Mechanisms of Soybean Host-Plant Resistance Against Megacopta cribraria (Hemiptera: Plataspidae).

A number of soybean varieties traditionally bred for resistance to various soybean arthropod pests have been identified as resistant to Megacopta cribraria (F.) (Hemiptera: Plataspidae). However, the mechanisms of host-plant resistance (HPR) in this system are not understood. The goal of this study was to identify the mechanisms of resistance by examining the role of plant volatile organic compounds (VOCs) and free amino acids (FAAs) among 16 soybean varieties. Choice and no-choice cage experiments identified several soybean varieties that demonstrated antixenosis as well as antibiosis. However, resistance varied over time in certain soybean varieties, such as N02-7002 and PI567352B. Mean nymph number from choice experiments had positive correlations with the FAAs asparagine, tryptophan, alanine, phenylanaline, and serine; negative correlation with leucine and threonine. Four plant volatiles, hexanal, 2-pentylfuran, beta-cyclocitral, and cis-9-hexadecenal, were positively correlated with subsequent nymph development, whereas n-hexadecenoic acid was negatively correlated with nymph number only, in adult choice cage experiments. This study contributes to understanding the mechanisms of HPR through associations with plant VOCs and FAAs in relation to M. cribraria development and provides useful knowledge for developing soybean varieties for M. cribraria management.

Soybean Host Plant Resistance to Megacopta cribraria (Hemiptera: Plataspidae) and the Potential Role of Leaf Trichome Density.

Megacopta cribraria (F.) (Hemiptera: Plataspidae) is an invasive pest of soybean that has spread across the southeastern United States since its initial discovery in 2009 in Georgia. Previous studies in the southeastern states have documented both the population dynamics of this pest and host plant resistance (HPR) among soybean varieties, although the specific mechanisms of HPR remain unknown. The objectives of this study were, therefore, to 1) quantify field resistance to M. cribraria in multiple soybean varieties in two states previously affected by severe M. cribraria infestations, North Carolina (NC) and South Carolina (SC); and 2) study the role of soybean trichome density in imparting resistance against M. cribraria. Soybean variety 'Camp' was least attractive to M. cribraria, through time and locations, suggesting consistent resistance. Other varieties showed variable performance among the locations and sampling dates. A significant difference in trichome density was evident. However, there was no correlation between trichome density and M. cribraria infestation. Compared to a previously published study in the same location, when M. cribraria adults emerging from overwintering dispersed into soybeans, in our study only first-generation adults dispersed into soybeans. Considering the current trend of significantly lower M. cribraria infestation rates in North and South Carolina, this pest may be finally succumbing to indigenous natural enemies and should be managed by incorporating integrated pest management tactics, such as HPR, that help conserve natural enemy populations.

Effects of Diamide Insecticides on Predators in Soybean.

Predatory arthropods can be important for preventing insect pests from reaching damaging levels in soybean. However, the predator community can be compromised when pest control strategies include the application of broad-spectrum insecticides. The use of selective insecticides such as diamides could conserve predators while still providing necessary pest control. We evaluated two selective diamide insecticides, chlorantraniliprole and flubendiamide, and a broad-spectrum insecticide, lambda-cyhalothrin in combination with chlorantraniliprole, for impact on predators in soybean. We applied insecticides to field plots and documented predator abundance prior to and up to 3 wk postapplication using sticky card, beat sheet, and sweep net sampling methods. In sweep net samples, total predator abundance in plots treated with the selective insecticides was not significantly different from untreated control plots. For beat sheet samples, there were no significant differences in the abundance of total predators on any day postapplication between the selective diamide insecticides or the untreated control, but abundance decreased after application of lambda-cyhalothrin + chlorantraniliprole and did not recover. For sticky cards, there were no differences in predator abundance among treatments on any day postapplication. Over all, results showed that there were no significant differences in the abundance of total predators, Anthocoridae, Araneae, or Geocoridae after application of flubendiamide or chlorantraniliprole compared with the untreated control for up to 3 wk after application. All insecticides significantly decreased populations of lepidopteran pests compared with the untreated control, but only lambda-cyhalothrin + chlorantraniliprole reduced predatory arthropod abundance.

