• Spider mites in Indiana field corn are uncommon.
  • Use caution while interpreting spider mite information from western states.
  • Mite species/environments vary considerably across the Corn Belt.
  • While scouting, must go beyond the end rows to determine infestation.

Spider mite damage in field corn is a rarity in the Eastern Corn Belt, and very little is understood about their potential effect on yield in the Midwest even after some experience in the drought of 2012. As we visit with, or read our colleagues’ information out west, we feel that we’re comparing apples to oranges. Mite species and humidity levels are two of the glaring differences.

Two-spotted spider mites (Tetranychus urticae) are our enemy in field crops, including soybeans, as well as many homeowner and greenhouse plants, while out in the Western Corn Belt, their primary species is the Bank’s grass mite (Oligonychus pratensis) with some two-spotted mites mixed in. There are several subtle differences in identification, biology, and damage between the two species, but the most important one to readers is that two-spotted spider mites are more difficult to kill. In addition, two-spotted mites are prone to building pesticide-resistant populations. In short, we’ve got the tougher pest. And because of that, pesticide carrier volumes are recommended to be increased to at least 5 gpa by air and 20 gpa by ground.

Drought is the root of spider mite problems in Indiana. Though not all areas of the state are impacted by the dry conditions, all counties have had higher humidity in recent days. Though high humidity (>50%) doesn’t stop spider mites, it certainly slows down water loss from plants, and therefore lowers plant stress and the mites’ reproductive rates. It also makes mite populations prone to epizootics (“plagues” of fungal disease), which we saw locally during the last week.

Spreading through a cornfield is not as easy for spider mites as in soybeans. Mites can either walk from plant to plant via touching leaves or they “balloon” with spun webs, allowing the wind to transport them. Certainly this must be a slow process, because wind movement in the lower canopy of cornfields is quite limited. Make certain that spider mites have moved beyond the end rows, walking well into the field to determine their presence and colonization of leaves is crucial.

We still remain doubtful that spider mite treatments in yellow-dent corn are warranted. However, if fields with decent yield potential (150+ bu/a) have spider mite colonies established on lower leaves (discolored), and spreading to the ear leaf or above, then treating before the dent stage may be justified. It will not be easy to get treatments down to where the mites are most numerous though. As previously mentioned, high amount of carrier (5 by air, 20+ by ground) is strongly recommended. Consider that spider mites are usually most actively colonizing the underside of lower leaves so canopy penetration is necessary. We have no experience to draw from in treating corn for spider mites, but those products registered for use in Indiana are dimethoate, etoxazole (Zeal), hexythiazox (Onager), propargite (Comite), and spiromesifen (Oberon).