Management of Spider Mites in Corn
Jack Campbell, West Central Research & Extension Center
Gary Hein, Panhandle Research & Extension Center
Updated July 29, 1996
Spider Mite Increase in FieldsTwo species of spider mites, the twospotted spider mite and the Banks grass mite, damage corn in Nebraska. They are somewhat similar in appearance but differ in the amount of damage they cause and their susceptibility to chemical controls. Weather and natural enemies appear to be important determining factors in spider mite abundance. Spider mites are most likely to develop economically damaging populations in fields that are moisture-stressed during June and July, particularly if weather is hot, windy, and dry. Mite build-up can occur even in irrigated fields, especially if irrigation problems exist or if irrigation is delayed during stress periods prior to the blister stage of corn. Other fields likely to develop mite problems are those that have received foliar applications of certain insecticides for European corn borer, western bean cutworm or other pests and fields situated next to grasses, ripening wheat or alfalfa. Watch these situations closely for a rapid mite increase.
Detecting the Difference in Mites
Banks Grass Mite
Twospotted Spider Mite
Proper mite identification is important since the twospotted spider mite is much more difficult to control. The most useful characteristic for differentiating between these two species is the pattern of pigmentation. Generally, in older twospotted spider mite females, pigmentation appears as a well-defined spot on each side of the body, ending abruptly just beyond half the length of the body. Banks grass mite females tend to have blackish-green pigmentation extending the full length of the body. Banks grass mites appear earlier in the season and are more likely to remain on lower leaves. Twospotted spider mites appear later in the season and may spread rapidly over the entire plant.
Impact of InsecticidesBefore deciding to treat for spider mites, consider the benefits of that application. Most insecticides have a detrimental effect on spider mite natural enemies. These same chemicals, however, often vary considerably in their effects on Banks grass mites and twospotted spider mites. Some products cause little mortality to either species, while others are somewhat toxic to Banks grass mites. Fewer insecticides/miticides are toxic to twospotted spider mites.
Since products differ in their effects on the two species, it is important to determine the mite species present in the field before making an application. Products that have sometimes been associated with both Banks grass mite and twospotted spider mite problems after their use include permethrin (Pounce, Ambush) and to a lesser extent, carbaryl (Sevin), which under some circumstances may even reduce Banks grass mites. Other products, including parathion, are most likely to be associated with mite population increases only when twospotted spider mites are present. Parathion seems to suppress Banks grass mites but not twospotted spider mites. Still other chemicals have only a slight effect on spider mites or tend to suppress them to some extent, such as Lorsban.
Reducing Spider MitesSpider mite control decisions are based on many factors including the mite species present, level of infestation, growth stage of the crop, cost of application, and market price of the crop.
Recent research in Texas has produced economic injury levels for Banks grass mites and twospotted spider mites on corn (see Table below).
Economic Injury Levels For Spider Mites on Corn Based on Percentage of Infested Leaves and Percentage of Total Leaf Area Damaged
|Control Cost per Acre||Market Value ($)|
TreatmentIt is important to watch corn fields to determine the status of the mites and to watch the weather since it will be a big factor in treatment need and timing. It is best to delay treatments as long as possible to allow for better weather conditions and a chance for natural enemies to have an impact on the mites. However, do not delay treatments after the thresholds have been reached unless the weather or natural enemies are in your favor.
Three chemicals are available for mite control in corn. These are dimethoate, Comite II, and Capture 2EC.
Dimethoate is the cheapest product and should control Bank's grass mites in all of Nebraska. Dimethoate does not provide long residual activity and early treatments may need to rely on two applications -- the first to control the adult mites and a second about a week later to control those mites not controlled as eggs by the previous treatment. These two dimethoate treatments should reduce the mite populations and may provide control for the season without further treatment, but they also will increase the cost to be comparable to a Comite II treatment. Another advantage of dimethoate is it will provide good grasshopper control at field edges.
The best timing for Comite II is just prior to tassel emergence. Comite II will control both Bank's and two-spotted spider mites, provide longer residual activity and be easier on natural enemies. The disadvantages of Comite II are its higher cost and its slower action in controlling mites (difficult to control established colonies). If mite colonies are present beyond the lower few leaves, adding dimethoate to Comite II will help to control these established colonies while Comite II will provide the residual control.
Capture is the most expensive mite product, but it will control both mite species and several other corn insect pests. Capture will provide good residual activity and should be applied when the threshold is reached.
Control with any of these products will be less than adequate if infestations are allowed to become extreme before treatment.
Management ConsiderationsWhen managing infestations of either Banks grass mites or twospotted spider mites, consider the following:
- Reduce moisture stress through timely irrigation.
- Treat only heavily-infested areas of the field to allow for recolonization by natural enemies.
- Increased gallonage (5 gallon minimum) and multiple insecticide applications may improve the degreee of mite suppression.
- In areas where water pH is high (7-9), a buffer such as li700 may help to extend the residual action of some pesticides.
- Corn that has reached the full dent stage is unlikely to benefit from treatment for spider mites.
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