influence aphid within-plant distribution. Understanding soybean aphid distribution within the soybean canopy is important both in developing sampling programs and in evaluating injury. Because natural enemies have the potential to alter aphid dispersion, we examined how within-plant distribution of soybean aphid was influenced by different natural enemies. Field cages of different mesh diameters were used to exclude different sizes of natural enemies from aphid infested plots. Plots were surveyed and both natural enemy density and soybean aphid density, by strata, were recorded twice weekly. Cages were found to have minimal effect on temperature and soybean plant development. Aphid densities were significantly different each year among dates, treatments, and strata. In 2004 aphid densities were concentrated on the top part of the plant and in 2005 aphid densities were concentrated in the middle of the canopy. Treatments had significant impacts on aphid density in both years of the study. The total exclusion treatment had the highest aphid densities in both years. Treatment also had a significant effect on which stratum aphids were most commonly found. The factor that was found to best explain the differences in aphid distribution was the abundance of natural enemies, primarily Orius insidosus (Say). In treatments left exposed to natural enemies the percentage of the total aphid population was found lower in the canopy. It is clear that aphid densities and distributions are being affected by the presence of natural enemies. Understanding how natural enemies affect the distribution of this economically damaging aphid species is an important component of its management, specifically the development of accurate scouting protocols and pesticide efficiency.

Development and Validation of Node-Based Sample Units for Estimating Soybean Aphid (Hemiptera: Aphididae) Densities in Field Cage Experiments

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is currently the most important insect threat to soybean, Clycine max (L.) Merr., production in the North Central United States. Field cage studies are a key tool in investigating the potential of natural enemies and host plant resistance to control this pest. However, a major constraint in the use of cage studies is the limited number of treatments and replicates that can be used as aphid densities frequently become so large as to limit the number of experimental units that can be quantified. One way to overcome this limitation is to develop methods that estimate whole-plant aphid densities based on a reduced sampling plan. Here, we extend an existing method, node-sampling, used for estimating aphid populations in open field conditions and apply it to caged populations. We show that parameters calculated under open field conditions are inappropriate to estimate caged populations. In contrast, using four independent data sets of caged populations and a cross-validation technique, we demonstrate that a three-node sampling unit and a weighted formula provide accurate and robust estimates of whole-plant aphid density. This method reduced the number of aphids counted per plant by and average of 60%, with greater reductions at higher aphid densities. We further demonstrate that nearly identical statistical results were obtained when whole-plant or node-sampling estimates were used in the analysis of two case studies. The reduced sample unit method developed here saves time without sacrificing efficiency so that more plants, replications, or studies can be conducted that will lead to improved soybean aphid management.

Two‐year oscillation cycle in abundance of soybean aphid in Indiana

The present study evaluated the population dynamics of the heteroecious soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae) during an 8‐year period in Indiana, shortly after its detection in North America. Sampling conducted at multiple locations revealed that A. glycines exhibited a 2‐year oscillation cycle that repeated itself four times between 2001 and 2008: years of low aphid abundance were consistently followed by years of high aphid abundance. Similar patterns of abundance of A. glycines and coccinellids (Coleoptera: Coccinellidae) in soybean fields, both within and between‐years, suggest that late season predation by coccinellids plays a role in the oscillatory cycle of aphids. Insidious flower bugs Orius insidiosus (Say) (Hemiptera: Anthocoridae) were numerically more abundant than coccinellids, although the lack of synchrony between aphids and predatory bugs suggests that O. insidiosus has a limited influence on between‐year variations in aphid density. The inverse relationship between aphid densities before and after the start of the autumn migratory period changes direction in alternate years. High aphid density on soybean in the summer is associated with a reduced number of alate migrants produced in the autumn. Conversely, years with low density aphids on soybean in the summer are characterized by high numbers of alates that migrate to the primary host in the autumn. From a pest management perspective, the 2‐year oscillation cycle of A. glycines is a desirable attribute with respect to population dynamics because it implies that aphids cause significant economic damage only in alternate years (as opposed to every year). Cultural practices enhancing the conservation biological control of Coccinellidae may help to preserve the periodicity of aphid infestation and restrict the pest status of A. glycines.

