Kelley J. Tilmon

Performance and prospects of Rag genes for management of soybean aphid

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean‐growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field‐testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north‐central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid‐resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid‐resistant soybean, and thus a strategy for optimal deployment of aphid‐resistant soybean is needed to ensure sustainability of this technology.

Management Recommendations for Soybean Aphid (Hemiptera: Aphididae) in the United States

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is the primary pest of soybean, Glycine max L., in the north central region. After more than a decade of research and extension efforts to manage this pest, several consensus management recommendations have been developed for sustainable and profitable soybean production. A summary of integrated pest management (IPM) tactics for soybean aphid are discussed, including cultural, genetic, economic, and chemical controls. To date, sampling and timely foliar insecticides are routinely recommended to protect yield and delay genetic resistance to insecticides. Host plant resistance is a new tool that can regulate populations and reduce the reliance of insecticides to control soybean aphid. A combination of these management tools also will reduce overall production costs and minimize negative environmental effects such as human exposure, and mortality of beneficial insects and other animals.

Direct effects of soybean varietal selection and Aphis glycines-resistant soybeans on natural enemies

The direct effects of three soybean parentages, each represented by an Aphis glycines-resistant and susceptible isoline, on the fitness and performance of two key predators (Orius insidiosus and Harmonia axyridis) were evaluated in the laboratory. Predators were reared from hatch through adulthood in Petri dishes with cut trifoliolate leaves of the designated soybean variety, using eggs of Ephestia kuehniella as surrogate prey to eliminate prey-mediated effects of the host plant. Preimaginal survival and development, sex ratio, adult longevity, fecundity, and size were compared among treatments and a no-plant control. An additional experiment compared life-history parameters of predators caged with soybean versus Ipomoea hederacea (ivyleaf morning glory). Aphid resistance reduced the adult longevity of H. axyridis, but O. insidiosus was unaffected by resistance traits. However, adult O. insidiosus lived longer on soybeans with Group C base genetics than the other soybean varieties. Other parameters were not affected by soybean base genetics or resistance, but both predators generally performed worse on soybean than on I. hederacea or no-plant controls. The results suggest that soybean varietal selection, particularly with respect to A. glycines-resistance, may directly affect biological control agents. Also, implications of the generally poor suitability of soybean for natural enemies are discussed within the context of current crop production practices.

One gene versus two: A regional study on the efficacy of single gene versus pyramided resistance for soybean aphid management

ABSTRACT The soybean aphid (Aphis glycines Matsumura) is a threat to soybean production in the Midwestern United States. Varieties containing the Rag1 soybean aphid resistance gene have been released with limited success in reducing aphid populations. Furthermore, virulent biotypes occur within North America and challenge the durability of single-gene resistance. Pyramiding resistance genes has the potential to improve aphid population suppression and increase resistance gene durability. Our goal was to determine if a pyramid could provide improved aphid population suppression across a wide range of environments.Weconducted a small-plot field experiment across seven states and three years. We compared soybean near-isolines for the Rag1 or Rag2 gene, and a pyramid line containing both genes for their ability to decrease aphid pressure and protect yield compared with a susceptible line. These lines were evaluated both with and without a neonicitinoid seed treatment. All aphid-resistant lines significantly decreased aphid pressure at all locations but one. The pyramid line experienced lower aphid pressure than both single-gene lines at eight of 23 location-years. Soybean aphids significantly reduced soybean yield for the susceptible line by 14% and for both single-gene lines by 5%; however, no significant yield decrease was observed for the pyramid line. The neonicitinoid seed treatment reduced plant exposure to aphids across all soybean lines, but did not provide significant yield protection for any of the lines. These results demonstrate that pyramiding resistance genes can provide sufficient and consistent yield protection from soybean aphid in North America.

Spatial Distribution of Aphis glycines (Hemiptera: Aphididae): A Summary of the Suction Trap Network

ABSTRACT The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an economically important pest of soybean, Glycine max (L.) Merrill, in the United States. Phenological information of A. glycines is limited; specifically, little is known about factors guiding migrating aphids and potential impacts of long distance flights on local population dynamics. Increasing our understanding of A. glycines population dynamics may improve predictions of A. glycines outbreaks and improve management efforts. In 2005 a suction trap network was established in seven Midwest states to monitor the occurrence of alates. By 2006, this network expanded to 10 states and consisted of 42 traps. The goal of the STN was to monitor movement of A. glycines from their overwintering host Rhamnus spp. to soybean in spring, movement among soybean fields during summer, and emigration from soybean to Rhamnus in fall. The objective of this study was to infer movement patterns of A. glycines on a regional scale based on trap captures, and determine the suitability of certain statistical methods for future analyses. Overall, alates were not commonly collected in suction traps until June. The most alates were collected during a 3-wk period in the summer (late July to mid-August), followed by the fall, with a peak capture period during the last 2 wk of September. Alate captures were positively correlated with latitude, a pattern consistent with the distribution of Rhamnus in the United States, suggesting that more southern regions are infested by immigrants from the north.

Variation in the Responses of Spotted Alfalfa Aphids, Therioaphis maculata Buckton (Homoptera: Aphididae) and Pea Aphids, Acythosiphon pisum Harris (Homoptera: Aphididae) to Drought Conditions in Alfalfa (Medicago sativa L., Fabaceae)

Spotted alfalfa aphids (Therioaphis maculata, Buckton) are a major pest of alfalfa in the arid western United States, yet are rarely encountered in Wisconsin. In Wisconsin, where severe drought conditions are rare, pea aphids (Acyrthosiphon pisum, Harris) are the predominant aphid herbivore in alfalfa. An unusually dry growing season in south-central Wisconsin in 2003 presented us with an opportunity to test two ideas. First, we conducted surveys to determine whether, due to the dry growing season, A. pisum density was lower than normally observed and the density of the more drought-tolerant T. maculata (Mittler and Sylvester, 1961) was greater in these dry areas. Second, we designed an experiment to determine whether these patterns were the result of environmentally mediated competition. We define this as competition between multiple species where the superior competitor is not fixed, but rather determined by the state of a variable environment (in this case, whether the water content of alfalfa determines which aphid species is the superior competitor). This type of competition is common in nature (Huston, 1979).

Laboratory Evaluation of Soybean Plant Introductions for Resistance to Aphis glycines (Hemiptera: Aphididae)1

Abstract  The soybean aphid (SA), Aphis glycines Matsumura, is a major pest of soybean in the north-central United States and south-central Canada. It is controlled primarily with insecticides, but the development of aphid-resistant soybean cultivars may provide an alternative management tactic. The viability of this management tactic depends on a diverse set of resistance sources in order to counter various resistance-breaking biotypes of SA, and the identification of new sources of resistance necessitates additional testing of soybean germplasm. The current study used no-choice tests to identify SA resistance in seven early maturing (maturity group I) soybean plant introductions (PIs) that had been advanced from free-choice screening trials. The tests showed PI 437353 and PI 612759 C had an intermediate level of resistance against avirulent SA, whereas PI 437282, PI 437658, PI 437733, PI 548417, and PI 548530 exhibited no significant resistance. Additional research is needed to determine if the source of resistance in PI 437353 and PI 612759 C is due to novel resistant genes, which would help diversify resistance to SA in soybean. Screening and follow-up tests of additional soybean germplasm is warranted in order to ensure the development of durable, SA-resistant cultivars.