By Christina DiFonzo, Entomologist
As expected, armyworms and cutworms are being sighted in northern Indiana and southern Michigan. Rod King with Brodbeck Seeds reported armyworm in corn and soybean, especially in fields that had a heavy cover crop. Bruce Mackellar with Michigan State University Extension in southwest Michigan reported armyworms west of Paw Paw, in corn planted into a terminated rye cover. In southern parts of the state, larvae will be large enough now to do obvious leaf damage, and black cutworms will be starting to cut plants.
Now is the time to check fields that had a cover crop or a lot of weed growth prior to planting, as well as wheat. Remember that armyworms and cutworms won’t be up on the plants conveniently waving at you to find them — both typically hide during the day at the base of plants and under residue. A trowel helps a lot for scouting.
As in previous years, cereal leaf beetle numbers in wheat are creeping up. A research area at the Kellogg Biological Station in southwest Michigan apparently had large numbers last week, although I did not see these fields myself. I’m not talking about outbreaks of cereal leaf beetle larvae, but instead a slow, steady increase in incidence.
When I started at MSU 20 years ago, it was several years before I even saw my first cereal leaf beetle in wheat. Now, it’s relatively easy to find beetles in sweep samples or larvae scraping leaves this time of year. This isn’t just my observation — experienced extension educators have the same impression.
Cereal leaf beetles used to be controlled entirely by parasitism. As the climate has become warmer, perhaps parasitoids are a bit out of sync with larval development. We can’t control the weather, but something we can control is insecticide use in wheat.
As part of adopting principles of intensive management, the wheat industry is doing a better job managing head scab and other diseases in Michigan. When a sprayer is already going over a field to apply fungicide, it is now tempting to add an insecticide in the tank to “clean up” whatever insects are out there; in fact, many websites and bulletins on intensive wheat management routinely recommend this practice.
However, there are repercussions to insurance insecticide applications to wheat that may not be obvious. One is that spraying wheat disrupts the biological control that keeps most wheat pests in check. Parasitoids are especially important for armyworms, aphids and cereal leaf beetles. Spraying wipes out beneficials and disrupts biocontrol; this may be part of the reason cereal leaf beetles are trending upward in the last few years.
Another repercussion is that spraying eliminates the natural enemies that build up in wheat, then move to other crops later in the season. The low numbers of aphids, thrips and other insects in wheat aren’t something to clean up — it’s the fuel that generates populations of beneficials for July and August. This is one of the hidden benefits of having wheat in rotation on your farm. I’m not saying don’t spray wheat — just scout and use an insecticide only if needed.