Managing armyworm in forages with IPM

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Fall armyworms
Fall armyworms are on the prowl in forage fields. (Alabama Cooperative Extension System photo)

By now, many of you have heard about the issues in forages and lawns caused by fall armyworm. Over the last couple of weeks, Jefferson County has seen several fields infested that have extensive feeding damage caused by the caterpillar while neighboring fields have seen hardly any armyworms at all.

With the recent armyworm invasion, now is a good time to review some basic integrated pest management strategies, or IPM, when dealing with insect pests such as armyworm. Steps in the IPM process include setting action thresholds, monitoring and correctly identifying pests and reviewing prevention and control options once a pest is identified at levels above the threshold.

What is IPM

An IPM program focuses on the long-term control of pests using multiple methods to both prevent and control outbreaks. Approaches for IPM include cultural and mechanical controls, such as maintaining soil fertility and forage health and creating an environment unsuitable for the pest in question. It also includes biological controls, like natural enemies that consist of many species of wasps, spiders and birds, and chemical controls, like insecticides. IPM does not emphasize one approach over all others, rather these approaches are to be used in combination of one another.

IPM for armyworm

Back to the fall armyworm as an example. Before we treat a field, we first need to confirm that what is causing the damage is in fact armyworm, and ideally, we would catch an infestation early.

Armyworm larvae have three light-colored thin lines that run down the dorsal side of the insect with two wider dark stripes that run along the sides. What distinguishes fall armyworm, however, from other armyworm species is the Y-shaped suture on the back of the head capsule.

Feeding damage includes complete defoliation of grasses and alfalfa. True armyworm larvae also appear earlier in the growing season, while fall armyworm, as the name suggests, migrates to Ohio in the late summer and early fall. During this time, we may see multiple generations before the moth dies during the winter months. Fall armyworm does not overwinter in Ohio.

OK, so we have identified that we have armyworm, but do we have enough to justify treatment? The typical threshold for treatment is two-three fall armyworm larvae per square foot. If the number of armyworms exceeds the threshold, we can move on to control measures that include insecticides.

What about cultural and mechanical controls for fall armyworm? Harvesting of hay and alfalfa crops should occur as soon as possible to minimize feeding damage. For alfalfa, if the crop is within about a week of harvest, early cuttings can reduce population numbers. It’s important to come back after a cutting through the fall to monitor regrowth for caterpillars. If a field has been severely damaged by fall armyworm, these fields should rest for the remainder of the season after being cut.

Sometimes the “wait and see” method works just fine. Keep in mind natural enemies such as tachinid flies, parasitoid wasps, beetles, earwigs, predaceous insects, entomopathogenic bacteria and certain fungal pathogens, among many others, are known to attack these insects. Birds also love them. You may notice a general decline below threshold levels after finding the younger larvae.

Results vary

The effects of natural enemies on armyworm populations may be inconsistent depending on population size and environmental conditions. Insecticide applications may be warranted if you are finding large numbers of the larger caterpillars. Insecticide applications are most effective when larvae are 1/2 inch in length.

When using insecticides effectively, timing is of the greatest importance – whether timing refers to the stage of development for the insect or how close harvest is.

In the case of armyworms, caterpillars larger than 3/4 inch may be more difficult to kill with insecticides, and some insecticides have preharvest intervals (PHI). Many products have anywhere between a 0-14 day PHI for hay and pasture. Remember to calibrate your sprayer prior to making applications and follow all label directions with any pesticide application.

A listing of insecticide options for pasture and hayfields can be found in the Michigan State/Ohio State Field Crops Insect Pest Management Guide in the forages chapter (https://go.osu.edu/2021fieldinsectmanagement) or contact your local Extension office for information on management options for pests in the pasture or field.

There are also many great resources available to learn more about this pest, including an article in a recent issue of the CORN newsletter, available at https://go.osu.edu/2021fallarmyworm.

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Erika Lyon is the OSU Extension educator for agriculture and natural resources in Jefferson and Harrison counties. She can be reached at lyon.194@osu.edu or at 740-264-2212, x.203.

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