Capture of armyworm (AW) and black cutworm (BCW) moths continue at a considerable, if slightly lower, number compared to last week. The AW trap captured 449 moths while the BCW trap captured 22 moths. This weekly catch includes at least one night of heavy rain, when moths could not fly. There is no direct relationship between AW and BCW, so don’t make the mistake that BCW is less of a problem because there is a relatively smaller capture than AW. This is a normal occurrence. The importance of these capture numbers are relative to our historic catch for the same species. Don’t compare AW numbers to BCW numbers. Both pest captures just happen to be larger than their historic numbers this year. In addition, even though the trap captures of both pests decreased this week, they are still well above the rolling 5-year average. It is likely the trap captures for both pests will decrease over the next few weeks. The real question now is: when will the offspring (caterpillars) of these moths turn up and start eating on our plants?
Estimating When Caterpillars May Appear
Because we are depending upon a relatively small sample, only two traps in two counties, I cannot tell you IF or WHERE an outbreak may occur. I do know that people are trapping in other counties, and those moth captures are also elevated, but I do not have immediate access to those data. However, we can use a basic IPM tactic to estimate when an outbreak might occur. Since the rate of insect development is almost completely dependent on temperature, we can use the IPM pheromone baited trap captures as a starting point (biofix), along with daily recorded high and low temperatures to estimate when the damaging stage (caterpillar, aka worms) might appear.
Using Degree Day (DD) calculators available through the UK College of Agriculture, Food and Environment, we can begin to add up the daily heat units to predict the appearance of the caterpillars in much the same way as agronomists use growing degree days to estimate the appearance of specific plant stages in corn. Because we have different levels of information for the two insects, the answers will be a bit different.
We are using the April 9 trap captures as our biofix, and this model was run on April 19. So, all dates previous to April 19 captures have known high and low temperatures. Dates after April 19 are estimated temperatures, which are based on historic temperatures for these dates at the Princeton location.
For AW, our target number is 239 DD. This is the number of temperature units needed for the insect to develop from the moth stage through hatch of the egg stage, thus the appearance of caterpillars. With our current information, our estimate is the first young caterpillars will appear on or about May 3. These caterpillars will be so small that we will not likely see them or their damage for a few more days, but it does make a good mark for beginning to scout for AW.
BCW is a little different because we know more about its development. In this case, 300 DD are needed for caterpillars to reach 4th larval stage, which is the stage in which caterpillars are large enough to cut plants. There will be leaf feeding by the 1st to 3rd instars (caterpillar stage), which may be detected before cutting begins by scouting. With our current biofix of April 9 and temperature data we expect that 300 DD will be reached by about May 11.
Be aware that these are estimates, not exact dates. Changes in temperature and other factors can alter insect development time. In addition, my estimates are based on temperature data from Princeton, Caldwell County. KY. Other locations will vary from this, based on local temperatures. Nevertheless, this gives you a ball park date to try to stay ahead of this insect. Additionally, remember that the closer in time we get to the actual caterpillar emergence, these estimates are based on a greater number of days for which we have real temperature data, and a fewer number of days for which the model is estimating the temperature data. Hopefully, this will make for more accurate estimates as we get closer to the events.
Remember to factor in the Bt traits that will help control cutworms (see previous post). For armyworm, the Bt trait of most use is Viptera™.
By Doug Johnson, Extension Entomologist