Adult spider mite destroyers are tiny (1/16") oval, convex, shiny black beetles covered with sparse, fine yellowish to white hairs. They are very active when in fruit trees and if disturbed will often fall to the ground. They are good fliers and therefore tend to concentrate in areas of the orchard where mites are plentiful and disappear when the mite population becomes low. The pale white, oval eggs are very small in size (1/64"). They turn blackish just before the larvae emerge. Eggs are laid singly on their sides and adhere tightly to the leaf. Females lay 1 to 10 eggs per leaf depending on mite density. Most of the eggs are laid on the underside of the leaf, close to the primary veins of the leaf. The newly hatched larva is gray to blackish and has many long branched hairs and black patches. As the larva matures it becomes reddish, at first on the edges of the body, and just prior to pupation the entire larva turns reddish. The larva passes through four larval stages and pupates in about 12-14 days. The pupae are black and flattened with the entire body covered with yellow hairs.
Spider mite destroyers overwinter as adults beneath leaves and other organic matter ("duff") under fruit trees and in other protected habitats near the orchard, such as fence rows or adjacent wooded areas. Adults can be found in the leaf litter immediately surrounding the trunks of fruit trees, in large numbers along the herbicide strip, and occasionally in the drive row. It is advisable not to disturb the area in the herbicide strip near the trunk of the tree from November 1 to mid- to late April. Adults begin to emerge at the tight cluster stage of apple development with peak emergence from the pink to bloom stage. Adult emergence is usually complete by the petal fall stage. The adults remain active in the orchard until September to late October. Egg laying occurs in most areas from May to mid-August. The peak periods of larval activity are dependent upon mite populations. There are usually three overlapping generations per year. The average period from egg deposition to the appearance of the adult is 23 days, and the adults feed for an average of 25 days before beginning to lay eggs.
There must be 2-5 motile mites per leaf to keep spider mite destroyer in an orchard, and "pockets" of 8-10 mites per leaf are required for reproduction. (See MBCN Vol. 1, Nos. 1, 2, 3, 4 for more information on spider mites.) You can monitor spider mite destroyer populations by making a three-minute observation around the periphery of a tree and recording all adult and larval beetles you see.
If you are interested in learning more about this predator, write to the author.
- Larry Hull, Penn State University, Fruit Research and Extension Center, Biglerville, PA 17307
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