Strawberry growers requested quality checks on the predatory mite products they were buying since they felt the numbers promised on the label did not always match the number of live mites they actually received. Here I summarize the samples we have received so far from January 2021 to May 2022.

We received a total of 55 bottles and one paper bag from growers, which totaled over 160,000 predatory mites from eight companies. Each bottle was labeled as 2,000-4,000 mites, and the bag contained over 500 stems from young bean plants. After submitting a sample, both the grower/PCA and the company received copies of the results. However, the results presented here will not directly show the company’s counts. This is because growers or their staff handled the samples before my lab received the samples and, in some cases, may have been unintentionally mishandled. Therefore, the quality checks presented here represent what arrived from the grower’s hands from their shipments. We have conducted separate counts from samples received from companies who were willing to submit directly. We will continue to work with those companies interested in feedback on their processes to help them improve the quality of the mites they provide to California strawberry growers.

Table 1. Predatory mite samples received for quality testing in 2021-2022 by company.

The samples received in paper bags were counted in the same manner as the bottles (see part 1 of this discussion) and were given as the number of live predatory mites and the number of live two spotted spider mites per bean leaf stem. Since there is no published or accepted quantity comparison using this method of predatory mite delivery, we will report the average numbers here.

Please note that the bottle average counts shown here are only a tiny proportion of the bottles the growers received and represent <5% of a typical shipment a grower will receive for a given field at one time.

Table 2. The average number of live mites per bottle for each species and company for 2021 and 2022.

For the only bean leaf sample submitted, there was an average of 17 predatory mites and 5 two spotted spider mites per <14 inch stem (217 stems counted). Depending on where/when the bean leaf plants were harvested from the greenhouses, the average numbers in different batches of bags may be different so we hope to receive more bean leaf samples for comparison in the future.

Overall, there was a lot of variation in the samples we received in terms of live predatory mite counts, the health of the mites, the sex ratios, and the ratio of adults to nymphs. For simplicity of this post, Table 2 shows the average live counts we received so far since 2021. The quality of the predatory mites may change over time as the demand ebbs and flows during the season for different crops in our region. The size of the company and the amount of resources they can put towards quality control varies among companies and may be reflected in those samples with greater variance. COVID and the labor shortages have, and continue to, affect these companies as well.

We will continue to tease apart issues affecting quality and what the best management practices are to keep those good mites going in your fields, so stay tuned!

If you would like to submit a predatory mite sample, please contact the Zukoff lab (szukoff@calpoly.edu) or your local CA Strawberry Commission representative.