We have been dealing with more problems associated with twospotted spider mite, Tetranychus urticae, this year than in previous years--mainly due to the dry conditions we have been experiencing throughout most of Illinois. However, another reason for the abundance of twospotted spider mite populations may be an indirect result of the heavy regional use of pest-control materials such as carbaryl (Sevin) and/or cyfluthrin (Tempo) for managing Japanese beetle adults this year. Japanese beetle adults were extremely abundant in many portions of the state, and carbaryl and cyfluthrin were used routinely (sometimes daily) to kill as many Japanese beetle adults as possible. Many of the heavy mite infestations are on silver maple, sugar maple, and oaks--trees typically not damaged or damaged only lightly by Japanese beetle. Commonly, in these situations, all of the trees and shrubs were sprayed for Japanese beetle regardless of the level of infestation or damage.
Carbaryl and cyfluthrin are very effective in man-aging the adult stage of Japanese beetle; however, both pest-control materials have broad-spectrum activity or kill many types of insects and mites. As a result, they are also very harmful to natural enemies, including predatory insects and mites that "naturally" regulate twospotted spider mite populations. In the absence of these predatory insects and mites, twospotted spider mites can increase in great numbers. This results in pest-control materials being needed regularly to prevent twospotted spider mite injury to plants. Also, natural enemy populations take much longer to build up in numbers to have any influence on the mite populations. This scenario is often referred to as a secondary pest outbreak.
Secondary pest outbreak, or pest replacement, is a situation in which a major pest (that is, Japanese beetle) is suppressed and continues to be suppressed by a particular pest-management tactic such as the use of pest-control materials. But it is then replaced in importance by another pest (that is, twospotted spider mite), previously a minor pest. Secondary pest outbreaks often occur in landscape settings after applications of pest-control materials. For example, spraying malathion for mosquito control in residential areas has led to outbreaks of pine needle scale on mugo pine because the malathion sprays kill the natural enemies of the scale.
There is a classic saying that"for every action there is a reaction." Nature doesn't exist in a vacuum, and once one organism or a group of organisms is removed, something else will occupy the niche. Although in the short term, it is important to control a given pest, it is also imperative to consider the long-term implications of a pest-management tactic. Proper stewardship of pest-control materials is important to prevent secondary pest outbreaks.