Armyworms, cutworms, cereal leaf beetles and refuge-in-a-bag concerns

An update on armyworms, cutworms and cereal leaf beetles in Michigan field crops, as well as concerns for using refuge-in-a-bag corn.

Armyworm and cutworm update

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, Michigan, 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 cutworm 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.

Cereal leaf beetle update

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. If so, we can’t control climate change, 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!

Refuge-in-a-bag corn may speed up Bt resistance

I urge you to read “Bt Corn Raises Questions: Refuge-in-a-Bag Corn May Speed Up Bt Resistance” by DTN. Emily Unglesbee, a reporter with DTN, writes about why moving to refuge-in-a-bag for Bt corn hybrids in the end may increase the rate of Bt resistance by ear-feeding Lepidoptera. Recall that seed companies were able to shift to refuge-in-a-bag because most corn hybrids now have multiple Bt toxins, which is important for resistance management. I compare it to having multiple hammers to smash a caterpillar; if the worm escapes one hammer (it is resistant to that toxin), it still is smashed by the second.

The potential failure of refuge-in-a-bag isn’t for corn borer, but for secondary caterpillar pests attacking corn ears (in the south it’s earworms and fall armyworms, in the north it’s western bean cutworms). When ears on refuge plants are pollinated by surrounding Bt plants, the resulting kernels express no, single, or multiple Bt toxins. From a caterpillar standpoint, this Bt smorgasbord is actually survivable, because larvae can find and feed on kernels with no or low Bt. This not only results in ear damage (there is an increase in western bean cutworm damage in the last few years, even in Bt corn), but this is how resistant populations are born. Unglesbee’s article does a nice job of summarizing research findings and explaining the potential fallout from using refuge-in-a-bag for resistance management.

Need an insect identification or a question to ask? I will be at the Michigan Wheat Program Field Day in Frankenmuth, Michigan, on June 15, 2016, and the MSU Weed Tour in East Lansing, Michigan, on June 29.

Dr. DiFonzo’s work is funded in part by MSU’s AgBioResearch.

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