Density-dependent intraguild predation of an aphid parasitoid

A growing body of research has examined the effect of shared resource density on intraguild predation (IGP) over relatively short time frames. Most of this work has led to the conclusion that when the shared resource density is high, the strength of IGP should be lower, due to prey dilution. However, experiments addressing this topic have been done using micro- or mesocosms that excluded the possibility of intraguild predator aggregation. We examined the effect of shared resource density on IGP of an aphid parasitoid in an open field setting where the effects of prey dilution and predator aggregation could occur simultaneously. We brought potted soybean plants with 2, 20, or 200 soybean aphids (Aphis glycines) and 20 pupae ('mummies') of the soybean aphid parasitoid Binodoxys communis into soybean fields in Minnesota, USA. We monitored predator aggregation onto the potted plants, predation of parasitoid mummies, and successful adult emergence of B. communis. We found that predator aggregation was higher at the higher aphid densities on our experimental plants and that this coincided with lower adult emergence of B. communis, indicating that even if a prey dilution effect occurred in our study, it was overcome by short-term predator aggregation. Our results suggest that the effect of shared resource density on IGP may be more nuanced in a field setting than in microcosms due to predator aggregation.

Quantifying aphid predation: the mealy plum aphid Hyalopterus pruni in California as a case study

Summary 1. Generalist insect predators provide an important ecosystem service in the regulation of agricultural pests. However, the role of predation in the dynamics of aphid populations has been difficult to quantify as any change in aphid density results from a balance between two opposing processes, aphid recruitment through reproduction and aphid mortality through predation. 2. Our goal in this study was to determine the degree to which aphid consumption by naturally occurring densities of aphidophagous predators is responsible for observed seasonal changes in aphid densities. We quantified consumptive predation using a mechanistic approach that incorporates the balance between aphid recruitment and mortality. We used the mealy plum aphid Hyalopterus pruni in Prunus domestica plum orchards as a model system. 3. Our approach was to compare two quantities: a demographic‐based estimate of predation and an observation‐based estimate of predation. The demographic‐based estimate used a simple model of aphid population growth and predation. It estimates the mortality that must have occurred to give rise to observed changes in aphid densities between sample dates. The model uses independent estimates of aphid population growth rate in the absence of predation. The observation‐based estimate combined field data on predator densities in plum orchards with stage‐ and species‐specific per capita predation rates estimated from field observation. 4. Predation may have reduced the rate at which mealy plum aphid populations increased early in the season and rebounded from mid‐season declines, but was not sufficient to prevent high aphid densities or to accelerate rates of decline later in the season. Moreover, predation at naturally occurring densities did not appear sufficient to drive the directional changes in mealy plum aphid densities at any point through the season. 5. Synthesis and applications. Our study shows that natural densities of generalist predators can be too low to provide effective biological control services for an aphid pest. Generalist aphid predators could provide more effective control if early‐season densities were increased through augmentative releases or encouragement of crop colonization, but the commercial viability of these options remains to be tested.

Effects of integrating companion cropping and nitrogen application on the performance and infestation of collards by Brevicoryne brassicae

AbstractSustainable management of cabbage aphids, Brevicoryne brassicae (L.) (Hemiptera: Aphididae), is a major goal for collard, Brassica oleracea (L.) var. acephala (Brassicaceae), growers globally. Host finding ability of insect pests is significantly affected by diversified cropping systems, and this approach is being utilized currently as a pest management tool. Soil nutrition and its interaction with the cropping systems could have a significant effect on the general performance of collards and the infestation by cabbage aphids. In a search for a sustainable cabbage aphid control, a two‐season field experiment was carried out with two intercrops, collards and chilli, Capsicum frutescens (L.) (Solanaceae), and collards and spring onions, Allium cepa (L.) (Alliaceae), and a collard monoculture. For each of the cropping systems, nitrogen (N) was applied to the soil as a top‐dress at 20, 25, 30, and 35 g per collard plant. The response factors monitored were collard yield (fresh weight) and aphid infestation on collards. Spring onion‐collard intercrop had the lowest aphid density and the highest yield. Collard monoculture had the highest aphid infestation and the lowest yield. High levels of N led to increased infestation of collards by aphids, but also led to a significant increase in the yield of collards. Significant interactions between the N rates and the cropping systems were observed on some sampling dates, with the highest yield being realized under a combination of spring onion‐collard intercrop at a N rate of 30 g per plant. High aphid density led to a decrease in the yield of collards. It was concluded that with a spring onion‐collard intercrop, the soil N level could be raised from the blanket rate of 20–30 g per plant and this would lead to an increase in yield.

