Breakout Season for Pest Mites

Entomology News and Information:
Breakout Season for Pest Mites

The Stethorus lady beetle (aka the spider mite destroyer, top) is one of many important mite predators. Both the small adult (left) and larva
(right) are predators of spider mites (bottom).

The hot, dry summer of 2012 was favorable to pest mites on a variety of crops in Utah. An early summer visit to a peach orchard in Weber County revealed trees with a silver sheen to the leaves, the classic representation of damage caused by the peach silver mite. These trees were also found to be teeming with the more cosmopolitan two-spotted spider mite. Visits to corn, melon, pumpkin fields and turf fields from Millard to Cache counties also showed flare-ups of two-spotted spider mites and the Banks grass mite.

Spider mite damage is commonly seen as “stippling” or yellow to white speckling where the mites have fed and killed the plant cells. Eventually, heavy feeding results in the yellowing and bronzing of leaves. Spider mites and their webbing can best be detected on the undersides of leaves with a hand lens.

Mites are usually held in check by predatory arthropods such as the Stethorus lady beetle, minute pirate bug, and predatory mites. One of the main contributors of mite outbreaks is insecticide applications that have non-target effects. Some insecticides, such as pyrethroids and organophosphates, kill predatory arthropods that would normally feed on pest mites. Other insecticides, such as carbaryl or neonicotinoids, can trigger increased mite activity and egg production that lead to mite outbreaks.

Recent research suggests that neonicotinoid insecticides alter plant chemical defenses, making plants more susceptible to mites. During warm, dry conditions, like in 2012, mites have shortened generation times and the population can build up rapidly. Mites feed more in dry conditions because low humidity evaporates excess water that they excrete. Unfortunately, predators are unable to keep up with this rapid increase in the population in part because they are less effective in hot, dry conditions.

It is not uncommon in our region, particularly during an extended, dry season, for plants to become drought-stressed, which benefits mites. Water-deprived plants undergo changes in plant chemistry such as increased availability of amino acids, allowing mites to thrive. Proper irrigation practices can reduce plant stress and help plants from succumbing to mite feeding and damage. Also, frequent overhead irrigation and rain can physically wash mites from plants and help decrease mite populations. Note that overhead irrigation will not reduce mites that are already at economically damaging levels.

Mite management needs to be proactive and preventive. Strategies include conservation of mite predators by avoiding broad-spectrum insecticides, correct identification of the pest mite, monitoring the mite’s population growth, and using action thresholds when available. Dust buildup on foliage from dirt roads, which benefits pest mites and hinders predators, should be kept to a minimum by washing plants or altering the road surface.

When mite populations exceed predator populations, a selective miticide will provide the best suppression, such as: Acramite, AgriMek, Oberon, Portal, and Zeal for commercial, and insecticidal soap or horticultural oil for residential (check label for listed crops). Be aware that some insecticides such as bifenthrin products list mites on the label as a pest that it controls; however, the results may be less than expected compared to a selective miticide and in some cases, may be worse due to the non-target effects previously described. Crop fields with a long history of miticide applications may also find less favorable results with these products due to resistance in the mite population.


-Ricardo Ramirez, Extension Entomologist

References:

Alston, D.G. and M.E. Reding. 2011. Web spinning spider mites. Utah State University Extension Fact Sheet. ENT-151-06

Fleischer, S. 2012. Mite Outbreaks in Hot Weather. Penn State University Extension. Vegetable and Small Fruit Blog.

Szczepaniec, A., et. al. 2011. Neonicotinoid insecticide imidacloprid causes outbreaks of spider mites on elm trees in urban landscapes. PLoS ONE 6(5): e20018.