It is easy to spot California pest control advisers who scout coastal strawberries. A hand lens dangles down the front of their work shirt.

And they are always peering at leaves, looking for spider mites.

The strawberry two-spotted spider mite is the most persistent and potentially damaging pest strawberry growers face each year, according to Mark Bolda, University of California farm advisor for strawberries in the heart of Central Coast strawberry production, Santa Cruz, Monterey and San Benito counties.

“After soil pathogens, mites are the biggest problem strawberry growers face,” he said. High populations early can have season-long impact on strawberry production. Reducing mite populations early and keeping them low is imperative.

“The effects of damaging mite feeding can last all the way to the end of the season. A little bit of damage early can have a big impact,” said Bolda.

Coastal strawberries are one of the most expensive California crops to grow, costing on average about $40,000 per acre. Most of that is labor for a crop that is picked as often as every other day from April to October, producing up to 6,000 trays or more per acre.

Growers spend an average of about $1,500 per acre (excluding fumigation) for pest control, including mites, insect pests and diseases. Resistance pressure can intensify with each pest treatment.

Fortunately, two-spotted mites can be controlled effectively in coastal strawberries using one of the most successful integrated pest management (IPM) programs in California.

Pest control adviser and field research director for Plant Sciences, Watsonville, Calif., Mike Nelson, was almost ecstatic as he looked through his hand lens recently in a test plot for a new miticide, Zeal, from Valent.

“Look at the Persimilis moving,” he proclaimed.

Predator mite

Persimilis is a commonly heard word in coastal strawberries fields. Persimilis are specifically, Phytoseiulus Persimilis, one of the most effective predator mites ever introduced into California agriculture.

Coastal strawberry producers release from 20,000 to 40,000 Persimilis per acre at a cost of about $6.50 per 1,000. It saves them two to three miticide applications per season, according to Bolda.

The predator mites are easy to spot. They have an orange color to them and when Nelson sees them moving after a miticide treatment, he is well pleased. Persimilis feed only on two-spotted spider mites.

“It is very important that any new miticide that comes into the strawberry market fit into the integrated pest management systems which utilize Persimilis,” said Nelson.

Just as important as saving money, it reduces the chance of resistance buildup to several relatively new, highly selective and effective acarcides.

“Strawberry growers have some super selective miticides right now. They are effective against the two-spotted spider mite and do not have a negative impact on Persimilis” said Bolda.

In that category are Acramite, Savvy and Agri-Mek. Zeal will be a fourth and it will bring a new mode of action into the mix.

“It will be a welcome rotation partner with the other miticides,” said Bolda. Zeal, a reduced risk pesticide, now has a federal label and Valent expects it to be fully registered late this fall in California.

Sterilizes eggs

Zeal is a molting inhibitor ovicide that sterilizes eggs and prevents eggs from hatching. Like many new reduced risk products, the label will limit product use to one application per season.

The predaceous Persimilis mite has long been in strawberry fields, but was not an economical predator until a mass release strategy was developed.

“Most of the early research on the Persimilis mite was in small plots in the middle of strawberry fields,” explained Nelson.

The mites would be released and monitored in those small areas. However, Persimilis are very mobile and they would move from those small plots into other areas of the field. That would dilute their number in the test plots, rendering them ineffective for control of the two-spotted mite.

However, it was discovered when they were released in large, sufficient numbers over an entire block, they proved highly effective.

Two companies are the primary providers, Koppert Co. in Holland and Syngenta BCM (Beneficials for Crop Management) in Southern California. Plant Sciences is the exclusive Koppert distributor for Persimilis.

“The key to integrated control of two-spotted mites is to get Persimilis released early in the season when the pest mites just begin to show up,” said Nelson.

Once a predator/prey ratio of approximately 1:20 is achieved, the pest population will be reduced to near zero within seven to 10 days when environmental conditions favoring the predator are ideal. Ideal is typical coastal cool temperatures and high humidity.

“At times a selective miticide application is necessary to take the peak off the two-spotted mite population, particularly when there is warm, dry weather favoring the pest mite,” said Nelson.

“When you can release high numbers of Persimilis you get a predator/prey ratio that gives good control,” said Nelson.

“We have some good miticide tools right now for mite control. Zeal will add another important element to this IPM strategy,” said Nelson.

e-mail: hcline@primediabusiness.com