“If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.”
― Sun Tzu, The Art of War

Author’s note: This is a continuing project, data analysis is not yet complete, and more trials are planned in 2026. We feel it is important to share progress on our findings that provide evidence for uncommon practices that may be rooted in biological and economic reality.

Spider mites have long been a headache for California strawberry growers, causing substantial yield reductions and economic losses if left unmanaged. The main culprit is the two spotted spider mite (Tetranychus urticae, TSSM), which is characterized by the signature two spots on their back. However, Lewis mites (Eotetranychus lewisi), a newer pest first found in Ventura in the early 2000s, have become a bigger issue since the early 2010s and have continued to spread north to the Santa Maria and Watsonville/Salinas growing districts. To the naked eye, they look almost identical to TSSM and a microscope or hand lens is usually required to tell the difference. (You can read more about the differences between the two here.) 

It is a well-known fact within the strawberry industry that the predatory mite Phytoseiulus persimilis is a good management option when trying to control TSSM. The catch is that P. persimilis is a specialist, it feeds only on Tetranychus species and ignores Lewis mites. This is where the generalist Neosieulus californicus comes in. In 2013, Dr. Anna Howell from UC Extension was able to show that N. californicus will feed on both TSSM and Lewis mites, making it a great fit for California’s mixed mite pressure. The challenge now is determining how to use these predators most effectively and economically.  

In a trial with Biobee, a producer of biological agents, we tested preventative high rates of N. californicus to manage spider mites. By “high rate,” we mean releases of around 600,000 mites/acre per season, which was far beyond anything typically practiced here in CA, but appears to be common in Israel. We decided to test that high-rate preventative strategy under local conditions. From December 2024 to May 2025, we evaluated three treatments replicated four times: the grower control, preventative high rate of N. californicus, and a curative release program of both N. californicus and P. persimilis which were both at lower rates. Preventative N. californicus was released four times starting in December at 200,000 mites/acre. Grower control N. californicus applications began in January 2025 at 30,000 and 180,000 mites/acre with P. persimilis released once at 100,000 mites/acre in April 2025. The curative release strategy which released N. californicus once at 150,000 mites/acre in February 2025 was followed by P.  persimilis released four times beginning in February 2025 at 40,000 mites/acre. 

Throughout the trial, fields that received preventative high rates of N. californicus releases maintained lower populations of both TSSM (Fig. 1) and Lewis mites (Fig. 2) for the longest time compared to the curative and grower standard applications. However, despite this, mite levels in all treatments eventually surpassed the economic threshold of 10 mites per leaflet  

The NDVI imagery clearly told the same story we were seeing on the ground. Plots treated preventatively with N. californicus showed healthier, more vigorous plants, while the rest of the field displayed overall poor health (Fig. 3). In fact, the contrast was so distinct that many of the field workers believed the treatments were different varieties.  

These results highlight the potential of high-rate preventative releases of N. californicus for improving spider mite suppression in strawberries. It also reinforced how important it is to keep testing and sharing what works, so growers have practical guidance on using biological control effectively.  

Caveat and Critiques

There are major caveats to this first year of the study. Our grower control plots were not infested in a timely manner due to logistical issues from the grower’s end. The high rate of N. californicus is a cause of concern in terms of cost. An economic analysis will accompany the next iteration of this trial. The study site was criticized by some PCAs for being a very suboptimal area for organic strawberry production. Another critique was that we did not verify the number of mites within the bottles we used. In future trials we will randomly select a bottle and do a volumetric sample to confirm numbers, especially if we are comparing two or more predatory mite producers. Despite these caveats, we present this information to the industry because of the concern we have with Lewis mite outbreaks becoming an increasing threat to organic production. Phytoseiulus persimilis will not feed on Lewis mite and the need for a dual or triple mite strategy may be necessary for a) dealing with both species of mites and b) preparing for any ill-conceived regulatory strategies that may limit miticides in the future.

Next Steps in 2026

We know some PCAs already use a combination of N. californicus and P. persimilis in response to joint Lewis mite and TSSM infestations in the field. However, if Lewis mite levels increase across more acreage, we may need to use both species of predatory mites more frequently. We need to repeat this experiment with increased replications or a larger area to have more confidence that the high N. californicus rate is viable and our 2025 results were not a fluke.

Next year’s version of the experiment will be done in a conventional field with a grower on the west side to avoid the challenges we faced in the organic field. We are currently planning a trial with two insectaries to examine field performance of testing two N. californicus releases and four P. persimilis releases against a standard miticide only program. Together, these studies will help confirm our initial findings and refine how timing, release rate, and source influence the consistency and success of biological mite control in California strawberries against Lewis mite and TSSM. These 2026 studies will be presented at the California Strawberry Commission meeting in the late spring. We enthusiastically look forward to presenting this data.