MSU Extension Field Crops

It’s been dry across most of Michigan so far this summer and a drier than normal forecast was made for the rest of summer. Under current conditions, watch for the following piercing-sucking pests. The pests are listed in order of how concerned Chris DiFonzo, field crops entomologist for Michigan State University Extension, is based on a combination of how common the pest is and how hard it is to control. For specific insecticide recommendations, see the free Field Crops Insect Management Guide.

Tarnished plant bug and related species (generically called lygus bugs) are always present in field crops in the state. Plant bugs suck juices from leaves and may pierce beans later in the season. They usually go unnoticed except in sugarbeets, where fed-on leaves turn a characteristic bright yellow or brown at tips and along the edges (some refer to this injury as stinging). Tarnished plant bug injury is most common in beets adjacent to alfalfa. When the forage is cut, adult bugs move into adjacent fields in large numbers. Beets that are nearby get fed-on for a period, as the bugs redistribute across the landscape.

Plant bug injury is frustrating from a management standpoint. By the time the bright yellow symptoms are seen on emerging leaves, the damage occurred a week or two before. The bugs are usually gone, and spraying doesn’t help. 

Thrips are tiny torpedo-shaped insects that feed by punching into plant cells and sucking the contents. Punctured cells dry out, creating silvery patches where cells collapsed. Under heavy populations, whole leaves dry up and die. Thrips are always present in crop fields in Michigan, but they are kept in check by predation. In fact, thrips are an important early-season food source for the beneficial insects. Occasionally, leaf injury can be found in bean fields adjacent to wheat because adult thrips move out of drying grain fields into neighboring crops. But damaging outbreaks in field crops are very rare—those in the Thumb may remember a thrips infestation on dry bean, soybean and beets during a dry spell in 2018.

Thrips are such a rare issue in Midwestern field crops that management recommendations are limited. My best guess is to spray only if the upper part of the plant (newer growth) is being affected by feeding and thrips are easy to find. To complicate matters, thrips are semi-beneficial because they are predators of spider mite eggs. Spraying for them may be counterproductive, as it may help to increase spider mite populations later.

Potato leafhopper arrived from the south in May and has been reproducing on crops and weeds for over a month. Potato leafhopper feeding causes leaf yellowing (hopperburn) on sensitive crops like alfalfa, dry beans and many vegetables. Both adults and nymphs cause this injury, and symptoms can appear quickly as populations build.

Potato leafhopper is relatively easy to manage if caught in time. Scouting is relatively easy. In dry beans, examine trifoliate leaves and count the number of adults and nymphs per threshold: one potato leafhopper per trifoliate. In alfalfa, use a sweep net to determine the number of adults and nymphs per 100 sweeps per threshold (by plant height): 50 (3-8 inches), 100 (8-12 inches) or 200 (over 12 inches).

Soybean aphid hasn’t caused many issues the past few years, but this season is a bit different. I have sampled fields with 75% of the plants infested and seen pictures of plants with many juicy aphids on the new growth. Don’t assume your fields have aphids—most fields won’t. Fields infested now will be those planted early (April, maybe early May) and without a seed treatment. These fields emerged in time to catch the flight of aphids coming out of overwintering areas. Fields planted a bit later (with or without a seed treatment) will still be clean. However, seed treatments are already running out and later-planted fields can get infested when aphids begin to move out of the early-planted fields.

The good news is that there are predators and wasp parasitoids already attacking the aphids. The bad news is that without rain, plants are just sitting there, and aphids may increase too fast for beneficials to catch up. It’s still a bit early; generally, I think about hitting July (or R1 stage) to decide about spraying. The threshold for aphids is an average of 250 aphids per plant (essentially all plants with a couple hundred juice aphids on the new growth). The threshold is robust, meaning that it applies under a lot of conditions and has some time built in before the true economic injury level is reached. No need to panic but be watchful.

Spider mite is the most worrisome pest to me because it is so hard to manage, especially if it increases early in the season. Infestations start innocently enough on field edges near ditch banks or dirt roads. But mites reproduce parthenogenetically (without mating), so populations can explode in hot, dry weather. Spraying for another insect can exacerbate the situation by killing predators and releasing the mites from biocontrol. Spraying for mites themselves can be a losing game, especially if coverage is poor or the wrong product is used. Finally, selective miticides can be expensive on low-margin field crops. Spider mites are just a pain in the butt. For more information, go to Field Crops Entomology Extension Resources and scroll down to the Spider Mite heading for corn and soybean decision sheets.

Why sucking insects do better under warm, dry conditions

Development is faster. Insects and mites develop faster from juvenile to adult with increasing temperature going through more generations. Development is even faster if both day and nighttime temperatures are elevated. Thus far, it’s often been cool in the morning, but during a hot spell, the favorable temperature for development probably occurs for a longer period, not just during the day but for 24 hours, as both day and night temperatures are elevated. 

The diet might be better. Insects feeding on a drought-stressed plant may have a better-quality host. Concentrations of nitrogen-type compounds such as amino acids typically are higher in the sap of drought-stressed plants. For sucking pests, this is ideal. Insects that suck plants usually get enough sugar in their diet, but they are nitrogen-limited. A stressed plant with a higher concentration of nitrogen compounds is a steak dinner instead of a fast-food burger. 

The injury is worse. Feeding of sucking pests under dry conditions just adds insult to injury. Thrips and spider mites feed by poking into individual plant cells, which lose water and dry up quickly when there is low humidity. Potato leafhopper and tarnished plant bug saliva disrupts plant cell growth and disrupts water flow, something the plant can’t afford under dry conditions. With adequate moisture and irrigation, plants can replace the liquid sucked by insects or compensate for their injury, but under drought conditions this is less likely. 

Biocontrol may be less. Entomopathogenic (insect-killing) fungi naturally control potato leafhopper, aphids and spider mites. In dry weather and low humidity, these fungi don’t have the right conditions to infect and wipe out a growing pest population. This is especially true along field edges and dirt roads. Dust particles on the leaves dry out the plant surface so that entomopathogens are less active. A dusty back road is a good location to look for yellowing along field edges that could be the first sign of spider mite. 

Insecticides have limits. Pyrethroids tend to work better under cool conditions, so control with this group of products may be less. UV light breaks down pesticides, and under sunny conditions insecticides don’t last as long on foliage and residue levels drop. At the same time, most insecticides wipe out beneficial insects. If insecticides break down fast and the biocontrol is gone, pest insects or mites can recolonize or rebound quickly, much quicker than their natural enemies can recover. This is especially true of parthenogenic pests like mites and aphids. Remember, a mom aphid gives live birth to pregnant daughters without even mating. 

Final thought before pulling the trigger on an insecticide spray

Spraying too early or for insurance (throwing insecticide in the tank as you go over the field) can be just as bad as not spraying, especially under dry conditions that are favorable for aphids, thrips and mites. Think about how spraying now will impact your fields later if it stays dry. Spraying now might lead to a spider mite outbreak or higher aphid populations in a few weeks. That is because the insecticide application will wipe out beneficial insects.

If you do spray, choose the right product. Use something with a lower chance of flaring mites such as dimethoate, orthene/acephate, Lannate or bifenthrin. Check that your crop is on the label for that active ingredient. Be sure to get good coverage, because surviving aphids, mites and thrips especially can reproduce fast and rebound.