Big Jump in Fall Armyworm Moth Capture

Doug Johnson, Extension Entomologist, University of Kentucky

The trap week of July 17-24 showed a large increase in fall armyworm (FAW) moths captured in the western UK-IPM traps. The capture increasing from virtually none to 155/trap-week, 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 Graphs at: http://www.uky.edu/Ag/IPMPrinceton/counts/fall/fawgraph.htm) 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 1-2 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 1/3 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 as they will kill the beneficial insects.

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 Bermuda grass. They 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.

Figure 1. FAW caterpillar; note inverted

 Y.

FAW caterpillars vary from light tan to nearly black with three thin light yellow lines down the back. There is a wider dark strip and a wavy yellow, red-splotched strip on each side. They resemble both armyworms and corn earworms but fall armyworms have a prominent white inverted Y mark on the front of the head (Figure 1.).

 So, producers should begin doing two things. First scout your crops for the presence of theses caterpillars and 2^nd check the trap capture numbers for the next several weeks to see if the population continues to increase. A good place to scout first would be 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. The 15” sized 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 UK-IPM Pest Info at: http://www.uky.edu/Ag/IPM/scoutinfo/corn/insects/faw/faw1.htm.