Mid-Season Insect Watch: Spider Mites, Grasshoppers and Fall Armyworms

Keeping control of mid-season insect pests in your corn crop is vital to protecting yield potential. We’ve discussed how corn rootworm, European corn borer and western bean cutworm can inflict harm to your crop, so you’ll want to stay on top of pressure with good scouting and timely applications when needed.

Let’s turn our attention now to three more pests—spider mites, grasshoppers and fall armyworms—that, while less prolific, can nevertheless impact your bottom line. 

Spider Mites

Spider mites are a pest to watch for as corn plants approach VT growth stage or better. Adult mites are the size of a grain of salt and are prolific. 

There are two common types of spider mites found in the Corn Belt: Banks grass mites (BGM), which can be found on the lower leaves of corn plants and do not normally cause economic losses; and two-spotted mites (TSM), which are identified in the adult stage by a yellowish-brown body with two dark spots on their backs and can cause substantial yield loss. 

How to control spider mites in corn

TSM populations favor drought conditions and windy dusty areas. Scouting should start at VT and focus on field borders on the south and west sides of your fields. Dusty roads adjacent to these borders can also further movement of the mites and can create a larger infested area. 

The webbing the TSM creates to protect the egg and nymph stages is difficult for an insecticide to penetrate, so a systemic insecticide such as bifenthrin, lambda-cy or chlorpyrifos may be required to gain control. Spot treatment for TSM is possible if they are found early enough.

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Grasshoppers

While not a typical pest, grasshoppers can potentially become an issue in corn when drought is present. These conditions push grasshoppers to scour for food sources, causing damage by feeding on corn leaves, silks and ears. In extreme cases, they can strip leaves to nothing but a midrib.

Grasshopper eggs overwinter in clusters in the soil, with nymphs hatching in late May or early June. If food is scarce, nymphs will move to nearby vegetation or crops for food. Areas prone to grasshoppers such as grass waterways should be scouted for any that may be migrating into corn fields. 

How to control grasshoppers in corn

If grasshoppers are present in the field, check five separate locations, noting the number of nymphs and adult grasshoppers per square yard. If levels of 15 nymphs or 8 adult grasshoppers per square yard near field borders—or 3 adults per square yard within a part of the field—are found, treatment with bifenthrin or chlorpyrifos may be needed. You may be able to perform a targeted spot treatment without having to treat a whole field, depending on where the populations are located.

Fall Armyworms

Fall armyworms are a common pest that rarely meets the economic threshold for treatment. Adult moths are attracted to late-maturing corn, where they will lay their eggs. Larvae arrive late in the season and feed on corn leaves, causing damage in patches throughout the field that is similar in appearance to that of hail injury.

Applying an insecticide is usually not economical for control unless larvae are less than 1.25 inch long with plants are under stress and 75 percent of plants have whorl feeding damage.

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