Biological Control Project

Optimizing spider mite management through biological control in walnuts and vineyards

The web-spinning spider mite (Tetranychus spp. and Eotetranychus willamettei) is a pest that causes severe leaf drop and reduced photosynthesis and plant growth. Walnut and winegrape growers routinely struggle with spider mite management, but have limited low-risk effective options. Furthermore, dependence on limited chemical options is of particular concern for spider mites as they can rapidly develop resistance to pesticides. These factors increase the need for alternative management practices that focus on biological control, such as:

In partnership with CDFA, UC Cooperative Extension, Pest Control Advisors (PCAs), and commodity boards, this project will implement research trials on eight farms in the Sacramento and San Joaquin Valley to investigate the efficacy and cost comparison of these three practices. Over two seasons, the Ecological Pest Management (EPM) team will monitor for web-spinning spider mite and its natural enemies. In addition to this research, the EPM team will provide technical assistance and robust outreach and extension to promote knowledge-sharing about these biological control practices.

Predatory Mite Release

Mites of the Phytoseiidae family, or commonly known as predatory mites, are a key predator of web-spinning spider mites. When making treatment decisions, it is not only important to consider the population of spider mites, but also the ratio of spider mites to predatory insects. Maintaining an adequate ratio can help keep spider mite numbers low enough without having to spray a miticide. Releasing predatory mites can supplement the already existing predator population to keep the predator-to-spider mite ratio favorable. These releases can be done by hand, leaf blower, or drone.

Predator Attractants

Plants respond to herbivore damage by releasing chemical signals called herbivore-induced plant volatiles (HIPVs). The natural enemies of the herbivore pests use these HIPVs to direct themselves to their prey. A synthetic HIPV can be applied to attract beneficial insects earlier in the season or when pests are first detected. One such HIPV, called methyl salicylate (MeSA), has been shown to attract various natural enemies, including the predatory mite N. californicus, big-eyed bugs, hover flies, minute pirate bugs, spider mite destroyers, lacewings, predatory mirid bugs, dance flies, parasitic wasps, and flesh flies. Applying this plant volatile enhances the beneficial predator and parasitoid populations, reducing reliance on miticides that disrupt biodiversity.

Predator Food Sprays

For natural enemies to properly thrive on a farm, they require a proper habitat and food source. For omnivorous predators, this food source can include nectar, honeydew, and pollen. Predator food sprays act like these sugar and protein rich plant-based foods to support beneficial insect populations when pest populations are still low. By providing this supplementary food source, growers can keep natural enemies in the field for longer periods and prevent any flare ups that would normally need to be treated with pesticides. This is a preventative strategy rather than a reactive one, and will work best when the beneficial insects are not disrupted by non-selective pesticide applications.