Spatial distribution of flower vs. honeydew resources in cereal fields may affect aphid parasitism

The potential impact of nectar availability for biological pest control is of increasing relevance due to the disappearance of non-crop plants in intensified agricultural landscapes. However, little is known about how floral resources and their spatial distribution in and around crop fields influence cereal aphid parasitoid (Hymenoptera, Aphidiidae) survival and population dynamics. Cereal aphid parasitoids may be less dependent on flowers as they can use honeydew from their aphid hosts as an alternative food resource. However, honeydew appears to be less suitable as a food source for parasitoids when compared to nectar. Here the effects of flower distribution (nectar), aphid abundance (honeydew quantity) and honeydew quality on parasitoid life history parameters were explored using a spatially explicit, individual-based simulation model. In addition, effects of the availability of floral resources in field margins on parasitism levels in winter wheat were investigated in a field study. In the simulation study three flower distributions were compared: (i) flowers restricted to the field margin (most common in agricultural landscapes) or (ii) flowers in strips within the crop, and (iii) random flower distribution in the field. Simulations suggest that nectar feeding increases parasitoid longevity and parasitism at low aphid densities, but only in fields with randomly distributed flowers. In contrast, the spatial distribution of flowers did not affect the parasitoid longevity and parasitism rate at high aphid densities, where parasitoid feeding was restricted to the abundant honeydew resources. In simulations, parasitoid survival and the number of aphids parasitized were also affected by the nutritional quality of the honeydew, with low quality honeydew having a less positive effect on parasitoid longevity and parasitism rate. The field study did not indicate evidence of a significant relationship between flower cover in field margins and parasitism rates in wheat near the margins. This finding is compatible with results from simulations and suggests that honeydew feeding is more prevalent in cereal aphid parasitoids than nectar feeding. In conclusion, nectar may be an important factor for the potential biocontrol success of cereal aphid parasitoids, but most likely only at low aphid densities and especially when flowers are broadly distributed in the field.

The spatio-temporal relationships among aphids, the entomophthoran fungus, Pandora neoaphidis, and aphidophagous hoverflies in outdoor lettuce

Worldwide, aphids are the key pests of lettuce, and they coexist with natural enemies in different guilds. In our study, the spatio-temporal patterns of aphids colonizing lettuce were assessed and compared with the distribution of an entomopathogenic fungus and hoverfly larvae during two spring lettuce growing seasons. Counts of aphid density, entomopathogenic fungal-infected aphids and hoverfly larvae present on lettuce plants were recorded and their spatial distribution analyzed using the Spatial Analysis by Distance IndicEs (SADIE) method to determine aggregation and association indices. Three aphid species ( Nasonovia ribisnigri, Macrosiphum euphorbiae and Acyrtosiphon lactucae) colonize lettuce plants in early spring; however, their natural enemies arrive several weeks later. Pandora neoaphidis was the only fungus identified to cause aphid mortality, and Sphaerophoria scripta was the most abundant syrphid species preying on lettuce aphids. In both 2007 and 2008, the aphids followed ephemeral spatio-temporal dynamics at the field scale. The maximum aphid aggregation was concurrent with the highest aphid density and with the complete development of the rosette lettuce leaf-stage. P. neoaphidis-infected aphids showed more aggregation over time than hoverfly larvae. The maximum spatial structure was recorded for both natural enemy species close to the harvest date and at the time when the aphid populations were distributed at random. Significant spatial associations between P. neoaphidis and hoverfly larvae were only detected close to the time of harvest, which reduces the risk of intraguild predation between these two natural enemies of aphids at the field scale.

