Mites in Corn?

This article was originally published on July 23, 2012 and expired on July 30, 2012. It is provided here for archival purposes and may contain dated information.

Area growers are battling spider mites in bean fields as we write this article, and many of those growers are beginning to recollect information about spider mites that grew a little dim over the years. One spider mite fact that may have been forgotten entirely is the pest's ability to feed on other crops – including corn.

Spider mites are not insects. They emerge from the egg as a six-legged, smaller version of the adult (termed a "larva"), progress through other stages with a total of eight legs (some of which are termed "protonymph" stages and some of which are termed "deutonymph" stages), and end life as an eight-legged adult (the overwintering stage). Their body can be segmented into two distinct regions. Because insects have six legs and three segments, spider mites are classified as being "arachnids" or are often generically referred to as "insect-relatives." They feed via sucking mouthparts, removing sap from the plant when the plant cannot afford to lose moisture and nutrients.

Beans are typically the row crop that growers think of when scouting conversations shift to the subject of spider mites. However, droughty conditions can also bring symptoms of spider mite injury to cornfields. Such injury is beginning to appear in a few central Illinois fields. Two species of mite are noted as possible candidates. The first is the two-spotted spider mite, the species that occasionally appears in beans. The second is the banks grass mite. The species can be distinguished from one another via their appearance and via the type of injury they cause.

Examining mites requires the use of a hand lens or microscope. They are literally that small. When observed with such tools, two-spotted spider mites will have two dark spots on either side of the abdomen while the banks grass mite has a more uniform region of darkened tissue located around the backside of the abdomen. The darkened tissue is somewhat caused by food deposits within the body cavity.

Mite injury symptoms in beans resemble yellow specks. While such specks are present in corn, corn plants often display a dirty, yellowish-brown discoloration. Some might even say that leaf material develops a slightly bronze discoloration. Two-spotted spider mites will cause a uniform discoloration of the corn leaf tissue while grass banks mites will cause discoloration that progresses outward from the leaf midrib and leaf tip (Source: Nebraska). Two-spotted spider mites are more prone to infest the entire plant while banks grass mites often restrict their feeding to the lower portion of the plant. From a distance, the symptoms of spider mite injury will resemble "firing up" symptoms commonly associated with moisture stress (Source: U of I Extension Pest Management Bulletin).

Yield losses have ranged from the single digits to the double digits in the western United States when mite populations have been exceptionally high. Mite infestations in corn are more prone where dry conditions prevail, where nitrogen rates are high, and where insecticides have been used (Source: Colorado State). The latter point requires more detail. Insecticides and miticides have the nasty habit of killing other things – including the natural controls that nature uses to keep mite populations in check. Minute pirate bugs, certain species of lady beetle, and predatory mites are examples of the type of beneficial insects that can be compromised when pesticides are applied. If beneficial insect populations are compromised, spider mite eggs can hatch after chemical products have broken down and the resulting spider mite population can explode in number minus any natural controls. For this reason, growers should not apply pesticides in the absence of a pest and limited areas of the field (those areas with the worst symptoms) may need to be treated when possible. The latter technique eliminates mites and mite predators in the treated area while leaving mite predators alone in the untreated area. Mite predators may then re-inhabit the treated area increasing the long-term benefit of an insecticide/miticide application (Source: Nebraska and Colorado State). Because mites tend to develop where insecticides have been applied, growers should practice caution when applying insecticides minus insect pressure. One might argue that the tank mixing of insecticides and fungicides earlier this growing season predisposed some fields to mite infestations via eliminating mite predators.

Thresholds for spider mites in corn are a little more complicated than they are in beans. In beans a simple discoloration threshold of 10 to 20 percent is often used (depending upon the stage of the plant). Nebraska has developed a threshold system that varies based upon the price of corn and expected yield. Growers estimate the percent of leaves displaying symptoms of mite injury. If the percent of leaves displaying symptoms reaches a critical threshold within an expected income range, one then estimates the average percent of individual leaf area displaying symptoms. If the average area of infestation per leaf reaches a second critical threshold, miticides are applied. The current price of corn and expected yields for 2012 likely mean that 20-25% of the leaves in an area would have to display evidence of mite injury to justify a miticide application. Furthermore, the average area of discoloration per leaf would need to rest around 10-15% before the grower could expect a return on investment (Source: Colorado State). The chart can be accessed at http://www.ext.colostate.edu/pubs/insect/05555.html.

Source: Matt Montgomery, Extension Educator, Local Food Systems and Small Farms, mpmontgo@illinois.edu

Pull date: July 30, 2012