Mites vs Predators vs Adjuvants vs Assumptions

My master’s student Colin Koubek has finished the first phase of a series of experiments examining a selection of fungicides used in 2023 and their interactions with selected adjuvants on the survival of Phytoseiulus persimilis. This effort is part of a larger research project to examine P. persimilis quality from the bottle to the field.

As many of you know, biological control agents are more finicky than a five-year-old at Fuddruckers. From rearing through release, there are many points of failure that can cause reduced efficacy that results in a rageful grower.

**Figure 1.** DALL-E generated image request by author of Schwarneggar’s Predator film using mites instead of humans.

I am sharing data from one of the rounds of experiments below to offer a glimpse as to what Mr. Koubek has found so far. The fungicides he tested were Captan, Switch, Topsin, Microthiol Sulfur, Luna Sensation, and Thiram in combination with Widespread, Broadspred, Silwet, Dyne-Amic, and Kinetic.

**Figure 2.** *Phytoseiulus persimilis* mortality rates when tested against Captan Gold, Switch, Broadspred, Widespread, and Silwet. Fungicide only treatments are peach colored. Adjuvant only is blue. Mixes of fungicide + adjuvant are in red. Data generated using a Potter spray tower. Data is not peer reviewed. **This data is from early rounds of testing. Graph was updated from earlier published version. Full updated data set will be presented at meetings in 2025.**

The data right now suggest that the fungicides Captan Gold and Switch by themselves are not harmful to P. persimilis, which confirms previous research. The harm is coming from not realizing that the tank mix with an adjuvant such as Silwet may pose a problem. There are other combinations that are surprising which will be further discussed.

Initial rounds of testing seemed to suggest that Broadspred was toxic to P. persimilis but later rounds of testing found it was not. These assays require feeding two spotted spider mites (TSSM) to P. persimilis to avoid starvation as a confounding factor. In the initial rounds the Broadspred had killed the TSSM which inadvertently killed the P. persimilis. This is one of the reasons why we have to take our time with this type of research.

Colin will be presenting the rest of the data, new conclusions, and further expansions on this project on April 25th, 2025 at the California Strawberry Commissions’ production meeting in Santa Maria.