Cereal aphid control by generalist predators in presence of belowground alternative prey: Complementary predation as affected by prey density

Summary Generalist predators are important antagonists of pest species in agroecosystems. Increasing populations of alternative prey through detrital subsidies is one way to foster these predators. However, alternative prey may also distract generalist predators from their prey thereby diminishing the efficiency of biological control. To develop reliable predictions for biological control, it is essential to evaluate the relative importance of generalist predators, pests, alternative prey and their respective interactions. We investigated the effects of an assemblage of generalist predators on the grain aphid Sitobion avenae in winter wheat. Treatments with 10, 100 and 1000 aphids were established inside 2 m 2 sized caged plots with control and reduced density of predators. Three weeks after the experiment started, samples were taken to estimate the size of aphid populations and those of alternative prey. Three prey taxa were significantly reduced by generalist predators: the grain aphid S. avenae (Aphidina), the click beetle Adrastus pallens (Elateridae) and the springtail Isotoma viridis (Collembola). Springtails were decreased by generalist predators independent of aphid densities, indicating complementary predation of springtails and aphids. At high aphid densities, grain aphid population peaks were decreased to the threshold level of economic damage, demonstrating efficient aphid suppression by the predator community. Click beetle numbers declined only at low and medium aphid densities. The results suggest that generalist predators preferentially fed on click beetles at low and medium aphid densities and switched to aphids at high aphid densities. Early-season predators likely had the greatest influence on aphid suppression. Our results indicate that alternative prey from the belowground system forms a substantial food resource for generalist predators, suggesting that the belowground subsystem modulates predator–prey interactions above the ground.

Direct and indirect effects of ants on seed predation in moth/yucca mutualisms

Strong indirect effects are important in community and ecosystem processes, particularly in aquatic communities where predation and competition are the dominant processes. In this study, we examine an obligate pollination/seed predation mutualism between moths and yuccas and the balance of direct and indirect effects of ants, aphids, and cheater moths on this mutualism. Cheater moths (Tegeticula corruptrix) do not pollinate yuccas but oviposit into developing fruit, where their larvae consume yucca seeds. The overall effect of ants and aphids on yuccas was positive. Ants and aphids had no negative impact on the number of fruit retained per plant, and they had a positive effect on the average number of viable seeds per fruit. Yuccas with high ant and aphid densities had a 60% increase in viable, uneaten seeds. This increase in seed production was largely due to the negative interactions between ants and T. corruptrix. The reduction of cheater ovipositions saved yuccas almost 46 seeds per fruit, plus an additional 26 seeds from the reduction of cheater larvae. Aphids also had an additional indirect benefit, because ants were less likely to damage yucca fruit when aphids were present. This study shows that strong positive interactions, such as the aphid/ant mutualism on yuccas, can also generate strong indirect effects on another positive interaction, the yucca/yucca moth mutualism.

Seasonal Abundance of Resident Parasitoids and Predatory Flies and Corresponding Soybean Aphid Densities, with Comments on Classical Biological Control of Soybean Aphid in the Midwest

