Insects Causing Problems for Alfalfa

By Bruce Anderson
Extension Forage Specialist

Worms and bugs are causing growth problems for alfalfa in all corners of our region. From alfalfa weevil larvae and adults to potato leafhoppers to armyworms, cutworms, and alfalfa caterpillars to all sorts of aphids, somebody, somewhere has had enough of each of these insects feeding on their alfalfa to hurt new seedlings or regrowth. If you’ve been lucky enough to avoid these problems so far, don’t assume you are safe for this year. Large numbers of butterflies and moths are flying around most fields, so another generation of many insects could occur any time.

I’d love to generalize and tell you exactly how many insects per square foot or sweep of a net is needed to economically justify a control treatment, but each insect is different. Thresholds can range from one spotted alfalfa aphid per seedling to 100 pea aphids on 20-inch alfalfa. Or from one or two armyworms per square foot in new stands to at least 10 alfalfa caterpillars per sweep in established stands.

What we use for control also varies. Simply cutting the alfalfa often works for many soft-bodied insects. Control using natural organisms like the Bt in Dipel and Thuricide will work for some insects and insecticides work for most. Which insecticide to use varies with the insect. For further information check the 2017 Insecticide Table from the Nebraska Extension Resource, Guide for Weed, Disease and Insect Management in Nebraska.

Summary
So my take-home message today is to scout your alfalfa if you’re seeing slow regrowth or weak alfalfa seedlings. Look for insects that might be causing the problem. Dig in the soil and dead litter to find insects hiding during the day. Then identify exactly what the insect is, taking it to a nearby extension office if necessary. Remember, many of the insects you’ll see will be beneficial or unimportant and not require treatment.

Once the insect is correctly identified, an appropriate treatment can be selected to protect your alfalfa.

Source: unl.edu