By John Tooker
It seems that a lot of people have caterpillars on their minds at the moment, so I thought I capitalize on the moment and address two species that may be influencing your productivity. I will first address western bean cutworm in corn ears and then share a few thoughts on fall armyworm, which is causing widespread damage in hay and cover crop fields.
Figure 1: Western bean cutworm caterpillar, showing the distinctive longitudinal lines in its collar (or pronotum) behind its head. (Image Credit: Eric Bohnenblust, Penn State)
As the season begins to wind down, it is a good time to learn what caterpillars are infesting field corn ears. In most years in Pennsylvania, growers will find corn earworm, which tends to be most abundant, but you can also find European corn borer, fall armyworm, and western bean cutworm. Most Bt corn hybrids offer some protection against these species, but control is not 100%. I find that farmers can be disappointed when they find worms in ears of their field corn, but they should keep in mind that infestations tend to be patchy and are not usually uniform across entire fields. My advice is usually to ignore typical, minor ear damage. Research that we conducted at Penn State clearly indicated that earworm infestations across PA tend to be very low and were unlikely to be causing meaningful yield loss, even in non-Bt corn.
However, if the caterpillars in your corn ears are western bean cutworm, you may have a more severe problem. From 2009-2015, Penn State Extension educators trapped for western bean cutworm, but the moth populations never amounted to much and scouting revealed only a handful of caterpillars (Figure 1), so we discontinued our monitoring efforts. In other Great Lakes states, however, western bean cutworm has been causing economic damage to ears, even in Bt hybrids meant to control it (e.g., the Herculex trait, Cry1F protein). In fact, research from 2017 revealed that western bean cutworm has evolved resistance to the Cry1F protein. If you have heavy ear infestations, it will be important to know which caterpillar species is responsible. We have been finding western bean cutworm in ears at our research farm in central Pennsylvania, so infestations may be common across the state. Particularly if you are in the northern part of the state, I encourage you to determine if your ear caterpillars are western bean cutworm or something else. The identity of the species matters a great deal and may explain the level of damage. Western bean cutworm lacks the strong longitudinal stripes of corn earworm and has an obvious brown collar (called its pronotum) behind it head that has three small stripes that parallel the body (Figure 1).
Now for fall armyworm. As you may have heard, we are amidst an unprecedented outbreak of fall armyworm, with infestations in alfalfa, grass hay fields, small grains, and cover crops ranging from Oklahoma to Virginia and north to Ontario. Most growers were caught by surprised by the amount of damage that they encountered and the number of caterpillars in some of their fields. Last week, I wrote about the challenge and some control options ; this week I wanted to offer a few more thoughts. Most folks want to know if the caterpillars will continue to linger in our region and cause damage, possibly needing to be controlled again. When fall armyworm caterpillars get large (~1.5 inch long), they will dig underground soon to pupate to become moths. These moths will emerge in about 10-14 days, when temperatures are likely to be cooler than now and day length will have shortened significantly. As a result, I expect that some portion of this population will fly south to escape our impending fall, but some may stick around and lay eggs. When those eggs hatch, the young caterpillars will feed on grasses, hay or whatever else they can find, but it will be difficult for those caterpillars to get big enough to cause any significant damage prior to our first frost, which occur in mid to late October. The bottom line is that I would not expect additional damage from the next generation. Nevertheless, scouting your vulnerable crops would be wise, but preventative insecticide applications are unlikely to be necessary or productive. My advice is to watch and scout your fields for damage, but it seems likely to me that no further control will be necessary.
Source : psu.edu