Webspinning Spider Mites

These three spider mites are difficult to distinguish as adults, have similar life histories, and are controlled in the same manner. However, Pacific mite is often the most difficult to control with miticides.

The overwintering female mites are red- or orange-colored and are found under rough almond bark, in ground litter, and on winter weeds. During the season the color ranges from yellow to green to black depending on age and host food. All have dark spots. Adult males do not overwinter and are smaller than females. Eggs are laid on the foliage. Immature mites molt three times. Early in the season, mites are found in lower to central areas of the tree. The mites reproduce rapidly during warm weather between June and September. During favorable conditions, mites develop within 7 days, with 8 to 10 generations per season.

Mites damage foliage by sucking cell contents from leaves. The damage begins with leaf stippling. Leaves can turn yellow and drop off. High numbers of mites cover tree terminals with webbing. Crop reduction and reduced vegetative tree growth shows up the year after damage occurs.

Spider mites are often a problem in water-stressed orchards. Mites can also become a problem when their natural enemies are disrupted by the application of broad-spectrum insecticides including pyrethroids, which kill many types of predatory insects and predator mites. Almond trees can tolerate moderate mite numbers without suffering economic damage. Predators are important in managing mites, so consider their presence and relative abundance before treatments are applied. Orchards with high predator-to-pest mite ratios will not require treatment.

Monitor orchards for both predators and spider mites at least once every 2 weeks from March to early May; monitor at least once a week after that. When treatments are required, choose selective miticides that cause the least harm to predators.

Several species play an important role in mite control, including western predatory mite (Galendromus [= Metaseiulus] occidentalis), sixspotted thrips, the spider mite destroyer (Stethorus sp.) and minute pirate bugs (Orius sp.). In recent years the sixspotted thrips has become the most common insect predator of spider mites. Sixspotted thrips is an extremely good mite predator. It is highly mobile and both nymphs and adults feed on spider mites. However, movement of sixspotted thrips into orchards can be unpredictable and sometimes does not occur until mites have already exceeded treatment thresholds. If sixspotted thrips is present in an orchard, avoid using pyrethroid or spynosyn insecticides or miticides containing abamectin to preserve their presence and maximize biological control.

The most widespread predatory mite is the western predatory mite. It is about the same size as spider mites, but the western predatory mite lacks spots and its color ranges from cream to amber red. It often can be observed moving quickly over the underside of leaf surfaces in search of spider mites. This predator maintains good control, unless the proportion of leaves with spider mites is higher than the proportion with predatory mites.

Predators will typically control webspinning mites if presence-absence sampling indicates equivalent numbers of leaves with predators and with webspinning mites. When predator mites are present, but are not controlling the spider mites, a lower-than-label rate of a selective miticide may be applied to create a more balanced ratio (i.e., a 1:1 ratio of one leaf with a predatory mite for every leaf with a webspinning mite). If predatory mites are present in an orchard, choose a miticide with reduced toxicity to predatory mites. See RELATIVE TOXICITIES OF PESTICIDES USED IN ALMONDS TO NATURAL ENEMIES AND POLLINATORS for a list of pesticides used on almonds and their toxicity to the western predatory mite.

Predatory mites can be purchased from insectaries and released to augment natural populations of predatory mites. Before releases, be sure that insecticide residues have sufficiently broken down to allow the survival of predatory mites. For example, research has shown that some pyrethroid insecticides can be highly toxic to predatory mites for several months to more than a year after application. In some cases, predatory mites from insectaries have been reared from stocks with tolerance to one or more pesticides, such as organophosphates and carbaryl, enabling survival in orchards if residues are present. Keep purchased mites cool when they are received from the insectary and release them as soon as possible. Release predatory mites when spider mites are present (to allow predatory mites to feed) but well before treatment thresholds are reached. Predatory mite releases are not advised in areas where sixspotted thrips are present, since the predatory thrips are known to eat both spider mites and predatory mites. For more detailed information on predatory mite releases, see Integrated Pest Management for Almonds, UC ANR Publication 3308 and Natural Enemy Releases for Biological Control of Crop Pests.

Reduce dusty conditions in orchards by oiling or watering roadways and maintaining a ground cover. Prevent water stress, as this condition results in higher mite numbers and makes trees more susceptible to infestation and damage.

During the season, avoid using broad-spectrum pesticides such as pyrethroids, carbamates, and organophosphates (unless organophosphate-resistant predator mites are present in the orchard); the use of these insecticides will often result in spider mite outbreaks.

Biological controls, including predator releases, cultural controls, and various types of oil sprays are organically acceptable ways of managing spider mites.

• Monitor for mites at least weekly from May through August. Treatment is not necessary after August because mites begin to migrate off trees to prepare for overwintering.
• If the orchard has problem areas, such as trees along roads or water-stressed trees, monitor every few days.
• Before July 1, focus monitoring on hot spots—that is, the areas that develop mites first; these are often dusty or water-stressed areas of the orchard. Once the treatment threshold has been reached in these areas, sample the remainder of the orchard to determine if a spot treatment is sufficient or the entire orchard requires treatment.
• After July 1, monitor the whole orchard, dividing it into sampling areas that could be treated separately.

Within each sampling area, sample a minimum of five trees. Select 15 leaves from each tree, randomly picking leaves from both the inside and outside of the canopy as you walk around it. Examine both sides of each leaf under a hand lens looking for spider mites and eggs, western predatory mites or eggs, sixspotted thrips, and other predators.

To sample trees that have not yet been treated for mites during the current season, use the presence-absence sampling form (PDF)to note the number of leaves on each tree with pest mites or their eggs, and the number of leaves with predators. There is no need to count total numbers of mites. Once you have sampled 5 trees, compare your total to the numbers in the "Don't Treat" and "Treat " columns on the form. Be sure to take into account the presence or absence of predators as noted on the form.

If treatment is required determine which natural enemies are present and choose a product that is compatible according to the RELATIVE TOXICITIES OF PESTICIDES USED IN ALMONDS TO NATURAL ENEMIES AND POLLINATORS table.

• Monitoring spider mites during fruit development
• Photos of mites and natural enemies
• Webspinning spider mites monitoring form (PDF)