Spider mites in saturated strawberry fields

Twospotted spider mites prefer lower humidity, yet they are reproducing and even thriving in some strawberry fields with flooded furrows, even right now after the historical rain storms. Although the flooding causes higher humidity in the field overall, the microclimate around the plant is still hospitable to spider mites (Fig. 1). As long as the humidity under the leaves isn’t above 90%, the mites will do fine in the absence of insect-eating fungi like Neozygites (Fig. 2), which we don’t often get on the Central Coast due to the cooler climate during the rainy season. Excess water in furrows and saturated soils due to flooding may even stress the plants, making them more susceptible to spider mite damage. Flooded furrows also are an impassible barrier to predatory mites unless they take a chance and try to catch the wind to get to another area. In the absence of predators, the spider mites are allowed to reproduce unchecked.

Figure 1. Soggy soils in many fields are still harboring spider mites despite the higher humidity.

Predatory mites do well in higher humidity, and if released on plants with at least some spider mites for food, they should do well unless temperatures are outside optimal limits (Fig. 3). However, if predatory mites are not established on strawberry plants despite their basic needs being met, there may be other factors, such as chemical exposure, mite product quality issues, or other weather-related issues that are preventing predators from establishing. Our lab is working with the California Strawberry Commission research team and other collaborators to work out what these other factors are and how growers could tweak practices to make their plants safer for predators while lowering spider mite populations.

Figure 2. Neozygites species of entomopathogenic (insect-eating) fungi need several days of hot and humid weather to live in spider mite populations. Normally, the rainy season on California’s coast is too cool for this fungus to develop in spider mite populations on strawberries. When present, this fungus can kill entire fields of spider mites in just a few days. The fungus requires an average daily temperature of 85°F, dew in the mornings, and 12+ days of relative humidity over 80% to move quickly across spider mite populations.
Photo Credits: Predatory Mites: 1. Biotactics;  2. University of Florida;  3. naturalenemies.com; 4-5. Evergreen Growers Supply; 6. USpest.org; 7. BioPlanet. Pest Targeted: A. HealingCanna; B. UCANR C; UC-IPM; D. Biobest;  E. Nigel Cattlin

Questions or comments? Email author Dr. Sarah Zukoff @ szukoff@calpoly.edu

Sarah Zukoff, PhD
Entomology Program Leader & Research Entomologist
Strawberry Center
California Polytechnic State University
San Luis Obispo, CA