Solving A Midge of a Problem
Young soybean plants infested with soybean gall midge larvae wilt and die. (ARS photo)
Soybean is the second most planted crop in the United States, with an estimated agricultural impact of over $120 billion. Soybean is grown across the country, but the bulk of this crop is grown in the Midwest, where fertile soils and warm temperatures promote positive yields. Unfortunately, soybean isn’t the only thing that enjoys this weather — pests do as well.
Enter the Soybean Gall Midge (SGM), a new species that was identified as recently as 2018 and has feasted on soybean plants. The adult, a midge (a small fly), lays eggs in cracks that naturally form in stems of soybean plants, or cracks that develop when plants are damaged by weather events like hail or wind. The eggs induce the formation of a simple gall (dark, swollen part of stem), and the larvae feed and develop between living and dead plant tissue in the gall, reducing the uptake of water and nutrients by the plant and causing plants to wilt, abort pod development, and eventually die.
"Oftentimes, infestations go unnoticed until symptoms occur," said Adrian Pekarcik, a research entomologist at ARS’s Integrated Cropping Systems Research Unit in Brookings, SD. "It is also difficult to predict when and where localized infestations will occur."
Chemical insecticides have been evaluated but are largely ineffective against SGM. As a result, alternative management tools are vital for suppressing this hungry pest, which has spread across seven states. If SGM continues to feast and migrate unabated, the entire U.S. soybean industry could be at risk.
Soybean gall midge larvae of varying stages (and colors) are exposed after peeling back the stem of an infested plant. (ARS photo)
"Without effective management tools to target and control local populations, SGM will continue to increase in abundance and will have more opportunity to spread further," Pekarcik said. "There is growing interest in using entomopathogens — organisms like nematodes, fungi, and bacteria that cause disease to insects — for insect pest management in field cropping systems, like soybeans."
Entomopathogenic (insect-killing) nematodes are parasites of insects that naturally occur in agricultural soils. Insect-killing nematodes have been shown to infect and kill larvae and pupae of other gall midge species in laboratory and field trials and are able to move inside plant galls containing larvae of another pest. SGM spends a portion of its life cycle in the soil: older larvae move into the soil to pupate — or mature — and also survive winter, where they may naturally encounter insect-killing nematodes.
Pekarcik and his colleagues evaluated the susceptibility of SGM larvae to four insect-killing nematode species. Researchers collected entire soybean plants showing symptoms of SGM infestation, stored them in a dark cold room, and collected larvae from these plants. Researchers then introduced four commercially available nematode species suspended in water and evaluated the SGM larvae for nematode infection for 30 days. The number of dead and alive larvae were counted, and dead larvae were dissected to confirm the presence of nematodes.
Soybean gall midge adults are long legged tiny flies about ¼ inch long. (ARS photo)
Researchers confirmed that all four nematode species could infect and kill SGM larvae in the lab. Rates of SGM larvae infection were generally higher at higher nematode application rates. At the highest nematode application rate, more than 10% of SGM larvae became infected and died. This type of pest management was more effective than the only other known biological control agent, a parasitoid wasp, which was reported to kill only 1% of SGM larvae. Nematodes may also be more advantageous than insecticides, which cannot penetrate the gall.
"The ultimate goal of this research is to provide alternative management tools other than insecticides to suppress SGM populations and reduce economic losses," Pekarcik said. "Entomopathogens including nematodes have the potential to provide long-term pest suppression of insect pests while minimizing non-target effects, reducing pesticide applications, and increasing yield."
Because nematodes evaluated in this study were continuously maintained in the lab and were not adapted to the local environment, further research has begun to study locally isolated insect-killing nematodes at field sites for soybean gall midge management. — By Todd Silver, ARS’s Office of Communications
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