Variable Isotopic Compositions of Host Plant Populations Preclude Assessment of Aphid Overwintering Sites.

Soybean aphid (Aphis glycines Matsumura) is a pest of soybean in the northern Midwest whose migratory patterns have been difficult to quantify. Improved knowledge of soybean aphid overwintering sites could facilitate the development of control efforts with exponential impacts on aphid densities on a regional scale. In this preliminary study, we explored the utility of variation in stable isotopes of carbon and nitrogen to distinguish soybean aphid overwintering origins. We compared variation in bulk 13C and 15N content in buckthorn (Rhamnus cathartica L.) and soybean aphids in Wisconsin, among known overwintering locations in the northern Midwest. Specifically, we looked for associations between buckthorn and environmental variables that could aid in identifying overwintering habitats. We detected significant evidence of correlation between the bulk 13C and 15N signals of soybean aphids and buckthorn, despite high variability in stable isotope composition within and among buckthorn plants. Further, the 15N signal in buckthorn varied predictably with soil composition. However, lack of sufficient differentiation of geographic areas along axes of isotopic and environmental variation appears to preclude the use of carbon and nitrogen isotopic signals as effective predictors of likely aphid overwintering sites. These preliminary data suggest the need for future work that can further account for variability in 13C and 15N within/among buckthorn plants, and that explores the utility of other stable isotopes in assessing likely aphid overwintering sites.

Rag Virulence Among Soybean Aphids (Hemiptera: Aphididae) in Wisconsin.

Soybean aphid, Aphis glycines Matsumura, a pest of soybean, Glycine max (L.) Merr., and native of Asia, invaded North America sometime before 2000 and rapidly became the most significant insect pest of soybean in the upper Midwest. Plant resistance, a key component of integrated pest management, has received significant attention in the past decade, and several resistance (Rag) genes have been identified. However, the efficacy of Rag (Resistance to Aphis glycines) genes in suppressing aphid abundance has been challenged by the occurrence of soybean aphids capable of overcoming Rag gene-mediated resistance. Although the occurrence of these Rag virulent biotypes poses a serious threat to effective and sustainable management of soybean aphid, little is known about the current abundance of biotypes in North America. The objective of this research was to determine the distribution of Rag virulent soybean aphids in Wisconsin. Soybean aphids were collected from Wisconsin during the summers of 2012 and 2013, and assayed for Rag1, Rag2, and Rag1+2 virulence using no-choice tests in a greenhouse. One clone from Monroe County in 2012 reacted like biotype 4, three clones in different counties in 2013 responded like biotype 2, and eight others expressed varying degrees of Rag virulence. Rag virulence in 2013 was observed in aphids from 33% of the sampled sites and was accounted for by just 4.5% of sampled clones, although this is likely a conservative estimate. No-choice test results are discussed in light of current questions on the biology, ecology, and population genetics of soybean aphid.

Potential Overwintering Locations of Soybean Aphid (Hemiptera: Aphididae) Colonizing Soybean in Ohio and Wisconsin.

Soybean aphids, Aphis glycines Matsumura, depend on long-distance, wind-aided dispersal to complete their life cycle. Despite our general understanding of soybean aphid biology, little is explicitly known about dispersal of soybean aphids between winter and summer hosts in North America. This study compared genotypic diversity of soybean aphids sampled from several overwintering locations in the Midwest and soybean fields in Ohio and Wisconsin to test the hypothesis that these overwintering locations are sources of the soybean colonists. In addition, air parcel trajectory analyses were used to demonstrate the potential for long-distance dispersal events to occur to or from these overwintering locations. Results suggest that soybean aphids from overwintering locations along the Illinois-Iowa border and northern Indiana-Ohio are potential colonists of soybean in Ohio and Wisconsin, but that Ohio is also colonized by soybean aphids from other unknown overwintering locations. Soybean aphids in Ohio and Wisconsin exhibit a small degree of population structure that is not associated with the locations of soybean fields in which they occur, but that may be related to specific overwintering environments, multiple introductions to North America, or spatial variation in aphid phenology. There may be a limited range of suitable habitat for soybean aphid overwintering, in which case management of soybean aphids may be more effective at their overwintering sites. Further research efforts should focus on discovering more overwintering locations of soybean aphid in North America, and the relative impact of short- and long-distance dispersal events on soybean aphid population dynamics.

Trophic hierarchies illuminated via amino acid isotopic analysis.

Food web ecologists have long sought to characterize the trophic niches of animals using stable isotopic analysis. However, distilling trophic position from isotopic composition has been difficult, largely because of the variability associated with trophic discrimination factors (inter-trophic isotopic fractionation and routing). We circumvented much of this variability using compound-specific isotopic analysis (CSIA). We examined the (15)N signatures of amino acids extracted from organisms reared in pure culture at four discrete trophic levels, across two model communities. We calculated the degree of enrichment at each trophic level and found there was a consistent trophic discrimination factor (~7.6‰). The constancy of the CSIA-derived discrimination factor permitted unprecedented accuracy in the measurement of animal trophic position. Conversely, trophic position estimates generated via bulk-(15)N analysis significantly underestimated trophic position, particularly among higher-order consumers. We then examined the trophic hierarchy of a free-roaming arthropod community, revealing the highest trophic position (5.07) and longest food chain ever reported using CSIA. High accuracy in trophic position estimation brings trophic function into sharper focus, providing greater resolution to the analysis of food webs.

Methyl salicylate attracts natural enemies and reduces populations of soybean aphids (Hemiptera: Aphididae) in soybean agroecosystems.

