1) Determine the distribution and severity of soybean gall midge across the north-central region.
The goal of this study is to document the current distribution and relative damage of soybean gall midge in soybean across the north-central region. In 2018, researchers in Nebraska, South Dakota, Iowa, and Minnesota briefly documented the distribution of soybean gall midge in the four-state where significant damage occurred. Due to the rapid development of soybean gall midge as a significant pest of soybean, the distribution and damage of this pest were not documented using consistent methods. In addition, documentation of infested counties was primarily based on calls from stakeholders with only limited opportunities for systematic surveying of fields.
Sampling protocols for soybean gall midge have been divided into two categories depending on the historical occurrence of gall midge in an area. In states where soybean gall midge has not been found, researchers will actively engage stakeholders at field day and extension events to alert them about the pest. Postcards will be developed and distributed at these events to increase the likelihood of detection. Each state will utilize resources such as the department of agriculture, extension educators, and IPM scouts when it is applicable to check fields. With the exception of field reports, sampling soybean fields will begin at the R3 or early pod development and continue through R7 or beginning maturity. Conducting this survey during the mid- to late-reproductive stages of soybean will increase the likelihood of detection.
Researchers will target soybean fields adjacent to dense vegetation such as trees, shrubs or uncut grass. To assess a field, plants will be checked every 100 feet along a field edge (Fig. 6). Any plants along this transect showing early signs of death or wilting will be assessed for the presence of soybean gall midge larvae. Infestation by gall midge will be assessed on an individual plant by peeling back the outer tissue of darkened areas near the base of the plant. If larvae are found, they will be collected and stored in alcohol for species identification.
Areas with extensive damage and historical presence of soybean gall midge will be assessed by randomly selecting a soybean field every 5-10 miles on a transect through a county. In each field, samples will be taken at 1, 10, 20, 50, 100, and 400 feet from the field edge (Fig. 7). At each of these locations, 10 consecutive plants will be broken off at
ground level to determine if plants are infested. Each location will be scored as a proportion of plants infested at that location (6/10). For all sampling sites, the stage of development of soybean plants at the time of the sampling will be recorded. Documenting the distribution is critical for making stakeholders aware of the issue, directing future research efforts, and determining the ability of soybean gall midge to spread to other areas of the soybean growing region.
2) Determine the emergence timing and source of adult soybean gall midge.
Adult soybean gall midge emergence will be monitored using corn rootworm emergence cages. To determine the source for adult emergence, cages will be placed in soybean fields with signs of damage from the previous year, ditches adjacent to damaged fields (brome grass and other vegetation), and in the field being planted to soybean (Fig. 8). Monitoring will begin on April 1st and continue until adult populations cease to emerge. All current funding sources of adult soybean gall midge emergence data will end by the fall of 2019. This monitoring effort is the foundation for current management practices until other strategies can be established. A minimum of three cages will be used per site. Cages will be monitored every 2-4 days and adults will be collected and stored. After initial adult emergence, cages will be placed over infested soybean plants to determine the timing of emergence of the second generation. This coordinated multi-state effort will lay the framework for developing a model to estimate the timing of emergence for subsequent years. In the spring of 2019, a total of 24 emergence monitoring sites were established across Nebraska, Iowa, South Dakota, and Minnesota. With limited knowledge on soybean gall midge, farmers with significant historical damage can apply an insecticide as soon as possible following adult emergence. To facilitate communication on emergence, a rapid communication system was established through Blackboard connect to notify registered stakeholders via an automated phone call, text message, and email. As of October 31st, over 300 users have registered for the alert system. When an alert for the emergence of soybean gall adults is sent, stakeholders will be directed to the soybeangallmidge.org to get more information on the location to increase the potential efficacy of insecticide applications against the adult stage of soybean gall midge. These short-term tactics and notification system will greatly enhance control tactics against soybean gall midge and reduce the likelihood of excessive spraying at times when adults are not present or moving.
3) Screen soybean varieties for resistance or tolerance to soybean gall midge.
A total of 750 soybean lines representing the genetic diversity of the USDA Soybean Germplasm Collection in Maturity Groups I, II, III, and IV will be used for screening for plant resistance or tolerance to soybean gall midge. The soybean lines are landraces and improved cultivars collected from all areas of the world where soybeans originated or are grown. The diverse panel of lines already has DNA genotype information available in the 50K SNP data, and the selected lines are the same set used in current and previous NCSRP-funded research for the projects “Identifying high-yield genotypes in the USDA Soybean Germplasm Collection” (G. Graef et al., 2014-2016) and “Increasing the rate of genetic gain for yield in soybean breeding populations.” (L. McHale et al. 2016-2019). All 750 lines are being grown in a seed increase during 2019 to prepare for this experiment. Brief background and rationale for use of this set of lines is as follows: (1) About 90% of the current commercial soybean gene pool in the US is based on fewer than 17 soybean accessions; (2) There are nearly 20,000 unique soybean accessions in the USDA Soybean Germplasm Collection; (3) The diverse genetic types that exist in the collection have been a source of resistance genes for important soybean diseases and insect pests like phytophthora root rot, brown stem rot, soybean cyst nematode, Asian soybean aphid, and others, and may also possess genes for resistance to soybean gall midge; (4) The recent NCSRP-sponsored research with this set of 750 soybean lines used different scientific sampling methods to select diverse soybean genotypes from the collection, and we have shown that this set of lines represents the genetic diversity in the collection, particularly for maturity groups I, II, III, and IV.
For this study, the lines will be planted in three infested locations across the region: one field each in Nebraska, South Dakota and Iowa. The lines will be planted in single-row plots 5’ long, replicated four times in an incomplete block design. Locations for the variety trial will be next to areas of significant damage from gall midge in 2019. These sites will also coincide with locations that are monitored for adult soybean gall midge emergence as described in Objective 2 to evaluate whether or not a significant population was present. A susceptible commercial soybean line will be interspersed through the trial to determine if adequate soybean gall midge pressure occurred in the trial Each line will be evaluated for the presence of soybean gall midge damage and/or larvae approximately three to four weeks after adult emergence. Any soybean lines exhibiting damage across reps and locations will be considered susceptible and will be removed from the second year of testing. If sufficient variation in plant reaction to soybean gall midge is observed among the 750 lines evaluated in Year 1, we will be able to conduct a genome-wide association analysis using the insect reaction data and the genotype data of each line to identify genomic regions that are associated with that reaction (i.e., resistance). The same process will be repeated in 2021 with the lines that showed no signs of damage, or some kind of limited damage or resistance the previous year. Testing in the greenhouse on a subset of lines will also begin in 2020 with a minimum of three replicates per line. A susceptible commercial check variety will be run at the same time as experimental lines to determine host susceptibility. Soybean lines advancing to year three would be planted at all three locations to evaluate for damage and determine each line's yield potential. To determine yield, plot size will be increased to four rows wide, and all data collected on the center two rows of a bordered plot.
4) Disseminate information to stakeholders on current information and management for soybean gall midge.
Sixteen of the Co-PIs have extension appointments and are well suited for communicating details about soybean gall midge biology and management recommendations. During Years 1 and 2, we will create short summaries (e.g., 2-4 page publications) from our multi-state objectives 1-3 and disseminate findings to stakeholders. During Year 3, we will create a pocket-sized field guide that includes data generated from this project and other companion projects. Guides will be distributed to all participating Co-PI’s. Expected audiences include farmers, crop consultants, agribusiness, industry, and other extension personnel.