Got Fall Armyworm? Want Some More!!!

Doug Johnson, Extension Entomologist, University of Kentucky
Patty Lucas, Extension IPM Specialist, University of Kentucky

Fig.1 FAW moth capture in UK-IPM trap in Princeton, KY

For the past week or so we have been dealing with large populations of fall armyworm (FAW) caterpillars. These hungry little pests have devoured alfalfa, forage and lawn grasses while munching around the edges of soybean fields. I am sorry to inform you that while this problem is decreasing it may also reoccur.

The caterpillars that have thus far caused our damage are probably the result of an increase in FAW moths during the last week of July. You must remember that FAW has been trickling into KY for several months and is a normal part of the pest pressure in August and September, particularly in the western 1/3 of the state. You can see this “pulse” in Figure 1. Also note that this graph is updated every week and can be viewed on the IPM web pages at:

http://www.uky.edu/Ag/IPMPrinceton/counts/fall/fawgraph.htm


The current year is always depicted in green and the rolling five- year average for years without an “Outbreak” population is always depicted in blue. Comparing these two sets of data gives you an idea of when there is an elevated risk of FAW problems. We have also included two known and very wide spread outbreak years in black (2012) and red (2007) just for reference. The important comparison is always the green and blue lines. The other outbreaks were important but we are not able to compare the relative importance of these larger peeks one to another.


These trap counts give us a heads up on FAW caterpillar activity because we have about 20 years of data. Unfortunately, we have few sampling points and the data is only taken weekly. Additionally, it is not the moths but the caterpillars that are important, and the time between moth and caterpillar is governed by temperature. So we can never be any more accurate in predicting the caterpillar arrival than ± 1 week and that does not account for the differences in temperature. I will say, based on experience that we are at elevated risk of FAW caterpillars from the moth peak of August 31^st in about 2 to 4 weeks, depending upon temperatures.

So, what is likely to be at risk? In order of damage potential:

Newly seeded grasses of any kind including but not limited to: grass (wheat & rye) cover crops, grass and mixed grass / alfalfa hay fields, and early planted wheat. (Wheat is never recommended as a cover crop because of this insect and several others species whose populations can build up on early planted cover crop wheat then move to our production wheat fields.)

Established mixed grass and alfalfa hay, grass forages, lawns, parks & playing fields etc. This insect really likes Bermuda grass, but fescue can also be hammered and has a difficult time recovering in hot dry weather.

Possibly very late double crops beans. Soybean is not a preferred food, but FAW will feed on them. The most likely places and the first to be noticed, are field edges and waterways of grass; interior of soybean fields that have a significant grass weed population. If the caterpillars are already feeding on the grass weeds within a field and the weeds are killed, the caterpillars will move to the beans. Palatable or not they will try to eat anything to stay alive! 

Full season corn, grain sorghum and soybeans are probably too mature to be much damaged.

Double crop beans, especially those planed very late could be damaged, but even they should be close to maturity by the time the next caterpillar population arrives.

It is always possible that absolutely nothing will happen.