Methyl salicylate, an herbivore-induced plant volatile, has been shown to attract natural enemies and affect herbivore behavior. In this study, methyl salicylate was examined for its attractiveness to natural enemies of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), and for its direct effects on soybean aphid population growth rates. Methyl salicylate lures were deployed in plots within organic soybean [Glycine max (L.) Merr.] fields. Sticky card traps adjacent to and 1.5 m from the lure measured the relative abundance of natural enemies, and soybean aphid populations were monitored within treated and untreated plots. In addition, exclusion cage studies were conducted to determine methyl salicylate's effect on soybean aphid population growth rates in the absence of natural enemies. Significantly greater numbers of syrphid flies (Diptera: Syrphidae) and green lacewings (Neuroptera: Chrysopidae) were caught on traps adjacent to the methyl salicylate lure, but no differences in abundance were found at traps 1.5 m from the lure. Furthermore, abundance of soybean aphids was significantly lower in methyl salicylate-treated plots. In exclusion cage studies, soybean aphid numbers were significantly reduced on treated soybean plants when all plants were open to natural enemies. When plants were caged, however, soybean aphid numbers and population growth rates did not differ between treated and untreated plants suggesting no effect of methyl salicylate on soybean aphid reproduction and implicating the role of natural enemies in depressing aphid populations. Although aphid populations were reduced locally around methyl salicylate lures, larger scale studies are needed to assess the technology at the whole-field scale.

Effect of soil potassium availability on soybean aphid (Hemiptera: Aphididae) population dynamics and soybean yield.

Studies were conducted to examine the effect of potassium (K) on soybean aphid, Aphis glycines Matsumura, population growth. A laboratory feeding assay examined the effect of K-deficient foliage on life table parameters of soybean aphids, and field experiments were designed to determine the effect of three soil K treatment levels on aphid populations and their impact on soybean yields. The feeding assay found that life table parameters differed between aphids feeding on the K-deficient and nondeficient soybean leaves. Soybean aphids in the K-deficient treatment exhibited significantly greater intrinsic rate of increase (r(m)), finite rate of increase (lambda), and net reproductive rate (Ro) relative to aphids feeding on nondeficient leaves. No significant difference was observed in mean generation time (T) between the two treatments. However, the field experiment repeated over 2 yr showed no effect of K on soybean aphid populations. Soybean aphid populations were high in unsprayed plots and feeding resulted in significant yield losses in 2002 at all three K treatment levels: when averaged across 2001 and 2002, unsprayed treatments yielded 22, 18, and 19.5% less than the sprayed plots in the low, medium, and high K treatments, respectively. No significant interaction was observed between aphid abundance and K level on soybean yields in either year. This study therefore suggests that although aphids can perform better on K-deficient plants, aphid abundance in the field may be dependent on additional factors, such as dispersal, that may affect final densities within plots.

Determining the optimal timing of foliar insecticide applications for control of soybean aphid (Hemiptera: Aphididae) on soybean.

Field experiments were performed over 3 yr to examine the impact of insecticide application timing to control soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), populations and to prevent soybean yield losses. Experiments were conducted in early and late-planted soybean, Glycine max (L.) Merr. Insecticide applications were made based on soybean growth stages. In 2001, applications were made at V1, V3, R2, and R3 growth stages; in 2002 and 2003, applications were made at R2, R3, and R4 stages. Additional treatments consisted of an unsprayed control and a multiple spray treatment that received insecticide applications at 7-10-d intervals. Soybean aphid densities were recorded throughout the growing season, and yields were measured. Soybean aphid populations varied considerably across years and planting dates. In general, late-planted soybean exhibited higher aphid pressure than early planted soybean, and experiments in 2002 had lower aphid numbers than those in 2001 and 2003. The multiple spray treatment significantly increased yield over the control in four of the six experiments, the exceptions being 2002 late planted and 2003 early planted. This suggests that soybean aphid populations were not large enough to cause yield losses in these two experiments. The R3 spray treatment increased yield in three of the six experiments (2001 late planting, 2002 early planting, and 2003 late planting), the R2 spray treatment increased yield in two of six experiments (2001 and 2003 late plantings), and the V1 application increased yield over the control in the 2001 late-planted experiment. Results suggest that when aphid populations are high insecticide applications made at R2 and R3 plant stages are most effective in preventing yield loss.

The attachment and stylostome of Trombidium newelli (Acari: Trombidiidae), an ectoparasitic mite on adults of alfalfa weevil, Hypera postica (Coleoptera: Curculionidae).

Larvae of the mite Trombidium newelli Welbourn and Flessel are ectoparasitic on adult alfalfa weevil, Hypera postica (Gyllenhall), an insect pest of alfalfa. The mite larvae are found under the elytra, attached to the dorsal surface of weevils' abdomen. T. newelli larvae use their chelicerae to penetrate the pliable and weakly sclerotized areas of the host's integument, and to hold on firmly to the host. The attachment sites associated with larval mites of different degrees of engorgement were examined using both light and scanning electron microscopy. The ventral inside surface of parasitized host tergites revealed a characteristic injury as spots varying in color from white-yellow to golden-brown depending on the engorged size of the mite. In addition, spots associated with fully engorged mite larvae showed an aggressive dendritically radiating mass expanding outward from the site of cheliceral penetration. This mass, known as feeding tube or stylostome was branched, with each branch ending in a cluster of closed bulbs. Stylostomes appear to exist independently in the host's tissue. Stylostome mass corresponded with the engorgement state of the mite, increasing as the mite larva increased in size. The possible nature and mechanism of stylostome formation is discussed.