How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

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.

DAMAGE

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

MANAGEMENT

Spider mites are often a problem in water-stressed orchards. Orchards that are properly irrigated usually do not require treatment for mites. Almond trees can tolerate moderate mite populations 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 and once a week or more after that. When treatments are required, choose selective miticides that have least negative impact on predators.

Biological Control
Several species play an important role in mite control, including western predatory mite (Galendromus [= Metaseiulus] occidentalis), sixspotted thrips, and spider mite destroyer (Stethorus sp.). The western predatory mite is the most reliable mite predator. About the same size as spider mites, it lacks spots and ranges in color 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.

A good rule of thumb is that predator mites will control webspinning mites if presence/absence sampling indicates equivalent numbers of leaves with predators and 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 sufficiently abundant and resistance to selective miticides has not been identified, rates as low as one-eighth to one-tenth of the propargite (Omite) label rates can be used to balance predator/prey ratios, although actual control of the spider mites may only reach 50 to 60% (but only in orchards where sprays of materials that kill predator mites are not used to control other pests).

Monitoring spider mite and western predatory mite populations and using miticides sparingly will help to delay the development of resistant spider mites. The other mite predators are often not abundant until pest mite numbers are high, and therefore their populations are difficult to manipulate. While the best course of action is to conserve and promote existing western predatory mite populations in orchards, commercially available western predatory mites can be released to establish or augment resident populations. Organophosphate-resistant predator mites are available and can be very beneficial when established. (See Integrated Pest Management for Almonds, UC/ANR Publication 3308, for detailed information on releasing and managing predator mites. See APPROXIMATE IMPACT RATINGS OF VARIOUS PEST MANAGEMENT TOOLS AGAINST NATURAL ENEMIES for a list of pesticides used on almonds and their toxicity to the western predatory mite.)

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

Cultural Control
Reduce dusty conditions in orchards by oiling or watering roadways and maintaining a groundcover. Prevent water stress, as this condition results in higher mite populations and makes trees more susceptible to 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 materials will often result in spider mite outbreaks.

Monitoring and Treatment Decisions
From May through August, monitor for mites at least weekly. 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 5 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 (24 KB, PDF) on the online version of this guideline to note the number of leaves on each tree with pest mites or their eggs and the number of leaves with predators. 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, avoid pyrethroid, organophosphate and carbamate insecticides. All can cause upsets of biological control. Soft materials such as oils, acequinocyl (Kanemite) abamectin (Agri-Mek) and bifenazate (Acramite) are good choices for protecting natural enemies.

• Summary of Important Links

IMPORTANT LINKS

• Monitoring spider mites during fruit development
• Photos of mites and natural enemies
• Webspinning spider mites monitoring form (24 KB, PDF)

PUBLICATION

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431
Insects and Mites
F. G. Zalom, Entomology, UC Davis
C. Pickel, UC IPM Program, UC Cooperative Extension, Sutter/Yuba counties
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
D. R. Haviland, UC IPM Program, UC Cooperative Extension, Kern County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
R. E. Rice, Kearney Agricultural Center, Parlier
L. C. Hendricks, UC Cooperative Extension, Merced County
R. L. Coviello, UC Cooperative Extension, Fresno County
M. W. Freeman, UC Cooperative Extension, Fresno County

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