Seasonal abundance of resident parasitoids and predatory flies, and corresponding soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), densities were assessed in soybean fields from 2003 to 2006 at two locations in lower Michigan. Six parasitoid and nine predatory fly species were detected in 4 yr by using potted plants infested with soybean aphid placed in soybean fields. The parasitoid Lysiphlebus testaceipes Cresson (Hymenoptera: Braconidae) and the predatory flies Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae), and Allograpta obliqua Say (Diptera: Syrphidae) were most numerous. Generally, L. testaceipes was more abundant late in the soybean growing season, but it also occurred during soybean vegetative growth; A. obliqua was more abundant during vegetative growth; and A. aphidimyza was common throughout the season. Soybean plants were visually inspected to estimate densities of soybean aphid, mummified aphids, and immature predatory flies. From 2003 to 2006, parasitism rates were inversely correlated with aphid density: percentage of parasitism was always very low (< or = 0.1%) at high aphid densities (> 100 aphids per plant), and higher parasitism, up to 17%, was observed at very low aphid densities (< 1 aphid per plant). Populations of immature predatory flies, particularly A. aphidimyza, generally increased in soybean fields with increasing soybean aphid populations, but aphids always outnumbered immature flies by 100-21,000-fold when flies were detected. Rearing field-collected aphid in 2006 substantiated that parasitism varied widely, with parasitism in most cases < 10%. Based on findings of low parasitism and predation, positive response to changing aphid densities by predatory flies but not parasitoids, early season abundance primarily of predatory flies, and past findings on these taxa's diversity and abundance, we discuss the potential use of exotic parasitoids and predatory flies to enhance soybean aphid biological control.

Interspecific Variation Within the Genus Asclepias in Response to Herbivory by a Phloem-feeding Insect Herbivore

Induced plant responses to leaf-chewing insects have been well studied, but considerably less is known about the effects of phloem-feedings insects on induction. In a set of laboratory experiments, we examined density-dependent induction by the milkweed-oleander aphid, Aphis nerii, of putative defenses in four milkweed species (Asclepias incarnata, Asclepias syriaca, Asclepias tuberosa, and Asclepias viridis). We hypothesized that high aphid density would lead to increased cardenolide expression in species with low constitutive levels of cardenolides (e.g., A. tuberosa), but that there would be no induction in high constitutive cardenolide species (e.g., A. viridis). Based on previous studies, we did not expect cardenolide induction in A. incarnata. Contrary to our predictions, we observed feeding-induced declines of cardenolide concentrations in A. viridis. Cardenolide concentrations did not respond to aphid feeding in the other three milkweed species. Aphids also caused reductions in biomass accumulation by two of four Asclepias species, A. viridis and A. incarnata. High aphid density led to a decrease in A. viridis foliar nitrogen concentration. However, aphids had no effect on the defensive chemistry, growth, or nutritional quality of either A. syriaca or A. tuberosa. Our results highlight that congeneric plant species may respond differently to the same levels of herbivore damage.

Effects of habitat fragmentation on the three‐way interaction among ants, aphids and larvae of the giant purple emperor, Sasakia charonda (Hewitson), a near‐threatened butterfly

AbstractSurvival rates of both early and middle instar larvae of the nymphalid butterfly, Sasakiacharonda, were estimated to be lowest on test trees planted in a meadow (site A), intermediate in a small, narrow secondary deciduous broadleaf forest (small patch, site B) and highest in a large secondary deciduous broadleaf forest (large forest, site C). The larval mortality rates due to predation by tree‐climbing predators from the ground (tree climbing predator) such as ants and the larvae of carabids were estimated to be greater at sites A and B than those at site C. The number of predatory ants climbing test trees was significantly greater at sites A and B than at site C, and the ants harvested honeydew from aphids living on tree leaves at those two sites. Aphid densities were significantly higher on trees at sites A and B than at site C, and aphid densities and numbers of predatory ants were significantly and positively correlated at sites A and B. In an experiment controlling aphid density per branch on test trees, the numbers of ants and the mortality rates of S.charonda larvae were greater on branches with high aphid densities than on those with low aphid densities at both sites A and B. These results suggest that the aphid density per host tree was higher in the meadow and the small patch than in the large forest; at both sites these higher aphid densities attracted higher numbers of predatory ants to test trees, and as a result, mortality rates of S.charonda larvae were increased.

