Written by David Doll (UCCE Merced) and David Haviland (UCCE Kern)
As summer approaches almond growers need to be monitoring for webspinning spider mites. The most common species during the summer are Pacific and twospotted spider mite, though on occasion strawberry spider mite can be found. As adults, all three mites look and behave similarly, even though control of Pacific spider mite is more difficult to control.
Webspinning spider mites overwinter in the orchard under rough bark and ground litter. During the spring, usually in March and April, migration will occur from these places into the lower areas of the tree. These early-season populations are typically small, do not reproduce quickly, and often become prey to early-season natural enemies. However, as temperatures warm in late May through September, the populations increase rapidly, particularly if natural enemies are absent. In favorable conditions, a lifecycle can be completed in 7-10 days, with 8-10 generations a year.
Spider mites damage leaves by sucking cell contents and damage initially appears as a light stippling. As populations increase, mites and their eggs become more visible, and eventually “webs” will appear around spurs and leaves. Leaves with high populations will fall from the tree, reducing carbohydrate production. If leaf loss becomes severe there is a potential for crop loss the following year. Spider mites are most problematic in orchards that are dusty or where the trees are stressed. Stressing factors can include insufficient irrigation, nutrient deficiencies, or excessive crop load.
The most important factor in spider mite management is biological control. When natural enemies are abundant, miticides are not needed. When natural enemies are absent, control with even the best miticides can be short-lived. The goal is to find a balance that relies primarily on natural enemies supplemented by miticides as needed.
The most important predators of spider mites are sixspotted thrips, western predatory mite, and the spider mite destroyer. Sixspotted thrips are highly migratory and can quickly control spider mite populations. Over the past few years it has become the predominant mite predator in California almonds. The western predatory mite- although similar in size as spider mites- is whitish in color and often moves quickly across leaves. The spider mite destroyer is a small black ladybug that feeds exclusively on spider mites. It is found primarily during the second half of the season. These predators are very effective in controlling mite populations, and in orchards with high predator-to-prey ratios, treatment may not be required. Growers who want to conserve and promote predators should avoid broad spectrum insecticides, particularly early in the season, and avoid prophylactic and preventative miticide applications that do not provide enough food for mite predators to become established in the spring.
In determining when to time the first mite spray application, a presence/absence monitoring protocol has been developed. Monitoring should occur weekly, and prior to mid-June should focus on hot-spots within the orchards. After mid-June, the whole orchard should be randomly sampled.
When sampling trees, 15 leaves from a minimum of five trees should be selected. Leaves should be randomly chosen from the inside and outside of the canopy. Examine both sides of the leaves looking for pest mites and predators. Note the number of leaves on each tree with pest mites and their eggs, and the number of leaves with predators. There is no need to count the mites. Once completed, compare the numbers with the guidelines provided in the “Don’t Treat” and “Treat” columns on the sampling form provided at the webspinning spider mite link of the UC Almond Pest Management Guidelines.
Many miticide options exist for cases where a treatment is required. Each miticide has its strengths and weaknesses, and for that reason options should be discussed with a pest control advisor. For almond growers in the San Joaquin Valley that plan on using miticides containing abamectin, keep in mind that new regulations require that only low-VOC formulations be used between May 1 and 31 October 2015 and 2016. More details on this new regulation and all miticide options can be found in the UC Almond Pest Management Guidelines.