Selectivity of Insecticides and Miticides

Some pesticides are less toxic than others to natural enemies such as green lacewings, lady beetles, parasitic wasps, beneficial mites, and predatory bugs. When an insecticide causes less harm to the natural enemies than the pests, it is a selective insecticide. Insecticides that kill a wide range of insects, including natural enemies, are called broad-spectrum. Examples of broad-spectrum pesticides include most organophosphate (malathion, dimethoate), carbamate (Sevin, Lannate), pyrethroid (Baythroid, Danitol), and neonicotinoid (Admire, Assail) insecticides. To reduce the detrimental effects on natural enemies, use broad-spectrum insecticides in a selective manner (e.g., in baits, spot treatments, at times of the year when natural enemies are not active, or applied systemically through irrigation water).

Within each of the major groups of pests in citrus, there are selective and broad-spectrum pesticides that can be used to control them. For instance, lepidopterous pests, such as fruittree leafroller and citrus cutworm, can be controlled with broad-spectrum carbamate insecticides (Lannate), with selective Bacillus thuringiensis microbial insecticides (Dipel), or the stomach poison cryolite (Prokil Cryolite). Armored scale can be controlled with broad-spectrum carbamate insecticides (Sevin), selective narrow range oils, selective insect growth regulators such as pyriproxyfen (Esteem) and buprofezin (Centaur), spirotetramat (Movento), or by releases of Aphytis parasites. Botanical, microbial, and oil insecticides are not disruptive to most natural enemies because of their limited persistence. In addition, some broad-spectrum pesticides can be relatively selective when used infrequently, with outside coverage, or at low rates. For example, a low rate of a highly broad-spectrum pyrethroid applied as outside coverage can easily kill katydids without disrupting natural enemies needed for scale control.

Some insecticides are selective or safe for one group of natural enemies but not another. For example, lacewings are naturally tolerant of pyrethroids, but parasitic wasps, predatory mites, and beetles are very susceptible to this group of pesticides. Insect growth regulators such as pyriproxyfen (Esteem) and buprofezin (Centaur) are safe for parasitic wasps but are very toxic to beetles such as the vedalia beetle needed for cottony cushion scale control. The miticide and insecticide spirotetramat (Movento) is safe for predatory and parasitic insects but is quite toxic to predatory mites.

Both selective and broad-spectrum pesticides can be quite persistent, or residues may degrade quickly. For example, both sabadilla (Veratran D) and pyriproxyfen (Esteem) are relatively selective, but sabadilla persists only for a short period of time (several days), whereas residues of pyriproxyfen are quite persistent (can harm predatory beetles for 2 to 3 months or more). If a persistent, broad-spectrum pesticide has been applied, residues on the plant may be harmful to natural enemies for weeks or months. For example, if adult Aphytis wasps are placed in a jar with leaves that were sprayed in the field with the dilute rate needed to control California red scale, carbaryl (Sevin) affects adults for 5 months after the pesticide application. In the case of the predatory mite Euseius tularensis, some San Joaquin Valley populations have developed resistance to organophosphates and their populations are largely unaffected by this group of pesticides. Occasional, single applications of broad-spectrum pesticides are much less harmful to natural enemies than multiple selective pesticides.

It is important to carefully consider the selectivity of a pesticide when making a treatment decision. This includes the effect the pesticide has on nontarget species, its persistence in the environment, and whether or not resistance to the pesticide has developed. These factors are listed in the table below and under each recommendation in the guideline.

• Range of Activity is listed first and denotes the degree of selectivity each pesticide recommendation has, along with the group of organisms primarily affected by the application. For example, the range of activity for dimethoate is listed as "broad (insects)." This means dimethoate affects most groups of insects. It also harms beneficial mites as noted in the next column. On the other hand, Bacillus thuringiensis is listed as "narrow (caterpillars)" because it only affects caterpillars.
• Persistence, or the length of time a pesticide remains effective or toxic, is listed next and is categorized as short (days), intermediate (up to 6 weeks), or long (months).
• Resistance information is included in instances where it has been observed in either the pest or natural enemy.

Factors Affecting the Selectivity of Treatment Materials for Citrus Pest Management