Big Jump in Fall Armyworm Moth Numbers

The trap counts for the week of July 17 to July 24 showed a large increase in fall armyworm (FAW) moths captured in the western Kentucky UK IPM traps. The capture increased from virtually none to 155 per trap-week, which is a pretty major change and worth noting.

This capture level is not an emergence because:

1.) This level is not yet at the point where we know historically that problems have occurred (See Insect Trap Graphs for fall army worm) and

2.) These are the moths, and moths do not damage our crops; damage is done by their caterpillar offspring.

Offspring of these moths should begin appearing in about 1 to 2 weeks, depending upon temperature. So, we have time to check out the situation. NOTE: Though it can be found across the state, FAW is rarely a pest outside the western one-third of Kentucky. Remember some FAW may already be in the area at low populations, so you may find a few worms that are well below thresholds. Treating too early will not be of help and may make the problem worse since treatment will kill beneficial insects.

Figure 1. Fall armyworm. (Photo: Ric Bessin, UK)

Figure 1. Fall armyworm moth. (Photo: Ric Bessin, UK)

In addition, though we know these are FAW, we do not know which feed type this population contains. FAW which migrates from south Texas to the New England states over the course of summer and fall can be of two feeding types. The “Corn” type tends to feed on corn and sorghum while the “Grass” type tends to feed on lawn, pastures, and hay grass (including millet and preferring bermudagrass). This type will also feed on wheat, but that is not a concern at this date. There is no way to differentiate these groups by sight. They can only be distinguished by what they are feeding on or by genetic analysis. Though these are the major food for FAW, they will also feed on soybean and alfalfa. Alfalfa would be at particular risk if it is in a mixed stand with grass.

Producers should begin doing two things:

(1) Scout crops for the presence of these caterpillars and

(2) Check the trap capture numbers for the next several weeks to see if the population continues to increase.

A good place to scout first is in very late-planted grain sorghum (milo) or corn. FAW are easy to see in sorghum heads and in corn whorls. Grasses, alfalfa, and soybean can be easily checked using a sweep net. A 15-inch hoop is the standard on which most thresholds are based (if thresholds are available). Corn and sorghum are evaluated by the number of infested plants. (see Fall Armyworm Scout Info on the UK IPM Programs Web site)


By Doug Johnson, Extension Entomologist

Posted in Grains