Previous KSC support has allowed identification of resistance to soybean stem borer in soybean PI165673 (Niide et al. 2012), inherited as a quantitative (multiple gene) trait, making development of molecular markers to track these genes more difficult. Nevertheless, solid, steady progress has been made to obtain genes from stem borer larvae that can be silenced and expressed in soybean plants as a stem borer management tool that kills larvae. Lina Aguirre Rojas, PhD student, has established a soybean stem borer laboratory colony and used it to show that larvae fed artificial diet covered with ds(double-stranded) RNA to silence the expression of larval cuticle genes, causing delayed larvae development, and that larvae injected with these genes become deformed adults. Most recently, Ms. Aguirre Rojas has shown major differences in the types of genes expressed by larvae fed soybean compared to those fed sunflower or ragweed. Experiments are in progress to determine if silencing any of these genes will delay larval development or kill adult beetles. Previous support has also provided field data to show that stink bugs are aggregated, but extent of aggregation varies by species, life stage and location in a field. Sequential sampling plans were developed for each combination of species, life stage and location, and a more practical, combined sequential sampling plan was developed for growers to make management decisions. We also evaluated patterns of soybean stem borer aggregation however, there are no consistent or predictable aggregation patterns in the soybean fields studied. Therefore, site specific management of adults is not a suitable strategy for reducing harvest lodging later in the season. However, larval patterns from a previous soybean crop do explain adult infestations the following year in fields not rotated out of soybean. Given this result, we would expect to see issues arise under continuous soybean scenarios, but further studies are needed.

Update:
Kansas Soybean Commission
End of Project Final Report
April 15, 2020

Principal Investigators and Units: C. M. Smith, Dept. of Entomology, (785)-532-4700, Brian McCornack, Dept. of Entomology (785 532-4729, Jeff Whitworth, Dept. of Entomology, (785) 532-5656.

Project Title: Development of Genetic, Chemical and Population-Based Tactics to Manage Key Kansas Soybean Insect Pests

Amount of Funding: $60,000

Final Report: March 1, 2019 – February 28, 2020

Objective 1. Create soybean plants resistant to soybean stem borer by inserting borer RNA into soybean plants to interfere (RNAi) with genes necessary for borer survival.
Postdoctoral fellow Dr. Lina Aguirre made significant progress during the past funding year, documenting repeatedly that young borer larvae die after eating artificial diet containing a silenced version of the Lac2 cuticle gene. Aguirre is currently awaiting the results of a final set of experiments testing the effects of silenced Lac2 on older larvae, which will be available in May 2020. Aguirre has also discovered that larvae fed soybean turn on several genes that may help them overcome chemical defenses in soybean stems and is preparing a manuscript involving these data. She is now testing silenced versions of these genes on borer survival.

Objective 2. Improve insecticide efficacy by using host plant developmental stages and other environmental cues or conditions to adjust timing of application.
Stephen Losey, current PhD candidate on the project, examined the effects of sugar type and mate availability on the potential fecundity and longevity of soybean podworms. To date, results from laboratory experiments show a significant increase in fecundity (i.e., number of eggs deposited) when female moths were allowed to feed on sugar water versus just water. Statistically, there were no differences between types of sugar used. Additional experiments were conducted to determine how the timing of mating events affects the potential fecundity and longevity of female moths. This was accomplished by adding a male on day four of the experiment for the life of the experiment, which was not statistically different from the control (female only). Sugar statistically affected both fecundity and longevity of female moths, while mate availability only statistically affected the overall fecundity. If mating disruption within row crops could significantly affect male podworms for 5-6 days, then female podworms would not be able to produce nearly as many eggs. After completing the final block of the fecundity and longevity experiment, we use flight mills to examine differences in dispersal distances between the sexes and how flight affects the reproductive output of migrating moths. Our first question addressed whether there was a difference between the sexes in total distance flown. We followed published methods and moths that had not been fed and were within 2-3 days of eclosion. The moths were flown until death. There were no statistically significant differences between the sexes. Future experiments will test how flight affects fecundity and longevity of female soybean podworms, leading to better predictive models that focus on large, migratory flights and successive establishment of this species in soybean across Kansas.

We also participated in a large-scale defoliation survey this past summer. In 2019, 32 fields in 6 states were sampled for insect defoliation a total of 47 times following a common protocol. Most fields had an overall average of less than 2% defoliation; Kansas had the highest levels of defoliation and ranged from 2-30%. Most of these values are well below the established economic thresholds for defoliating insects in soybean, and none of the sampled fields would have lost yield due to insect defoliation. Ground-level imagery, as well as a limited amount of aerial drone imagery, were collected from these fields to develop sampling aids. This study will be repeated in summer of 2020.

Objective 3: Expand web pages and other educational materials associated with soybean insects.
During the past funding year, co-PI Whitworth presented information on soybean pest management in nine different areas of Kansas to interested soybean stakeholders, including growers, consultants, and aerial applicators. Approximately 500 total individuals attended these presentations. During the 2019 growing season, co-PI Whitworth determined the effect of different insecticides applied to soybean plots at plant growth stages R1 and R3 to develop improved timing of insecticide application for stem borer management. Stem borer larval infestation and yield results will soon be available in the 2020 Soybean Insect Management publication (https://bookstore.ksre.ksu.edu/pubs/Mf743.pdf).

Kansas Soybean Commission funding has enabled K-State researchers to make significant short- and long-term contributions to reducing soybean pest damage in Kansas. Transgenic soybean plants resistant to soybean stem borer are being created as a primary borer management tactic for future production. Completing the development of this technique will require several years of additional testing and regulatory approval. Basic research also continues to determine if disruption of soybean podworm mating can reduce podworm-related damage; and to improve predictive models of podworm migratory flights and establishment in soybean fields across Kansas. Experiments continue each year to improve the efficacy of insecticides used to manage various insect pests and these results are presented annually to interested soybean stakeholders, including growers, consultants, and aerial applicators; and used to update the Soybean Insect Management publication (https://bookstore.ksre.ksu.edu/pubs/Mf743.pdf).