“How many predatory mites are in there?” Spider mite biocontrol in California strawberries: Part 1

Many strawberry growers in California have happily embraced biological control to help them manage their ever-growing spider mite populations in both their organic and conventional fields by releasing commercially grown predatory mites (PM). However, some have expressed concern over the control they are seeing (Fig. 1) and are worried they will have to rely more heavily on miticides [whose effectiveness is also quickly dwindling due to resistance…but that’s a post for another day]. To start to address these concerns requires a thorough investigation into the many factors that could disrupt PM effectiveness both before and after the styrofoam boxes filled with predatory mites arrive at the field. This will be the first in the series of blog posts whose goal is to discuss this vital and expansive topic of predatory mite biocontrol use in strawberries.

Figure 1. Two spotted spider mites overtaking a strawberry plant in Ventura, CA despite repeated releases of predatory mites.

Just the numbers

The quality of the predatory mite product has been one factor that has been brought up amongst the growers and PCAs who work in CA strawberries. But what does a high-quality PM product look like? This is a harder question to answer than most would think. There are no industry-wide, agreed-upon quality measures published for assessing what the perfect PM product is. One publication that tried to address this topic came up with a measure for two popular predatory mites species and included 1) percent survival, 2) number of eggs laid, and 3) the number of mites in the container (Table 1). Because of this, companies all have their individual quality standard that tends to differ from organization to organization (and will be addressed in a later post) because commercial PM companies have great variation in their operation size, logistics, and their ability for R & D/quality control.

Table 1. Suggested guidelines were published in 2003 (van Lenteren) for quality control of commercially produced predatory mites.
Table 2. Zukoff lab assessment and rating scale for commercial predatory mite quality testing.

After discussing the growers’ concern with the PM companies that primarily serve strawberry growers in CA, my lab has modified the van Lenteren, et al. (2003) guidelines for assessing PM quality across all companies (Table 2). This process however may evolve in the coming year as more discussions are had with the PM industry and the continuing work to establish industry-wide best practices is further pursued. Some in the commercial PM industry have had the opportunity to test our methods and the reasons for this are two-fold: (1) the PM companies must have confidence in our method of quality assessment and have the opportunity to provide feedback; and (2) to generate a standard of commercial PM assessment that can be used across companies, consultants, and growers in the hope that it could ultimately make a positive difference in the use of PM in California strawberries through higher quality and greater consistency (and perhaps other crops as well).

Want to know how it’s done?

Figure 3. Equipment setup for determining predatory mite quantity (left) and fecundity (right).
  • To assess the number of live mites, we use a simple modified Berlese funnel consisting of a small plastic container with the bottom cut out then lined with soft tulle netting (Fig. 3 left). The whole plastic container sits on popsicle stilts with a 40-watt incandescent bulb above to help warm the mites so that they are moving after being stored in the chilled shipment containers. The entire assembly is positioned in the middle of a large sticky card. The mites move either through the fabric in the bottom of the container or will crawl out and down the container where they will get stuck onto the sticky card. The cap and inner cap sit in the plastic container and a sticky card is placed on the upright bottle to catch any mites left in the container. This type of container can be used to assess bean leaves, vermiculite, or sawdust samples. Up to 6,000 mites can be used in each container. Extra-large bottles of 20,000+ predatory mites often used in drone releases require several containers for a full assessment.
  • To assess fecundity or the number of eggs laid, we use insect rearing containers with bean leaf disks sitting aloft moistened cotton. Each cell has one predatory mite adult female and is provided with a bean leaf disk full of healthy spider mites and spider mite eggs (Fig. 3 right).

The assessments currently require about 8 hours per sample and we are working on ways of automating the process to make it more efficient and easily adaptable to cover various needs and possible surges in requests.

Want to know the results of the samples we received in 2021?
Stay tuned for next week’s exciting blog post!

If you need PM samples assessed, contact our lab or your nearest Strawberry Commission field rep.

Have questions, comments, or snide remarks? Feel free to contact me!
Dr. Sarah Zukoff, szukoff@calpoly.edu, 805-540-4876 (cell).

2 thoughts on ““How many predatory mites are in there?” Spider mite biocontrol in California strawberries: Part 1

  1. What a cool study Dr. Zukoff! It’s interesting to see that PM quality hasn’t been standardized yet but it’s promising to see your work in bringing greater consistency to PM shipments. Food product labels and chemical safety data sheets (SDS) are standardized which makes comparisons and expectations for products easier to gauge. I see this being especially important so we don’t have to count every mite during PM application! Important stuff for our growers! Keep up the good work.

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