Galendromus (=Typhlodromus, =Metaseiulus) occidentalis
Predatory mites are important control agents of pest mites in North American orchards. The western United States native, Galendromous occidentalis, is a primary biocontrol agent of pest mites on many crops, but its benefit to commercial apple, plum, peach, and cherry orchard farming is especially well known.
Adults are pear-shaped and slightly smaller than the European red mite adult. They are white until they feed when they take on the coloration of their prey, usually red or brown.
The eggs are pear shaped, almost transparent, but slightly larger than the round European red mite eggs. The larvae are also transparent and difficult to see without a microscope. Of the five G. occidentalis life stages, only the larvae are six legged. All other post-egg stages have eight legs. In all stages, G. occidentalis is indistinguishable from Neoseiulus fallacis and Galendromus pyri, other phytoseiid predatory mites, without a compound microscope.
Although G. occidentalis is a predator of mites in many systems, this page deals solely with orchard trees (apple, plum, peach, cherry, pear). G. occidentalis can thrive in hot, dry climates.
Two-spotted spider mites, McDaniel spider mites, yellow spider mites, apple and pear rust mites, Prunus rust mites, blister mites, and European red mites.
G. occidentalis overwinters as adult mated females in debris under the trees and in crevices on the tree itself. Many die during very cold winters. Emergence begins at first bud, and the mites disperse, looking for either spider mites or apple rust mites. They will also consume European red mites, but they do not seek them out. Eggs are laid on the undersides of leaves, in the opening buds and on the flowers and hatch in 1-4 days. Adults hunt along the leaf midveins during the day, and over the entire leaf surface toward nightfall.
Total development time, from egg to adult, is 6-12 days, depending on the temperature. Females survive about 30 days and lay about 21 eggs. G. occidentalis reproduces very quickly, and there may be 8-10 generations per year.
The presence of apple rust mites ensures a healthy population of predator mites which will be able to control spider mites and European red mites. Growing populations of apple rust mites in May and June encourage the presence of predatory mites, and at moderate numbers, they are not so damaging to the trees as spider mites. Therefore, controlling apple rust mites means keeping predator mite numbers low. (Apple rust mites are generally found on terminal shoots, whereas spider mites are usually on the lower and more central parts of the tree.) If there is about one predator mite per leaf and no more than 25 other mites per leaf, the pest mites should be controlled within 7-10 days.
Pesticide use can not only destroy the predator mite population, but it can destroy less harmful prey species so that the predators will starve. Careful management of G. occidentalis and apple rust mites provide the best control of pest mites. If miticides must be used, this should be done before introduction of the predators.
G. occidentalis is highly susceptible to permethrin, methomyl, and esfenvalerate. Refer to Insecticide Selectivity Guide in Beers et al., p. 226 for more specific information.
G. occidentalis is readily available commercially, including some pesticide resistant strains (see the off-site publication, Suppliers of Beneficial Organisms in North America, page of the California Department of Pesticide Regulation website).
Beers, E.H., Brunner, J.F., Willett, M.J., and Warner, G.M. (Eds.) (1993) Orchard Pest Management: A Resource Book for the Pacific Northwest. Good Fruit Grower, Yakima, WA. 276 pp.