Spider mites (family Tetranychidae) and predatory mites (Phytoseiidae) are tiny eight-legged arthropods (larval stage has only six legs). Persea mite is a key pest of California-grown avocados, whereas sixspotted mite is a sporadic pest. See AVOCADO BROWN MITE for details on how to manage this occasional pest.
Several beneficial mites are important predators of pest mites and certain insects. Natural enemies and certain management strategies vary among pest mites. Identify the pest and natural enemy species in your grove and learn their biology so you can manage these pests appropriately as needed.
How to Monitor
Both species feed on the underside of leaves, and the similar appearance of these mites as well as their damage can be confused. Monitor mites about every 7 to 10 days from about mid-March through October using one or more methods.
- Randomly pick current-season leaves of mixed age, one leaf from each of at least 10 trees.
- Looking at the underside of each leaf, start at the petiole end. Locate the second major vein that goes strongly from the midrib to the left leaf edge. Ignore any partial, small, or weak veins. Examine the upper (towards the leaf tip) vein edge through a hand lens.
- Count the persea mites adjacent to that upper edge of the second major vein.
- Count mites in webbed nests or exposed necrotic feeding patches that touch the vein.
- Count any other mites up against the vein.
- Do not count mite eggs; or any visible mites located away from the vein and outside webbed or necrotic patches.
- Total the number of persea mites counted and divide the total by the number of leaves sampled (typically 10). Multiply by 12 to derive the average mites per entire leaf.
- Also count the predaceous mites (e.g., Galendromus spp.) in the persea mite feeding patches. Divide total predator mites by the number of leaves sampled. Multiply this predaceous mite average by 6.
- Record results .
- Manage persea mite if warranted based on sampling results and past experience.
- Monitor for mite-damaged leaves; include locations from previous seasons that experienced premature drop of numerous green leaves with discolored spots caused by mite feeding. Pick leaves of mixed age at random from the tree, choosing 10 leaves from each of at least 10 trees per grove.
- Visually estimate the percent of mite-damaged tissue on each leaf by comparing it to standards with known levels of damage, such as colored photographs of avocado leaves showing 1 to 50% feeding damage. Record results .
- For each tree, total the percent damage on the 10 leaves, then total the percent damage on all trees.
- Calculate the estimated average percent damage per leaf: Total percent damage/Total number of leaves sampled (e.g., 100 leaves) = Average percent mite damage per leaf.
- Manage persea mite or sixspotted mite if warranted based on sampling results past and experience.
Although it has not been experimentally verified, the probability of leaf drop is believed to increase greatly once 7.5 to 10% of the leaf surface is damaged by persea mite feeding. Thus, control may be warranted before reaching this damage level.
- In multiples of five, select a number of expanded, but not very old leaves.
- Using a hand lens or other magnifier to examine the underside of leaves, identify whether there are any live persea mites on each leaf.
- Consider the leaf infested only if it contains any live pest mites.
- Be aware that mite nests (discolored spots and webbed patches) on leaves (especially older leaves) may no longer be inhabited.
- If an overall average of two or more out of five leaves have any living persea mites, some pest control advisors believe treatment may be warranted; however, there is no research-based evidence that mite density or treatment need can accurately be determined using this presence-absence sampling method.