Research Highlights

Threecornered Alfalfa Hoppers Becoming Problem Soybean Pests

By Laura Temple

When new insects become a problem in soybeans, farmers expect to hear about invasive pests. That is not the case with threecornered alfalfa hopper, a recent and spreading problem in Virginia and other states.

“This is a case of native insects gone bad,” says Dr. Sally Taylor, associate professor and entomologist for Virginia Tech. “Threecornered alfalfa hoppers are native to North America, found in most soybean-growing states. For decades, they have been present in Virginia fields without a detectable problem. But in the past few years, they’ve caused economic damage in Virginia soybeans, and the affected area is growing.”

Taylor received calls at harvest in 2016 about lodged soybeans, and she recognized the damage as early season feeding of threecornered alfalfa hoppers. Since then, the number of calls and impacted counties has increased.

In 2021, the Virginia Soybean Board began funding research led by Taylor to understand why this minor pest is becoming more significant and to find economic management options for soybean farmers.

Changes in Feeding Patterns

These small, green bugs have piercing sucking mouthparts. Taylor explains that they feed on tender shoots, petioles and peduncles by piercing the surface and injecting an enzyme that liquifies the plant so they can ingest it. Their feeding creates a scar that can girdle a stem, making it susceptible to breaking off later in the season.

“Threecornered alfalfa hoppers feed on a wide variety of plants and weeds,” she says. “They move as they need food.”

According to Taylor, the insects are most likely to migrate into soybeans fields after burndown applications, wheat harvest or cutting hay. Historically, feeding in soybeans has been minimal, and plants compensated for any damage.

“Soybeans are most vulnerable to this pest from emergence through the V6 growth stage, until they are about 8 inches tall,” she explains. “After that, soybean stems toughen up so the bugs can’t pierce it. They can still feed near growing points, but soybeans compensate so yield is not lost.”

Based on observation to date, Taylor attributes earlier infestations of threecornered alfalfa hoppers in soybeans to a variety of factors.

“Winters have not been as cold, allowing the bugs to breed earlier,” she says. “Then, spring conditions have been hot and dry, with more rain falling in fewer events. Soybeans may sit at early growth stages for longer periods of time, growing more slowly. They are no longer winning the race to grow past vulnerable stages before the bugs move into these fields. My colleagues in Kentucky are seeing similar trends.”

Exploring Management Options

While Talyor is confident threecornered alfalfa hoppers can be ignored later in the season, she has documented economic damage from early-season feeding, when most farmers aren’t scouting soybeans unless they are looking for slugs. However, the small, green bugs are hard to see in young soybeans, and the plants are too small for a sweep net.

“In our studies, we relied on yellow sticky cards attached to wooden stakes to scout and monitor insect populations,” she says. “We found that the bugs constantly move into soybean fields. That means timing protection can be challenging.”

The yellow sticky card scouting method helps identify treatment thresholds and efficacy of treatments and management practices.

In 2021, Taylor’s team found that increasing soybean planting populations could help protect soybean yield, but if the bugs don’t appear in the field, that isn’t economical. They also learned that seed treatment insecticides don’t provide effective threecornered alfalfa hopper protection in the heavy clay soils of Virginia, though she has heard that they can be helpful in other soil types. They also found that insecticide treatments can help preserve plant stands, but yield results were inconsistent.

“The benefits of spraying are apparent, but both timing and cost can be challenges,” Taylor explains. “During 2022, our study will look at timing aerial applications of insecticides in soybeans with corn herbicide applications for convenience and cost-effectiveness.”

Her team also plans to identify factors that lead to a high risk of threecornered alfalfa hopper damage. The surrounding crops and landscape may help predict if or when the bugs are likely to move into soybean fields.

In high-risk fields, determining ideal timing for control will also be part of her studies. Affordable insecticides have short residual periods, and products with longer residual control cost too much.

“Bugs change over time, and research like this helps us keep up with them,” Taylor says. “My hope is that we can figure out cultural practices to manage threecornered alfalfa hoppers in soybeans without insecticides. In this area, we prefer to raise soybeans without insecticides, saving those chemistries for other, more vulnerable crops in our rotations.”

Published: May 9, 2022