No. 23 Article 2/October 5, 2012

Fall Armyworms Reach Impressive Levels in Some Pastures and Newly Seeded Hayfields

Published online as an Alert on September 21, 2012

I have received scattered reports from producers in southern and central Illinois that fall armyworms have reached impressive levels in some pastured areas and newly seeded hayfields. University of Kentucky entomologists have reported impressive flights of this migratory moth through mid-September.

The fall armyworm is a tropical insect and a common and continuous pest throughout the Gulf Coast states. The moths migrate to northern states in late summer and the early months of fall. Tropical storms often increase the surge of moths northward. As female moths find suitable pastures, they begin to lay eggs on the blades of grasses. The overall length of the life cycle (egg to adult) is temperature dependent and typically takes about 30 to 50 days to complete.

Overwintering of partially grown larvae occurs in Gulf Coast states. The larvae currently feeding in some pastured areas of Illinois will not survive the winter, and frosts will take their toll on this population. In a sense, they are on a dead-end journey this far north; however, their current feeding activity can still lead to significant damage to newly seeded hayfields, pastures, and wheat.

Feeding by fall armyworm larvae typically occurs in the morning, late afternoon, or early evening. Densities of 5 to 7 larvae per square foot can cause significant damage to stands. Larvae that are 0.75 inches long or less are easier to control. Those that are 1.25 to 1.5 inches long are the most injurious and will consume the bulk of the foliage. They are also more difficult to control with insecticides. Producers are encouraged to scout pastures, newly seeded hay, and wheat fields. If fall armyworms are found in damaging levels, consider a rescue treatment, paying careful attention to harvest and grazing restrictions for the chosen insecticide.--Mike Gray

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