Field Crops News

Natural Enemies of Spider Mites on Field Corn

Spider mites are very small arthropods, usually less than 1 mm in length, that can be severe pests of field corn. The Banks grass mite (Oligonychus pratensis) and the twospotted spider mite (Tetranychus urticae) are the two most common mite pest species on field corn grown in the Midwest. The Banks grass mite feeds specifically on grass plants, whereas the twospotted spider mite will feed on a wide variety of plants including soybeans, alfalfa and apple trees. On many crops, including field corn, spider mite problems may be increased when natural enemies are destroyed by applications of broad-spectrum insecticides applied against other pests. Several species of naturally occurring insects and mites prey on spider mites. The most important of these include a predatory mitethe fallacis mite, Neoseiulus fallacis; a lady beetlethe spider mite destroyer, Stethorus punctum; a predaceous thripsthe six-spotted thrips, Scolothrips sexmaculatus; and minute pirate bugs, Orius spp. In addition to these arthropods, the fungal pathogens, Neozygites spp., are also important in reducing spider mite populations.

The fallacis mite is the most important spider mite predator found in corn fields. These mites are slightly larger than the pest mites, pear-shaped, and uniformly pale brown or straw-colored. In addition, they do not have the dark pigmentation characteristic of the pest mite species. Fallacis mite eggs are laid in pest mite colonies and under summer conditions hatch in 1.5 days. After hatching, the juveniles feed on 4 pest mite eggs per day for the 8 days which it takes them to reach the adult stage. Female mites will mate and begin to lay about 3 eggs per day for about 13 days. Adult mites eat about 15 mites per day for the 70 days that they survive. This consumption rate varys depending on the density of pest mites, but when they are present, the fallacis mite can quickly build its population and significantly reduce pest mite numbers.

The spider mite destroyer is a small (about 2 mm long) black lady beetle which can eat up to 40 mites or mite eggs each day it lives. The adult beetles lay eggs in active mite colonies. After 5 days, eggs hatch and larvae begin to feed on mite eggs. The larvae are gray and cylindrical, growing in about 17 days to 2 mm long at maturity. The adult beetles are relatively long-lived with a life span of over 1 year. Adults hibernate during the winter months in leaf litter and other organic debris. Females lay eggs for about 67 days and are capable of producing 750 offspring. These predators are usually most numerous when the pest mite population is also high.

The six-spotted thrips is a small (2 mm long), tan, cigar-shaped insect, with long narrow wings fringed with hairs. Thrips eggs are laid in the leaf where they hatch in 7 days. There are 4 larval stages, of which 2 are active. During the active stages the larvae eat 10 mite eggs per day over a period of 10 days. When adult thrips emerge, they feed on about 60 mite eggs per day over a 30 day life span.

Minute pirate bugs are small insects (2 mm long) but can be important predators of insect eggs and spider mites in both the immature and adult stages. The immature bugs (nymphs) are orange and wingless, whereas the adults are black, with white triangles at the tips of the wings. These bugs are usually found in protected places on the plant, such as the whorl, tassels, leaf axils, corn husks or corn silks. Eggs are laid in the leaf collar tissue and hatch in about 4 days. Nymphs go through 5 stages in 9 days. The adults live an additional 26 days and the females lay an average of 45 eggs. Each Orius adult or nymph can eat 30 or more spider mites per day. Orius are generalist predators which makes their presence highly desirable since they can survive on other prey (insect eggs, small caterpillars) or pollen if pest mite populations are low.

In addition to predatory arthropods, naturally occurring fungal pathogens can, under the proper conditions, easily provide complete control of a spider mite infestation. Neozygites floridana has been identified from spider mites in the southeastern portion of the country, whereas the predominant fungal species attacking Banks grass mite in western Kansas has been identified as N. adjarica. If several cooler (<80-85F), damp days occur together, these pathogens have an excellent opportunity to infect and kill spider mites. Infected mites become shriveled and brown, and die quickly. If weather conditions favoring fungal propagation occur, a re-evaluation of the pest mite population should be undertaken before making any pest control decisions.

At this time the full impact of these natural enemies on pest mite infestations it is not known, but during many years they are important in keeping spider mite populations below economic levels. They are particularly effective during cooler, moist periods in early and mid-summer when pest mite reproduction is slowed. Although high temperatures do not reduce the development rate of the fallacis mite, low relative humidity may be the cause of low adult survival and high egg mortality. Researchers have shown (using a simulation model) that under conditions of higher humidity, such as those found in well watered, non-stressed field corn, the fallacis mite can completely control Banks grass mite. Less is known about the effect of climatic conditions on the other natural enemies, but they also seem to be more efficient in cooler conditions with higher relative humidity. For these reasons, the presence and abundance of natural enemies, as well as long range weather forecasts, should be noted and considered when evaluating spider mite populations and making pest management decisions.

In addition to identifying and monitoring naturally occurring beneficial species, there have been some attempts to document the effectiveness of inoculative releases of insectary-reared predators. When released in cage studies, the predatory mites Phytoseiulus persimilis and Amblyseius californicus reduced spider mite infestations on corn by 60% and 28%, respectively. Predator releases in field plots by hand at a rate of 5 per plant became established well enough to provided adequate control of Banks grass mite. Finally, P. persimilis was released from an airplane to pre-tassel stage field corn, at a rate of 15,000 per hectare in corn cob grit. While this application did not successfully suppress the pest mite infestation, the predators did become established and were beginning to impact the pests. Additional research needs to be conducted on the delivery of insectary-reared predators, but their use as a tool in spider mite pest management programs is promising.

- Ron Seymour, West Central Research & Extension Center, North Platte, & Bob Wright, South Central Research & Extension Center, Clay Center, University of Nebraska-Lincoln.

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