Spidermite Control-N. Fallacis

A complex of predatory mites attack spider mites, the family Phytoseiidae being of the greatest importance. A complex of these species feed on spider mites in unsprayed trees; however, in sprayed orchards the phytoseiid Amblyseius fallacis generally assumes prominence. It is more tolerant of organophosphate-based spray programs than are many of the other species in the family, and several studies showed it was the only predatory mite to remain common in sprayed orchards throughout the year. Nevertheless, is subject to mortality induced by pesticides; it is important to note that some fungicides and herbicides are disruptive to populations of Fallacis, in addition to insecticides. For example, work in Michigan has shown that benomyl poses risk to Fallacis from direct contact toxicity (more pronounced in immatures), ovicidal action, and permanent reduction in oviposition caused by secondary poisoning (consuming treated prey mites). Toxicity of specific pesticides is provided in other extension publications. This predator is present throughout much of North America, but is most important in the eastern states, Resistant strains have been introduced from North America into New Zealand.

II. Hosts: Fallacis feeds on a variety of mite species, but exhibits little cannibalistic behavior. It is capable on feeding on all prey life stages. It prefers to feed on spider mites, but will also feed on the apple rust mite in the absence of more preferred prey.

III. Description: This mite (Plate 135) is similar in size to somewhat smaller than the European red mite. It is straw-colored to almost white, depending on age and recent food sources. It is flatter in shape than the relatively dome-shaped spider mites, and has fewer hairs protruding from the body than spider mites. Viewed from above, Fallacis is oval to somewhat pear-shaped.

IV. Biology: Fallacis spends the winter in the orchard ground cover, where it feeds upon overwintering twospotted spider mite and other mites (recent research in New York indicates that a portion of the population may also overwinter on the tree). Completely bare ground does not support the community including Fallacis; however complete ground cover is not required to support predators. Research in eastern New York indicates that as little as 20% ground cover will support the complex. During the spring (May and June) mites disperse upward into tree canopies, where they provide mid- and late-season biological control of European red mite. Research in Michigan has revealed that the three most important factors influencing dispersal of Fallacis into the canopy are degree-day accumulation, initial density of predators in the ground cover, and prey density in the tree. When spider mites or rust mites (15 ARM approximate one ERM in the diet of Fallacis) were present in adequate numbers, this predator appeared in the trees after 600±100 DD54 after January 1. Spring frosts or freezing rains may suppress activity of this predator while it is still in the ground cover.

Females represent 66-75% of the adult population. These females lay 1-5 eggs per day for their adult life, which lasts from 1-81 days (average 41) at 80 degrees F. Higher egg production is achieved with greater prey availability. At 70 degrees F, the life span is somewhat longer, 24-80 (average 62) days. The period from egg deposition until emergence of adults is 7.3 and 3.3 days at 70 degrees F and 90 degrees F, respectively. The complete life cycle generally takes 7-9 days.

Fallacis uses two alternative foraging patterns, as demonstrated by research in Nebraska. When prey are plentiful, a random walking pattern is used, allowing them to utilize mites on a leaf most efficiently. When prey are scarce, the predators walk along leaf edges, increasing the chance of moving to a new leaf. Research in Michigan has revealed that this predatory mite exhibits a dispersal behavior which aids in distribution among trees. Individual mites assume a stance that allows them to be carried off on a breeze; this behavior is most pronounced when prey are in low numbers. Massachusetts research has shown that Fallacis uses chemical signals from prey silk and feces to hunt prey, and furthermore uses its own marking pheromone to mark previously searched areas.

Research in Michigan has yielded tentative thresholds for predicting success of biological control by Fallacis. A predator-to-prey ratio of at least 1:10 presents a good probability of biological control. Higher ratios increase the probability of success. Lower predator-to-prey ratios (e.g., 1:20) may result in successful control on some apple varieties less conducive to spider mite reproduction than `Delicious'.

V. Injury: Predatory mites do not injure the trees or crop in any way.