What is in this article?:
- Staying on top of the spider mite threat
- Control methods
- Webspinning spider mites pose a threat at much lower densities than other mites.
Early in the season, choose one leaf between the 2nd and 4th nodes on each of these vines. Later in the season, choose the 4th expanded leaf from the growing tip.
A form for recording your findings is available at www.ipm.ucdavis.edu/PMG/C302/grape-leafhoprmite.pdf. This form also offers treatment guidelines for various combinations of Pacific mite injury levels and] predator-prey distribution ratios in Thompson Seedless vineyards.
Damage caused by each species can help in identifying each species. Pacific spider mite damage begins as yellow spots. As damage progresses, dead (necrotic) areas appear on the leaves. High populations can cause leaf burning and bronzing and large amounts of webbing. Damage is worse along the shoulder and tops of the vine canopies.
Willamette spider mite feeding in mid- or late season causes foliage to turn yellowish bronze. Usually, though, no burn occurs unless vines are weak. In red varieties, infested leaves may turn reddish.
Proper management of webspinning spider mites in a vineyard integrates biological, cultural and chemical controls.
Many natural enemies, such as thrips and predatory mites, help control pest mite populations. The western predatory mite, Galendromus (=Metaseiulus) occidentalis, is commonly present in vineyards and can be quite effective in reducing all stages of spider mite populations. This mite is translucent to light amber, pear shaped, and quite active. The effectiveness of this predator depends upon its ability to increase its population size as the season progresses. Disruptive sprays applied early will reduce the survival of this beneficial mite.
Naturally occurring predator mites will survive sulfur sprays and dusts, but released ones may not survive dusting sulfur unless they have sulfur resistance. Predator mites, including insecticide-resistant ones, are available commercially to augment populations in the field.
Other predators, including sixspotted thrips (Scolothrips sexmaculatus), can also be important. To preserve these natural enemies, avoid using disruptive materials, especially carbaryl.
Cultural control measures include applying water or other materials formulated to reduce dust on roads in the vineyard. If possible, maintain resident vegetation or other cover in the vineyard middles to further reduce dust. Irrigate in a manner that will avoid stressing vines. Although overhead watering has been shown to reduce mite problems, it can also increase some disease problems.
Biological and cultural control methods as well as oil or soap sprays are organically accepted methods.
When using chemicals to control spider mites, keep in mind that broad-spectrum insecticides can result in spider mite outbreaks. Consider the ratio of beneficials to pests before spraying. If there is one beneficial for every ten or less mites, hold off spray and continue monitoring.
Do not spray more than twice per season with the same pesticide and alternate types of spray. Follow all labeled instructions for safety, including when it is safe to reenter the vineyard after spraying with no protective clothing.
Note that using some pesticides to control other insects can actually favor mites. Avoid carbaryl (Sevin), organophosphates, and pyrethroids which increase nitrogen in leaves and can cause mite population explosions, especially in hot weather.
More information on pesticide selection and use is available at www.ipm.ucdavis.edu.