Spider mite pressure built up significantly in almond orchards in southern San Joaquin Valley and on the West Side of the valley in early July — but those are the only areas where numbers have been high.

This scenario highlights the value of carefully evaluating the need to apply miticides as a preventive measure, according to Walt Bentley, University of California Cooperative Extension IPM Entomologist, based at the Kearney Agricultural Center, Parlier, Calif.

Spider mites, especially the web-spinning species — Pacific spider mite and two-spotted spider mite — can damage almond trees. Even so, some growers may be better off waiting until mites actually show up in an orchard before applying a miticide.

That’s one alternative Bentley offered to about 40 growers who attended a mid-July almond field say near Firebaugh, Calif., sponsored by the San Joaquin Sustainable Farming Project under the direction of the Sustainable Cotton Project, a nonprofit based in Davis.

Farmers can chose from a number of effective miticides, he notes. Rather than risk crop damage, many growers routinely spray miticides before mites build up. But, that may not always be the best approach.

“In many areas of the state north of Fresno, a lot of unnecessary miticide applications are being made,” Bentley says.

Waiting until mite populations reach the treatment threshold level is cost-effective and may make more sense. More importantly, Bentley points out, it can also reduce the chance of the pest becoming resistant to the miticides.

“I’d hate to see that happen,” he says. “In the long run, it’s imperative that growers use these chemicals carefully so mites don’t develop resistance, even if that means suffering some defoliation in the short term.

“Trees can tolerate mites better than farmers can. Mites can develop quickly, forming their webs and defoliating the trees, but this late in the season, trees can handle mites pretty well.”

In the past, mite sampling procedures have required more expertise than many growers had. However, a simpler, more practical method has been developed. A description is available at www.ipm.ucdavis.edu (Under How to manage pests, click Agriculture and floriculture to go to the UC IPM Pest Management Guidelines page. Then, under Insects and mites, click Web-spinning spider mites.)

Still, there are times when a preventive miticide application makes sense, Bentley notes. One is on farms with a history of recurring mite problems where pyrethroids are used regularly and trees may be stressed for water. In such a case, spraying orchards in early May as the mites begin to move into the trees may be a sound approach.

A late season preventive spray may also be appropriate for orchards where mite infestations still exist despite a May treatment, or in historical problem areas. Such areas include orchards on sandy soils, where more alkaline soils components tend to favor spider mite problems, and where withdrawing water in preparation for harvest creates added stress on trees and leaves them more vulnerable to mites.

“At this point in the season, farmers are dealing with a lot of other activities, like irrigation cut off and lining things up for harvest,” he says. “It’s often easier to spray a miticide than to monitor and sample trees to see if a control treatment is necessary.

“I’d like farmers be aware of the benefits of sampling to determine a real need for a miticide before spraying and to do some introspection about what they’re comfortable doing in managing mite problems,” Bentley says. “If they felt like they could implement this kind of program, it would be worthwhile. We can do better.”