Controlling Spider Mites in Corn Crops

IFA crop consultant Krea Mecham offers insights on pest control solutions for Intermountain West farmers battling spider mites.

Spider mites continue to be a major pest in hot, arid areas of the Intermountain West. Mite infestations occur primarily in corn crops from mid-July to the first of September. They damage corn by sucking chlorophyll out of infested leaves causing them to die and thereby limiting kernel fill.

 Leaves become translucent and their underside occasionally shows strands of webbing similar to the cobwebs made by spiders. This webbing protects mites and their eggs. Mite populations propagate in windy conditions with temperatures over 90 degrees. They appear in areas with many acres of established cornfields that have been grown for consecutive years. Spider mites tend to establish on field borders first and thrive in dusty environments, i.e., where dirt roads run alongside the field. I’ve also observed increased mite populations in rye crops growing adjacent to corn crops. 

When scouting for spider mites, I start at the third leaf from the bottom of the plant, as the first two leaves from the bottom will most likely be lost for various reasons before or during the growing season. Look at the top of the leaf for a light green or gray color, then observe the bottom for “spider webs.” One can also hold a white sheet of paper under the leaves, shake the leaves and look for mites on the paper.  

Spider mites work from the bottom of the plant upward. If they reach the ear leaf, kernel fill can be affected due to reduced photosynthesis as these leaves are lost. Consider planting varieties with high ear placement on the corn stalk if fields are prone to spider mites.

Various universities recommend chemical control for spider mite treatment when there is injury to the bottom third of the plant and spider mites are starting to appear on the second third of the plant. When considering which chemical mode of action (MOA) to treat, try to use products that preserve the predators of spider mites. Products such as Oberon, Onager, Zeal, and Zara kill eggs and juvenile mites while preserving predators. Comite and Decimite are also safe on predators but target adult mites. Pyrethroids (Bifenthrin and Sniper) and organophosphates (dimethoates and chlorpyrifos) kill adult mites but will also kill predators; these chemistries may lead to spider mite flare-ups within seven to ten days. Pyrethroids and organophosphates should only be used in the last two to three weeks of the growing season. When applying any of these products, adequate coverage is key. As these pests become increasingly problematic, it is important to rotate chemistries with differing modes of action, as spraying the same MOA year after year creates resistance. It is also important to note that there is not an economic benefit to spray mites past the hard dough stage of the corn kernel.

Apply chemistries through sprinkler irrigation, ground sprayers, or aerial application by plane, helicopter, or drone. Consult with an IFA Crop Advisor in your area to create a spider mite program specific to your farm.