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A NEW PEST IN SOYBEANS

Soybean gall midge is a new pest in soybeans, first documented in Nebraska in 2011 and South Dakota in 2015. In 2018, damage was documented in 66 counties in four states (Nebraska, Iowa, South Dakota, and Minnesota) with that number increasing by 19 additional counties so far in 2019 including the northwest corner of Missouri. It was not until the fall of 2018 that the species was identified. According to McMechan, Hunt, and Wright (2018), the species is Resseliella maxima Gagné. This is one of 15 midge species known in the United States and the first to affect soybeans. Gall midge belongs to the same family as Hessian fly and wheat midge, which are pests in wheat (McMechan, Hunt and Wright, 2019).

GALL MIDGE INFECTIONS

Initial gall midge infections typically occur in June with multiple generations over the summer. It is still not known exactly how many generations can occur in one year. The midge life cycle is complete metamorphosis (egg, larva, pupa, and adult). The larvae are the stage that does the damage. The midge overwinters as a pupa. In traps set in eastern Nebraska, adults emerged over a relatively long period (McMechan, Hunt and Wright, 2019). The adults are weak flyers, and therefore they primarily impact the field edges adjacent to fields that were infected the prior year. The adults lay their eggs near the base of the plant in the soil or on the stem. The eggs hatch and the larvae then enter the soybean stem.

Gall Midge Larvae in the stem of a soybean plant

Adult Midge

GALL MIDGE IMPACT

Once in the stem, the larvae feed under the epidermis reducing movement of water and nutrients in the xylem and phloem causing the plant to wilt and die. The name gall comes from the formation of an enlarged area of the stem where the larvae are feeding. The plants also become brittle where the feeding occurs and can break easily. This damaged area of the stem can also be an entry point for plant diseases that also can lead to plant death. Yield impacts come from premature plant death and harvestability-related losses. It is common to see random wilted or dead plants on the outside rows around infested fields. The greatest plant death and yield impact is usually along the edges of the fields initially, but subsequent generations move further into the field leading to more of the field showing symptoms. The worst damage is usually in the outside 50-100 feet of the field.

WHAT CAN BE DONE ONCE GALL MIDGE IS PRESENT:

Researchers at universities and other organizations like the Iowa Soybean Association have studies to determine what types of control measures may reduce the impact. Each control strategy comes with specific challenges.

The soybean gall midge is a pest that likely will continue to spread in the Midwest and could become a very important economic pest in soybean production. With so little known currently about the pest, it will take time to determine effective control measures, but work being done by universities in the states affected will hopefully lead to best management practices in the near future.

Works Cited:
McMechan, J., Hunt, T. and Wright, R. (2019). Mid-Season Update on Soybean Gall Midge. [online] CropWatch. Available at: https://cropwatch.unl.edu/2019/mid-season-update-soybean-gall-midge [Accessed 23 Sep. 2019].
McMechan, J., Hunt, T. and Wright, R. (2018). Soybean Gall Midge: Adult Stage Identified. [online] CropWatch. Available at: https://cropwatch.unl.edu/2018/soybean-gall-midge-adult-stage-identified [Accessed 23 Sep. 2019].

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