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, and munched 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 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 Kentucky 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.
The current year is always depicted in green and the rolling 5-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 widespread 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 to one 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 an elevated risk of FAW caterpillars from the moth peak of August 31st for 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, the following: grass (wheat & rye) cover crops, grass and mixed grass / alfalfa hay fields, and early planted wheat.
- Established mixed grass and alfalfa hay, grass forages, lawns, parks and playing fields, etc. This insect really likes Bermuda grass, but fescue can also be hammered, and it 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, as well as interior of soybean fields that have a significant grass weed population. If the caterpillars are already feeding on grass weeds within a field and the weeds are killed, the caterpillars will move to the beans. Palatable or not, these insects will try to eat anything to stay alive!
- Full season corn, grain sorghum, and soybeans are probably too mature to be damaged much.
- Double crop beans, especially those planted 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.
Doug Johnson, Extension Entomologist and Patty Lucas, Extension IPM Specialist