July 25, 2013
Recognizing spider mite damage and identifying the species responsible.
We are starting to get more reports of spider mites damaging corn and soybeans in eastern Nebraska. Previously we had reports of Banks grass mites colonizing corn near wheat in central and western Nebraska.
UNL Extension Entomologist Bob Wright provides scouting and treatment recommendations for spider mites in a segment of this week's Market Journal.
Twospotted spider mite damage to soybean (top) and corn. (Photos courtesy of Daren Mueller, Iowa State University, Bugwood.org)
Two species of spider mites, the Banks grass mite and twospotted spider mite, commonly feed on Nebraska corn. Banks grass mites (BGM) feed almost exclusively on grasses, including corn, small grains, and sorghum. Twospotted spider mites (TSM) not only feed on many species of grasses, but also on soybeans, fruit trees, and a variety of vegetables and ornamental plants. Although these two species are somewhat similar in appearance, they differ in several biological characteristics and in their susceptibility to pesticides.
Damage Indicators
Mites damage crops by piercing plant cells with their mouthparts and sucking the plant juices. The first evidence of mite feeding, which usually can be seen on the top of the leaf, is a yellow or whitish spotting of the leaf tissues in areas where the mites are feeding on the lower leaf surface. Because many other things can cause similar discoloration, it is important to check leaves closely to make sure mites are actually causing the damage. Leaf discoloration caused by mite feeding can be easily identified by checking the undersurface of leaves for the presence of mites, eggs, and webbing. Both BGM and TSM produce webbing, and a fine network of silken webs likely will be associated with mite colonies. A magnifying glass or 10X hand lens is helpful in examining plants for the presence of mites.
As mite infestations develop, leaves may be severely damaged and the food manufacturing ability of the plants progressively reduced. If an infestation is severe, leaves may be killed. In corn, effects on yield are most severe when mites start damaging leaves at or above the ear level. Infestations may reduce corn grain yields due to poor seed fill and have been associated with accelerated plant dry down in the fall. The quality and yield of silage corn also may decline due to mite feeding.
Damage is similar on soybeans, and includes leaf spotting, leaf droppage, accelerated senescence and pod shattering, as well as yield loss. Early and severe mite injury left untreated can completely eliminate yields. More commonly, mite injury occurring during the late vegetative and early reproductive growth stages will reduce soybean yields 40%-60%. Spider mites can cause yield reductions as long as green pods are present.
Mites do not cause major economic damage every year in Nebraska. Several factors, which fluctuate from year to year, strongly influence spider mite numbers. Probably the most important of these factors are weather, natural enemies, and pesticide use. Overwintering sites that are close to corn and soybean fields, especially grasses, wheat, and perhaps alfalfa, also may increase the possibility of mite invasion.
Dry, hot weather favors mite reproduction and survival, especially if accompanied by drought stress in the crop. When the weather in June, July, and August is especially hot and dry, mites can reach damaging numbers in most corn and soybean growing areas of Nebraska. Major mite infestations are more likely to occur in central and western counties that normally experience less rainfall. Sandy soil types also may contribute to spider mite problems in these areas because crops grown on these soils are more likely to experience drought stress even when irrigated.
Impact of Beneficial Insects and Disease
Several species of insects and mites prey on spider mites. These predatory insects and mites play a major role in suppressing spider mites in most years. Many spider mite problems in corn and soybeans may be traced back to an earlier application of a broad spectrum insecticide that reduced populations of these natural controls. The most important of these include a predatory spider mite, the mite destroyer beetle, the six-spotted thrips and the minute pirate bug. In addition to these predators, a fungal disease also may be important in reducing spider mite populations.
Insecticide Choice is Critical
Most pyrethroid and organophosphate insecticides used in corn and soybeans have severe, detrimental effects on spider mite predators. Additionally, pesticides differ a great deal in their effects on BGM and TSM. Some cause little mortality of either species, while others are somewhat toxic to BGM. Fewer are toxic to TSM. Thus, great care should be taken to evaluate the benefits of an insecticide application before any material is applied for insect control in a field that also has spider mites. Even small numbers of mites can rapidly increase to damaging levels when conditions are favorable. In many cases, it is an earlier treatment for western bean cutworms or corn rootworm beetles that leads to a later spider mite problem in corn.
If a decision is made to treat an insect pest in a field that also has spider mites, the choice of products becomes very important. Because TSM and BGM differ in their susceptibility to various pesticides, it is important to determine which species is present. Spider mites (particularly TSM) are noted for their ability to develop resistance to chemicals that were once toxic to them. For this reason, it is very likely that some products now toxic to spider mites will become less toxic in the future.
Fig. 1. Note the difference in coloration pattern between the adult Banks grass mite (left) and the twospotted spider mite (right). (Drawings by James Kalisch)
Identifying the Mite Species
Proper identification of the mite species present in a field is essential for making control recommendations and selecting an appropriate pesticide. This is because colonies of TSM generally are more difficult to control than BGM, and some insecticides used to control other pests are more likely to increase TSM numbers than others.
Accurate identification of spider mites is difficult and requires specialized microscopes and specimen handling procedures. Since BGM and TSM now are the only species known to damage corn in Nebraska, a simplified method has been developed to help differentiate between these two species in the field. Using this method and a 10X hand lens, you should be able to determine the species composition of most mite infestations.
The characteristics used to identify the two species will apply to most specimens; however, there is considerable variation among individuals. Examine at least 20 adult female mites. In an established colony, adult females will be the largest individuals. The rear of their body is rounded whereas that of the much smaller male mite is more tapered.
The most useful characteristics for identification are the overall body shape and the pattern of pigmentation spots on the back. The dark green spots on both species are caused by food particles that accumulate in their gut. Because of differences in gut structure, these pigment spots accumulate in slightly different patterns. In BGM, the pigments accumulate along both edges of the body near the rear and along the sides of the body. In TSM, the pigments accumulate along the sides of the body in two distinct spots and do not extend back more than halfway on the body. The BGM is also slightly less robust than the TSM, and is slightly flatter from top to bottom.
In addition to the differences between individuals of the two species, there are some differences associated with colonies. There are exceptions, but TSM colonies tend to produce more webbing than BGM. BGM colonies often begin earlier in the season and remain longer on the lower leaves before moving up the plant. TSM usually appear later in the season and colonies can be found anywhere on the plant.
Spider Mite Treatment Thresholds and Recommendations: See Part 2 of this article.
Robert Wright
Extension Entomologist, Lincoln