Impact of Lepidoptera (Crambidae, Noctuidae, and Pyralidae) Pests on Corn Containing Pyramided Bt Traits and a Blended Refuge in the Southern United States.

Blended refuge for transgenic plants expressing Bacillus thuringiensis (Bt) toxins has been approved in the northern United States as a resistance management strategy alternative to a structured refuge. A three-year study (2012-2014) was conducted with 54 trials across nine states in the southern United States to evaluate plant injury from lepidopteran pests of corn and yield in a corn hybrid expressing Cry1F × Cry1Ab × Vip3Aa20 (Pioneer Brand Optimum Leptra) planted as a pure stand and in refuge blends of 5, 10, and 20% in both early and late plantings. Injury by corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae), and fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), was generally proportional to the percentage of non-Bt corn within each refuge blend. Across locations, ear injury in plots with 100% Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) corn ranged from no injury to a maximum of 0.42 cm(2) per ear in Mississippi in 2013. Leaf injury ratings in 100% non-Bt plots in early and late planted trials in 2014 were 86- and 70-fold greater than in 100% Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) plots. Plants in plots with blended refuges had significantly greater leaf injury in 2012 (5, 10, and 20% refuge blends), in the early-planted corn in 2013 (10 and 20% only), and in both early- and late-planted corn in 2014 (20% only) as compared with leaf injury in a pure stand of Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) seen during these years. Corn ears in plots with blended refuges also had significantly greater area of kernels injured in 2012 (5, 10, and 20%), in early- and late-planted corn in 2013 (5, 10, and 20%), and in early (10 and 20% only)- and late-planted corn (5, 10, and 20%) in 2014 as compared with ear injury in a pure stand of Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) seen during these years. Infestations of southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), were also significantly reduced by Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra). Despite these differences in injury, yield averaged across locations varied among refuge blends only in the late-planted trials in 2013, with greater yields in the 0% refuge blend than in the 20% blend; however, when examining yield separately by location, only two of nine locations had higher yields in the 100% Bt plots than in any of the blended refuge plots. As a complement to studying the contribution of blended refuge to delaying resistance, quantifying injury and yield in a range of refuge blends is a necessary step to provide management information on the range of lepidopteran pests that occur in the southern United States.

Host Plant Resistance to Megacopta cribraria (Hemiptera: Plataspidae) in Diverse Soybean Germplasm Maturity Groups V Through VIII.

Initially discovered in Georgia in 2009, the exotic invasive plataspid, Megacopta cribraria (F.), has become a serious pest of soybean ( Glycine max (L.) Merrill). Managing M. cribraria in soybean typically involves the application of broad-spectrum insecticides. Soybean host plant resistance is an attractive alternative approach; however, no commercial soybean cultivars have been identified as resistant. During 2013 and 2014, we compared 40 and 44 soybean genotypes, respectively, for resistance to M. cribraria in a split-plot design under natural insect infestation in small-plot experiments. Soybean genotypes were selected to maximize diversity with respect to maturity group, pubescence type, leaf shape, seed size, nitrogen fixation, drought tolerance, seed protein content, and pest resistance. Megacopta cribraria egg masses, nymphs, and adults were counted during the growing season to identify potentially resistant soybean genotypes. Soybean seed yield was measured in insecticide-protected and unprotected conditions to determine tolerance to M. cribraria feeding. In both years, a range of host plant resistance was observed. The fewest M. cribraria adults and nymphs were found on narrow-leaf, small-seeded cultivars 'N7103' and 'Vance,' as well as the nonnodulating cultivar 'Nitrasoy.' Additionally, N7103 and Vance were among the least susceptible genotypes to M. cribraria oviposition in the field. Most 'Benning' cultivar insect-resistant near-isogenic breeding lines also displayed moderate levels of resistance to M. cribraria . Seed yields of Vance and N7103 were less affected by M. cribraria in 2013 than most other soybean genotypes. These results may be useful to soybean breeders to develop cultivars with resistance to M. cribraria.

Lepidoptera (Crambidae, Noctuidae, and Pyralidae) Injury to Corn Containing Single and Pyramided Bt Traits, and Blended or Block Refuge, in the Southern United States.

Fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae); corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae); southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae); sugarcane borer, Diatraea saccharalis F. (Lepidoptera: Crambidae); and lesser cornstalk borer, Elasmopalpus lignosellus Zeller (Lepidoptera: Pyralidae), are lepidopteran pests of corn, Zea mays L., in the southern United States. Blended refuge for transgenic plants expressing the insecticidal protein derivative from Bacillus thuringiensis (Bt) has recently been approved as an alternative resistance management strategy in the northern United States. We conducted a two-year study with 39 experiments across 12 states in the southern United States to evaluate plant injury from these five species of Lepidoptera to corn expressing Cry1F and Cry1Ab, as both single and pyramided traits, a pyramid of Cry1Ab×Vip3Aa20, and a pyramid of Cry1F×Cry1Ab plus non-Bt in a blended refuge. Leaf injury and kernel damage from corn earworm and fall armyworm, and stalking tunneling by southwestern corn borer, were similar in Cry1F×Cry1Ab plants compared with the Cry1F×Cry1Ab plus non-Bt blended refuge averaged across five-plant clusters. When measured on an individual plant basis, leaf injury, kernel damage, stalk tunneling (southwestern corn borer), and dead or injured plants (lesser cornstalk borer) were greater in the blended non-Bt refuge plants compared to Cry1F×Cry1Ab plants in the non-Bt and pyramided Cry1F×Cry1Ab blended refuge treatment. When non-Bt blended refuge plants were compared to a structured refuge of non-Bt plants, no significant difference was detected in leaf injury, kernel damage, or stalk tunneling (southwestern corn borer). Plant stands in the non-Bt and pyramided Cry1F×Cry1Ab blended refuge treatment had more stalk tunneling from sugarcane borer and plant death from lesser cornstalk borer compared to a pyramided Cry1F×Cry1Ab structured refuge treatment. Hybrid plants containing Cry1F×Cry1Ab within the pyramided Cry1F×Cry1Ab blended refuge treatment had significantly less kernel damage than non-Bt structured refuge treatments. Both single and pyramided Bt traits were effective against southwestern corn borer, sugarcane borer, and lesser cornstalk borer.

Does feeding by Halyomorpha halys (Hemiptera: Pentatomidae) reduce soybean seed quality and yield?

The nonnative brown marmorated stink bug, Halyomorpha halys Stål (Hemiptera: Pentatomidae), has become an abundant pest of mid-Atlantic soybean since its introduction in the mid-1990s. Currently, there is little information indicating how this new pest should be managed in soybean or if economic thresholds developed for native stink bugs should be adjusted. In 2010 and 2011, field cage studies were conducted in Beltsville, MD, and Suffolk, VA, to evaluate H. halys injury to three different soybean reproductive development stages. Cages were infested for 2 wk using densities of zero, one, two, four, or eight stink bugs (fifth instars and adults) per 0.3 row-m. Cage plots were harvested, and subsamples were taken to determine pod losses and seed quality. Feeding injury to soybean caused by H. halys was similar to that of native stink bugs, as evidenced by seed destruction, punctures, and destroyed pods. Densities of four stink bugs per 0.3 row-m resulted in significant seed damage in three of four experiments. The full flowering (R2) soybean development stage was least affected by H. halys feeding. The full pod (R4) and the full seed (R6) stage were similarly sensitive to injury. There was no significant yield loss was associated with stink bug densities at either location, although there were significant differences among stages in two of four experiments. The data do not indicate that threshold densities for H. halys should be different than for the native stink bugs.

First-generation Megacopta cribraria (Hemiptera: Plataspidae) can develop on soybeans.

Megacopta cribraria (F.) (Hemiptera: Plataspidae) was first reported in 2009 near Atlanta, GA. The insect undergoes two generations per year. The first-generation is reported mainly in kudzu during May and June, with the second establishing on both kudzu and soybean during July and August. A greenhouse study was conducted to determine the suitability of two legumes as hosts for first generation M. cribraria. First generation M. cribraria successfully developed on caged potted soybean plants. Conversely, snap beans were not a suitable host under the conditions of this study. A range of 45-50 d was needed to transition from the egg to adult on soybean plants. Although this study was limited to the greenhouse, kudzu may not be an obligate host for the development of first-generation M. cribraria. An important implication of this finding is the establishment for this pest on spring-planted soybean and for the possible expanded geographic range for this pest beyond that of kudzu.