Apple Rust Mite, Aculus schlechtendali (Nalepa)

I. Introduction: Apple rust mite (ARM) is a tiny mite living an apple foliage. This eriophyid lives in apple orchards world-wide. Although more abundant in unsprayed trees, it is common in managed blocks. High population densities can cause injury, but more usual populations are beneficial because ARM is an alternative food source for the predatory mite, Amblyseius fallacis, and the black hunter thrips, Leptothrips mali.

II. Hosts: Apple, including crabs, and possibly pear.

III. Description: ARM are elongate triangular mites, pale yellow or off-white in color (Plate 49). They are barely discernible using a hand lens (7/1000 inch; 160-175 microns long, ca. 0.2 mm). ARM possess only two pairs of legs (mites generally have four pairs).

IV. Biology: Winter-form females overwinter in dormant buds and under loose bark on 1-year-old shoots. Mites move into fruit buds between tight cluster and pink, and into vegetative buds as they swell. Mites feed on flower receptacles and fruitlets in May and June. Females deposit eggs on the undersides of leaves, giving rise to males and summer form females. There are several generations before winter-form females appear in July, possibly in response to the hardening of buds. These females return to overwintering quarters at this time. Presence of apple rust mites can condition foliage so that it is less suitable for development of ERM.

V. Injury: ARM can cause a silvering of foliage (Plate 50), although population densities seldom reach damaging levels. If populations are very high, however, terminal growth will be affected and leaves will curl lengthwise and turn brown. ARM may feed on young fruit in May and June, damaging the epidermis and causing russeting.

VI. Monitoring: To assess summer populations, examine leaves with a hand lens. Although it is impractical to obtain accurate population counts with this method, if many are seen, consider more intensive sampling, ideally with a dissecting microscope. When concerned with russet, sample overwintering populations by examining under bud scales under magnification. Below these levels ARM is actually beneficial because it is an alternative food source for predatory mites. There is no threshold available for overwintering populations beneath bud scales.

This is taken primarily from a chapter by D.G. Pfeiffer, L.A. Hull, D.J. Biddinger, & J.C. Killian on apple indirect pests, reprinted with permission from Mid-Atlantic Orchard Monitoring Guide, published by NRAES, 152 Riley-Robb Hall, Ithaca, New York 14853-5701.
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