Oviposition Response of Green Lacewings (Neuroptera: Chrysopidae) to Aphids (Hemiptera: Aphididae) and Potential Attractants on Pecan

Pecan foliage is attacked by three species of aphids [Monellia caryella (Fitch), Melanocallis caryaefoliae (Davis), and Monelliopsis pecanis Bissell], resulting in damage that can reduce tree nut yield. In this study, we assayed the ovipositional response of the green lacewing Chrysoperla rufilabris (Burmeister) to M. caryella and M. caryaefoliae at high and low aphid densities and the development of C. rufilabris larvae when fed solely on each of the three pecan aphid species. During 2004 and 2005, combinations of attractants and food sprays were applied to pecan trees in an orchard to monitor green lacewing ovipositional response. We found that C. rufilabris laid more eggs on seedling trees infested with the M. caryella (at both high and low densities) than on seedlings infested with M. caryaefoliae. Development of C. rufilabris was unaffected by aphid species. At least one attractant/food spray treatment applied to trees in an orchard significantly increased green lacewing oviposition for three of the five treatment dates over both years. These results show that larvae of C. rufilabris will consume all aphid species attacking pecan, even though female ovipositional response can differ for aphid species. It is likely that combinations of attractants and food sprays can be used to enhance green lacewing populations in orchards.

Biodiversity as both a cause and consequence of resource availability: a study of reciprocal causality in a predator–prey system

1. One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '... is species diversity the cause or the consequence of resources that limit community biomass?' 2. Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. 3. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. 4. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey.

Development of a sampling plan for Myzus persicae (Hemiptera: Aphidoidea) and its predator Macrolophus costalis (Hemiptera: Miridae) on tobacco

A tobacco field in Greece was sampled during the 2001 and 2002 growing seasons to assess the seasonal trends in densi- ties and spatial distributions of the aphid Myzus persicae (Sulzer) and its predatory mirid Macrolophus costalis (Fieber). On repeated occasions between June (just after the transplantation) and September (just before harvest), 20 tobacco leaves (10 from the upper and 10 from the lower plant part) were taken from randomly chosen plants. These leaves were sampled for aphids and mirids. In both years, the highest aphid densities were recorded during July and August, while aphid numbers were low in September. In contrast, the majority of M. costalis individuals were found during September when aphid numbers were low. Significantly more M. persicae individuals were found in the upper part of the plants, whereas significantly more M. costalis individuals were found in the lower part of the plants. As indicated by Taylor's Power Law estimates, both species were aggregated in their spatial distributions among sampling units (leaves). Wilson and Room's model, based on the Taylor's estimates, was used to calculate the mean number of aphids and mirids, from the proportion of sampling units (leaves) that had > 0 individuals of each species. This model provided a sat- isfactory fit of the data for both the aphid and the mirid. In addition, Wilson and Room's model was successfully used to predict the mean number of aphids and mirids in a series of samples that were carried out in the same area between June and September 2003 for model validation. Finally, equations are given for the calculation of precision in estimating the mean number of aphids or mirids per sampling unit, and the required sample size for a given level of precision.

Hymenopteran parasitoids and dipteran predators of the invasive aphid Diuraphis noxia after enemy introductions: Temporal variation and implication for future aphid invasions

Shifts in prevalence and abundance of hymenopteran parasitoids and dipteran predators, Diuraphis noxia , and other aphids were measured in the west-central Great Plains of North America, April–September, in 2001 and 2002, corresponding to over a decade after first detection of D. noxia and first release of D. noxia enemies. Significant temporal shifts in enemy species prevalence and diversity were detected in this study and more broadly during an 11 year time span. At any given time, some species were relatively common. One parasitoid had been predominant throughout ( Aphelinus albipodus ), two had shifted in dominance ( Lysiphlebus testaceipes and Diaeretiella rapae ), three parasitoids had been detected infrequently ( Aphidius avenaphis , Aphidius matricariae , and Aphelinus asychis ), one parasitoid was detected in the 1990s but not during 2001 and 2002 ( Aphelinus varipes ), two predatory flies occurred at occasional significant levels ( Leucopis gaimarii and Eupeodes volucris ), and two parasitoids may have been minor members of the fauna ( Aphidius ervi and Praon yakimanum ). Aphid populations detected were usually very low or not detected, precluding estimation of percent parasitism. The best evidence of suppression was observations of parasitoids in the rare case of D. noxia exceeding economic thresholds, which complemented past studies using high aphid densities. The D. noxia enemies detected were primarily endemic or long-time residents derived from previous introductions. This enemy community may provide flexibility in responding to a future aphid invasion, allowing more strategic use of biological control and other pest management approaches.

