Identifying Spider Mite Damage and the Species Responsible
We are starting to get more reports of spider mites damaging corn and soybeans in Nebraska.
Two species of spider mites, the Banks grass mite and the 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. The carmine mite is now considered the same species as the twospotted spider mite. This spider mite species is a distinctive dark red color, but otherwise identical to the TSM (Figure 2).
Although these two species — the Banks grass mite and the twospotted spider mite — are somewhat similar in appearance, they differ in several biological characteristics and their susceptibility to pesticides (see Table 1).
Figure 1a-b. Twospotted spider mite damage to soybean (left) and corn. (Photos courtesy of Daren Mueller, Iowa State University, Bugwood.org)
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. Spider mites are very small in size (adult females are 0.016 inch in length and other life stages are even smaller) and can be difficult to see without a magnifying glass or 10X hand lens.
As mite infestations develop, leaves may be severely damaged and the food manufacturing ability of the plant may be 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.
Managing Spider Mites in Corn and Soybean for treatment thresholds, insecticides and potential impact of beneficial insects, diseases and insecticide applications.
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.
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 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.
|Appearance (adult females)
Drawings by Jim Kalisch
Dark green pigment spots extend down length of body; body is more elongated
Dark green pigment in two distinct spots on front half of body; body more rounded
|Webbing||Produces spider-like silk webbing||Produces spider-like silk webbing; tends to produce more webbing than BGM|
|Host Range||Almost exclusively grasses, such as corn and sorghum||Many grass species (corn, sorghum) plus soybeans, fruit trees, vegetables and ornamentals|
|Timing||Appears earlier in the season||Tends to appear mid- to late season|
|Location on Crop||Mostly lower leaves, moving upward as the infestation grows||Can feed over the entire plant|
|Overwintering Location||Primarily the crowns of winter wheat and native grasses||Primarily alfalfa and other broadleaf plants along crop field borders|
|Susceptibility to Insecticides||Moderately susceptible to many common miticides||Has developed resistance to some products; control is less consistent|
|Source: UNL Department of Entomology|
“Spider Mites in Corn and Soybeans” by Julie Peterson, Jeff Bradshaw and Bob Wright.