Spider mites showing

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Spider mite infestations are starting in some fields. Hot, dry weather favors this pest.

Mites move into fields from the edge, often by passive airborne movement. Damage may be noticed first near an obstacle, such as a tree-line or power pole, that disrupts wind near the edge of the field. Mites feed with a piercing mouthpart, inserting it directly into plant cells and sucking out the contents. This type of feeding not only injures or kills plant cells, but results in water loss through the feeding wounds. Mite feeding leaves behind tiny yellow dots or specks on the leaves. As mite numbers and feeding increase, the yellowing becomes more apparent across the leaf surface. Cells are disrupted, water is lost, and plant tissue begins to die. Under severe infestation, leaves turn brown and eventually drop off the plant. Mites are best seen with a hand lens, especially by shaking plant foliage over a white piece of paper. A heavy mite infestation, however, is fairly obvious to the naked eye, with leaf speckling and yellowing, obvious numbers of mites on the undersides of leaves, plus webbing. Treatment options insecticides with the OPs chlorpyrifos or dimethoate (Lorsban, Dimethoate, Cobalt) or bifenthrin (Capture, Hero).

Unfortunately, if you treat spider mites, populations can resurge due to:

Egg hatch: Mites lay eggs on the plant surface. Insecticides kill adults and nymphs, but do not kill eggs. Since Dimethoate and Lorsban have short residual, newly hatched nymphs survive and repopulate the plants.

Rebound or flaring:
Insecticides kill beneficial insects, but don’t kill 100 percent of the mites. The mites reproduce in the absence of predators, potentially leading to a rapid increase, or flaring, of the population. This is one of the reasons we recommend scouting and spraying only when mites have reached a threshold, avoiding insurance applications of insecticide for mites as well as soybean aphid.

Spider mites can become resistant to insecticides. This problem increases with the number of applications. This is another reason we recommend scouting and spraying only when mites have reached a threshold.

If you do plan to treat, check fields before you spray to make sure mites are still present, as populations can crash quickly. Rain itself reduces plant stress and replaces water lost to pest feeding. But more importantly high humidity is critical for promoting the growth of fungi that naturally infest and kill mites. Humidity must be elevated for an extended time, 48 hours or more, before naturally occurring fungi are active. Mite populations can crash in a matter of days once fungal pathogens become active. 

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