Differential Susceptibility of Poplar Hybrids to the Aphid Chaitophorus leucomelas (Homoptera: Aphididae)

After its recent introduction to Chile, the aphid Chaitophorus leucomelas Koch is becoming a serious pest affecting commercial poplar, Populus spp., plantations. The pattern of natural infestation of C. leucomelas among poplar hybrids with different pedigrees and the aphid intrinsic rate of increase (r(m)), of C. leucomelas were assessed in the field. In most of the hybrids, aphid abundance peaked in March (late summer). Among 12 types of poplar crosses, [(P. trichocarpa Torr. & Gray x P. deltoides Bartram ex Marshall) x (P. trichocarpa x P. deltoides)] and [(P. trichocarpa x P. maximowiczii Henry) x P. maximowiczii] showed the highest and lowest aphid densities, respectively. A trend to find more aphids in branch bases was apparent. The intrinsic rate of C. leucomelas increase was higher in [(P. trichocarpa x P. deltoides) x P. deltoides] hybrids, and lower in [(P. trichocarpa x P. maximowiczii) x P. trichocarpa] hybrids. Aphid density and performance were higher in hybrids with P. deltoides parentage, whereas hybrids with P. maximowiczii parentage showed lower aphid densities and performance. Hybrids with P. nigra L. parentage, namely, [P. trichocarpa x P. nigra], also had high aphid density, but aphid performance was lower compared with hybrids with P. deltoides parentage. These results suggest that among poplar hybrids studied, susceptibility to C. leucomelas is inherited through P. deltoides, whereas resistance seems to be inherited through P. maximowiczii. Thus, P. maximowiczii hybrids are recommended for commercial or ornamental planting programs in zones where there is a high risk of aphid infestation.

Alternative prey disrupt biocontrol by a guild of generalist predators

We examined the potential of a guild of generalist predators to control Colorado potato beetles (“CPB,” Leptinotarsa decemlineata) on potato. We were interested in determining whether beetle suppression would change with varying predator density, and with varying background densities of green peach aphids (“GPA,” Myzus persicae), which are common alternative prey for the predators in the field. We conducted two field experiments where we manipulated predator densities and measured the impact of these manipulations on CPB and GPA densities. In the first experiment, with low aphid densities, the predator guild significantly reduced beetle and aphid densities. In the second field experiment, with higher aphid densities, predators did not reduce beetle densities and only slightly depressed aphid increase. In both field experiments, we were unable to maintain elevated predator densities in cages where we added more predators, possibly due to intraguild predation. In laboratory microcosms, we further examined beetle predation by two common predators, Nabis spp. and Geocoris spp., in the presence versus absence of aphids. The two predators responded differently to a choice in prey. Geocoris spp. preyed upon aphids and eggs in rough proportion to the abundance of each, whereas Nabis spp. appeared to switch more readily to feeding on aphids. Overall, in both the field and the laboratory, we found evidence for a positive prey–prey interaction, with the presence of aphids reducing predation of potato beetles.

Density-dependent induction and suppression of soldier differentiation in an aphid social system

We investigated the mechanism underlying the control of soldier production in colonies of a social aphid, Tuberaphis styraci, which has a sterile soldier caste in the second instar. High aphid density was shown to induce soldier production in T. styraci. Analysis of natural colonies revealed that the soldier proportion tended to increase with aphid density but reached a plateau. Artificial diet experiments identified a similar plateau of soldier proportion under high-density conditions. In order to gain insights into the controlling mechanism of soldier production, the effect of soldiers on reproducing adult aphids was examined using the artificial diet system. It was experimentally demonstrated that soldier production was suppressed by coexisting soldiers, whereas coexisting non-soldiers facilitated soldier production. These results suggested that caste ratio in the colony of T. styraci is controlled by positive and negative feedbacks consisting of density-dependent induction and suppression of soldier differentiation.