Twospotted spider mites on underside of leaf
Photo: Raymond Cloyd, Kansas State University

insecticides and miticides are widely used to alleviate problems with insect and mite pests in greenhouse production systems. However, relying continually on insecticides and/or miticides can result in resistance developing in insect and/or mite pest populations. Therefore, greenhouse producers need to understand the mode of action of designated insecticides and miticides to develop effective rotation programs that will mitigate/delay resistance. Mode of action is how an insecticide or miticide affects the metabolic and/or physiological processes of an insect or mite pest. The mode of action of all insecticides and miticides can be found in the Insecticide Resistance Action Committee (IRAC) document entitled, “IRAC Mode of Action Classification Scheme,” which is available on the website irac-online.org This article discusses the IRAC mode of action group 10, which are referred to as mite growth inhibitors. The three mite growth inhibitors available for use in greenhouse production systems are: etoxazole (TetraSan 5 WDG: Valent U.S.A. Corp., and Beethoven TR: BASF Corporation), clofentezine (Notavo [formerly Ovation and Applause]: OHP), and hexythiazox (Hexygon: Gowan Comporation). Clofentezine and hexythiazox are classified as 10A; whereas etoxazole is classified as 10B.

Mite growth inhibitors have contact activity only with no translaminar properties. Therefore, thorough coverage of all plant parts is important, especially leaf undersides where spider mites typically reside. They are primarily active on spider mites (Tetranychidae) including: twospotted spider mite (Tetranychus urticae) and Lewis spider mite (Eotetranychus lewisi).

Close-up of twospotted spider mite (Tetranychus urticae)
Photo: Raymond Cloyd, Kansas State University

Mite growth inhibitors are less active on broad and cyclamen mites (Tarsonemidae) and eriophyid mites (Eriophyidae). The mite growth inhibitors are slower-acting than most contact or translaminar miticides, and they only affect eggs and the immature stages (larvae and nymphs) of spider mites, with no direct effects on adults. However, eggs laid by female spider mites may be directly killed by residues, or females exposed to residues may not produce viable eggs.

The mite growth inhibitors that are commercially available have long residual activity — up to 45 days (1.5 months) from initial application. If existing spider mite populations include adults, then the mite growth inhibitors should be mixed with a miticide with adult activity; such as, abamectin (Avid: Syngenta Crop Protection), bifenazate (Floramite: OHP), or chlorfenapyr (Pylon: BASF Corporation).

The mode of action of the mite growth inhibitors is associated with disruption of chitin synthesis during the molting process, thus inhibiting embryo development and larval maturation. However, the specific target site or proteins affiliated with inhibition are not known. Do not use mite growth inhibitors in succession (back-to-back) as a mutation in the chitin synthase gene has been shown to confer resistance to etoxazole, clofentezine and hexythiazox. Therefore, minimize applications or use in rotation programs to reduce the potential for spider mites developing resistance. In general, mite growth inhibitors can be used in conjunction with most predatory mites, parasitoids and predatory bugs.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits. rcloyd@ksu.edu or 785-532-4750