Field crop insect update for July 23, 2018

An update on armyworms, sucking pests and western bean cutworms in Michigan field crops, with a special concern for growers in the Thumb with the dry weather conditions and sucking pests.

Armyworm damage to corn field
Central Michigan corn field stripped by armyworm. Photo by Paul Gross, MSU Extension.

I am getting a lot of calls about insects. For those of you in the Thumb, note a special concern with the dry weather conditions and sucking pests.


Isolated, but spectacular, armyworm infestations have been found in the western half of Michigan and the Upper Peninsula. There was an initial flight of moths into Michigan in late April and early May. Not much came of that flight in either wheat or corn. There was a second unusual dump of moths in mid-June in the western half of Michigan, leading to infestations in isolated corn, sorghum and hay fields. Many of the affected fields seem to have weed problems (thus attractive for egglaying). The reports I received are from Van Buren County, further north to Isabella, Kalkaska, Montcalm and Osceola counties, plus the Upper Peninsula.

Now that the larvae are 1-1.5 inches, eating a lot and marching across the road, people are suddenly taking notice. Here are a few pictures of impressive defoliation by larvae. Most of us old-timers recall this type of feeding, especially back in the non-Bt corn days, but many younger people have not seen anything like it. Some have questioned the speed at which damage occurred.

For many insects, 90-95 percent of what they consume is in their last stage. In their early stages, they are toddlers that don’t each much, thus outbreak levels of small larvae can be present in a field with few signs from the road. But in the final stage, the larvae are like voracious teenage boys, eating continuously and a lot. Corn plants can be reduced to stalks seemingly overnight, and damage is obvious at 55 mph.

The lesson here is that most of these infestations were probably avoidable with better weed control and timely scouting. When weed control is poor in an area, armyworms are attracted for egglaying. If there are no local pheromone traps, moth flight goes unnoticed. When fields are not scouted, wimpy “toddler stage” larvae are missed, and they grow into harder-to-kill teenagers that do real damage.

Things that suck

Soybean aphids are generally low, a few fields here and there with visible numbers. Several small infestations I found two to three weeks ago in the Thumb already are gone, either eaten by other insects or parasitized.

Potato leafhopper adults and nymphs are in most crops now. No unusual concerns except for dry beans and alfalfa in dry areas like the Thumb. The impact of potato leafhopper feeding is worse under drought. The thresholds for potato leafhoppers are good: in dry beans, it is one potato leafhopper per trifoliate; in alfalfa, it is 20 potato leafhoppers in 100 sweeps for new growth, 50 potato leafhoppers per 100 sweeps in 3-8 inch growth and 1 per sweep in taller stands.

Over the last month, I have had calls about thrips in the Thumb, where the weather remains persistently dry. (Note that the word “thrips” is both plural and singular. One thrips is called a thrips.) Thrips were first reported in corn in Michigan and Ontario, but they are also in beets and dry beans, especially in fields adjacent to harvested wheat fields.

Thrips are tiny, torpedo-shaped insects that feed in a unique way. Instead of using two mandibles (i.e., chompers) like most chewing insects, the right mandible of a thrips is small and non-functional. The thrips uses its left mandible to cut into plant tissue, then it injects saliva and slurps up the plant juices from the damaged cell. This type of feeding is sometimes termed “punch and suck,” and it can be devastating. The punctured cells collapse or dry out. When a large number of thrips are present, leaves have silvery patches where many cells have died. Under the worst damage, leaves dry up and die.

Thrips are normally a pest in limited situations, for example in hot, protected greenhouses. Although thrips are always present in field crops in Michigan, they are typically in very lower numbers, and in fact they are an important food source for some of the beneficial insects we depend on to control other pests. In an open field situation, they are simply kept in check by environmental conditions and predation.

Thrips are such a rare issue in Midwestern field crops that there are few, if any, scouting guidelines, thresholds or control recommendations available for various crops. I confess am struggling to provide guidance on thrips management. My best guess is to delay spraying for thrips unless or until the upper part of the plant (newer growth) is being affected by feeding. A change in current weather patterns—rainfall and higher humidity—would go a long way to reducing thrips and help plants recover from feeding.

Why be cautious about, or delay, spraying for thrips? My concern is with the current dry conditions in the Thumb. For example, Michigan State University Enviroweather stations in Fairgrove, Sandusky and Pigeon recorded less than 2.5 inches of rain since June 1. Spraying thrips now might be the gateway to a spider mite outbreak in a few weeks, if it stays dry. An insecticide application will control thrips (at least for a time) but also kill beneficials insects that feed on both thrips and mites.

Something I haven’t revealed yet: In addition to feeding on plants, thrips are predators of spider mites, poking into mite eggs and sucking out the juices. One of my colleagues in Texas, where spider mite routinely is a problem, views thrips as semi-beneficial. Before spraying thrips, it would be wise to think about how spraying thrips now will play out in the next few weeks if it stays dry. That spray might be better used against mites four weeks from now, or in a worst case scenario, might be responsible for flaring mites in the first place.

If you must for potato leafhoppers in dry beans or if you don’t believe me about thrips, at least choose the right product. Use something with a lower chance of flaring mites—a dimethoate, orthene/acephate, Lannate, Lorsban, bifenthrin. Check that your crop is on the label for that active ingredient. Get good coverage. Note the honey bee warnings on the label. Some insecticides, like dimethoate, have had previous issues with bee kills, and under drought conditions bees may be more likely to forage in field crops if they can’t find other sources of nectar and pollen. View this miticide list for field crops, prepared in 2016, just in case it is needed in the next few weeks.

Western bean cutworms

Western bean cutworms are flying, with a couple of high catch locations in St. Joe and Branch counties, and very high trap catches to the east in southwest Ontario. See the current western bean cutworm trap catch map. Thus far, however, few egg masses have been found in my scouting, and anecdotal reports from field scouts also show low/no infestation. Under dry conditions it is harder for western bean cutworm females to survive and make batches of eggs. Irrigated fields might be attractive though.

If you are trapping western bean cutworms, remember to add your trap locations and weekly data to the regional trapping network website. The Canadian Corn Pest Coalition funds the site so it is free for trappers in Ontario, Quebec and Michigan. The site uses our data to generate a weekly interactive map for the region and provide a visual snapshot of western bean cutworm population levels and movement. But the mapping only works if people contribute trap catches on a weekly basis (ideally entered on Monday). Michigan data is sparse so far, although I know many people are trapping. Consider taking the time to create an account and enter numbers to flesh out the state map. If you have multiple locations that you trap weekly, but don’t have time to enter data, email me at Maybe we can bargain to have my student enter the data for you.

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