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Insect Management: Managing Worms, Bugs and Slugs in Soybeans Ben Thrash

By Laura Temple

Managing Worms, Bugs and Slugs in Soybeans

As farmers adopt management practices to improve soybean production, insects take advantage of favorable environments and food sources.

“In Arkansas, insect management continues to be a major focus for growers and consultants,” says Dr. Ben Thrash, assistant professor and Extension entomologist, focusing on integrated pest management (IPM) in row crops for the University of Arkansas System Division of Agriculture. “We are addressing various aspects of integrated management of problem pests in soybean production and finding solutions for farmers to effectively and economically protect soybean yield and quality.”

The research behind identifying solutions to insect challenges is being funded by the Arkansas Soybean Promotion Board. Thrash and his team are putting these soy checkoff dollars to work in various practical trials identifying and refining answers to current questions about managing in-season worms, protecting soybeans from potential insect infestations in cover crops, controlling slugs early in the season and more.

“Adoption of reduced tillage, cover crops, early season planting and other practices have caused changes in insect management,” Thrash says. “We want to be sure growers and consultants have the information and tools they need to adjust insect management appropriately.”

Identifying Thresholds and Guidelines for Virus Insecticides
Southern farmers have found great value in the use of a nucleopolyhedrovirus (NPV) strain to control corn earworm, also known as soybean podworm in soybeans or cotton bollworm in cotton. The viral insecticide carries the trade name Heligen, which works a bit differently than traditional insecticides.

“The virus sickens the worms so they stop feeding fairly quickly, but it takes four to six days for them to die,” Thrash explains. “We are used to insecticides that control target pests within hours, so this creates a big learning curve for consultants and farmers.”

Because of how the virus works, he has found that worm density and size should determine how best to control corn earworm in soybeans. Based on research, he recommends using the viral treatment when scouting finds three to five worms in 25 sweeps that are less than ½-inch.

“The virus should be applied earlier than standard insecticide timing,” he says. “It doesn’t work well in high worm density situations, so if there are more or larger corn earworms, we recommend a standard insecticide.”

Viral insecticides are less expensive, and they can be used in any cropping system, including organic production. They also persist in the field longer than traditional insecticides.

“Viruses don’t have true residual, but as worms sicken, the virus reproduces and is present to infect other worms,” Thrash explains. “I’ve seen viruses provide protection from new infestations for a month or more.”

He adds that viral strains are specific to a target pest, so they only control one species, leaving beneficial insects in the field. His team is also currently supporting development of viral insecticides to control soybean looper and fall armyworm.

Protecting Soybeans Following Cover Crops
Cover crops provide many benefits for long-term soil health and sustainability. However, cover crop biomass sometimes attracts insects that can damage the following crop.

“Cover crops don’t always cause insect problems, but it is hard to predict when problems may show up,” Thrash says. “Cereal cover crops can foster soil insects, while legume cover crops may attract cutworms or threecornered alfalfa hoppers. And, it can be hard for insecticides to penetrate cover crop residue in the field, reducing efficacy.”

His team compared use of an insecticide seed treatment in soybeans to manage potential insect problems. Regardless of active ingredient or field conditions, they found that insecticide seed treatments increased soybean yield 1.8 bushels per acre compared to plots without an insecticide seed treatment.

“We saw a benefit with insecticide seed treatments when planting soybeans into both cover crops and fallow ground,” he notes. “That has become our recommendation, especially in cover crop systems.”

Where insect pressure is a significant concern in cover crops, he also recommends timing burn down at least two to three weeks before planting to eliminate insect habitat and food source. Thrash says this reduces the risk of insects bridging to germinating and emerging soybeans.

Studying Economic Slug Control
According to Thrash, slugs have become an increasing problem in Arkansas soybeans.

“We’ve had very wet springs, creating a favorable environment for slugs,” he says. “Many fields are in no-till or minimum-till systems, and slugs like to hide in that crop residue. Tillage can control slugs, but that isn’t an option for some growers.”

Molluscicides, commonly called slug bait, can be very expensive, so Thrash and Dr. Nick Bateman have been investigating ways to reduce the cost of control. They are looking at reduced rates, banding applications over the row, edge-of-field slug control and more. Soy checkoff funding will accelerate the progress of this work in 2022.

“Slug bait can cost as much as $30 per acre at the recommended application rate,” he adds. “We’ve seen some promise with banded applications, and we will continue to explore both rates and timing. And, we will compare the costs of what we learn with the cost of replants to develop sound recommendations.”

Investigating Insect Control Improvements
Thrash constantly looks for additional ways research can help farmers improve insect management. For example, one of his students is currently investigating how water hardness and pH impact insecticides. Current greenhouse research is testing how water quality impacts the length of insecticide residual control.

“We know water hardness impacts herbicide efficacy, but we haven’t looked at similar issues with newer insecticides,” he explains. “We are currently trying to identify and define issues, like the potential for chlorantraniliprole to break down in water with high pH or high hardness. Once we know the challenges, we will look for solutions.”

In 2022, his team will focus on ways to scout lodged soybeans late in the season, when it is difficult to walk fields and swing a sweep net. They will also reevaluate established corn earworm and stinkbug thresholds.

“It is good practice to evaluate insect thresholds every 10 to 15 years because management practices and genetics change,” Thrash says. “We want to be sure farmers control pests to protect yield while still maintaining profitability.”

For more information about this project, visit the National Soybean Checkoff Research Database: here and here.

Insecticide seed treatments improve soybean seedling stand. The top row in the photo was treated, compared to the bottom row, which did not receive an insecticide seed treatment.
Source: Ben Thrash, University of Arkansas Cooperative Extension Service

A corn earworm sick following treatment with NPV stops feeding on soybeans shortly after treatment.
Source: Ben Thrash, University of Arkansas Cooperative Extension Service

Insecticide seed treatments improve soybean seedling stand. The top row in the photo was treated, compared to the bottom row, which did not receive an insecticide seed treatment.
Source: Ben Thrash, University of Arkansas Cooperative Extension Service

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