Twospotted spider mite (TSSM) is an important pest of strawberry throughout North America. Although more of a problem in warmer and drier climates, Midwestern strawberry growers may also experience damage from this pest, especially in drier years. Biological control of TSSM has been used by commercial strawberry growers in California for many years, using augmentative releases of various species of predatory mites.
Len Coop, Robin Rosetta, and Brian Croft of the Department of Entomology at Oregon State University have developed a web site with guidelines on how to release the predatory mite Neoseiulus fallacis for TSSM control in strawberry production in western Oregon. The web site, "Release Calculator and Guidelines for Using Neosieulus [sic] fallacis to Control Two-Spotted Spider Mites in Strawberry" is a culmination of four years of research including field trials. The authors advise that their recommendations have been tested only in western Oregon, and they make no claims as to the appropriateness of the methods in other regions. However, their suggestions may serve as a starting point for growers or extension personnel to develop similar practices in other areas.
After a brief introductory section, there is a section describing the predatory mite and reviewing its life cycle, then a detailed discussion of sampling for both the pest mite and predator. The authors correctly emphasize that sampling is critical for pest management decision making. A detailed and clear explanation is provided as to exactly how to sample for both types of mites, including the number of leaves to sample per site and the number of sites to sample per field. Frequency of sampling is also discussed; this varies depending on time of year and weather conditions.
The next section of the web site is a discussion of biological control. In western Oregon, N. fallacis is a common and important predatory mite in strawberry that often provides excellent natural biological control. [Note that in other regions of the country, the species of predatory mite(s) will differ.] If naturally occurring predatory mites are insufficient, or if they are killed by the use of incompatible pesticides, then they may be reintroduced by augmentative releases. Mite release guidelines are based on knowledge of the rate of dispersal of the predators from release sites, which, in turn, is based on degree-day accumulations. This has been developed into a model called the "predator mite release calculator", or MITECALC. Several specific examples of mite releases are provided, based upon TSSM populations, weather, etc. An economic assessment is given for each example. Specific release guidelines are provided, including information on sources of the predators. Methods are also suggested as to how to verify that the predator releases have been successful. A table is given of the effects of various pesticides on N. fallacis. The web site concludes with a list of pertinent references.
One of the problems in augmentation biological control is that, with few exceptions, the guidelines for usage of natural enemies are often rather vague. In many cases, there is insufficient detailed research to make specific recommendations based upon crop and pest conditions, which can vary substantially. The recommendations provided on this web site, and the underlying research, are an excellent example of the type of work necessary to make biological control more reliable and understandable for the user. The web site does have a few editorial deficiencies, such as the incorrect spelling of Neoseiulus in the title, and poorly constructed or punctuated sentences. However, these are minor and hopefully will be fixed in a future version.
- Dan Mahr, University of Wisconsin
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