Fruits & Nuts
During this time of year, peach growers are noticing peach leaves with a mottled appearance and fine web material covering leaves, twigs, and sections of branches. These are signs of an infestation of the two-spotted spider mite (Tetranychus urticae) (Fig. 1). If left unchecked, spider mites can go from being barely noticeable to infesting entire trees (Fig. 2).
For the past several weeks, Alabama has been experiencing dry, hot weather and these are favorable conditions for outbreaks of spider mites. As these conditions persist, the spider mite population increases.
As their name suggests, spider mites spin webs although they are not spiders nor are they insects. The web allows them to walk about and is a place for them to lay eggs (Fig. 3). Two large dark spots on either side of their bodies distinguish the mites from other species (Fig. 1). Spider mites are approximately 1/32 of an inch long, and the female is larger than the male.
Spider mites will infest many other plant species in addition to peaches. The mites will move up a host tree until it is entirely covered.
Signs of Infestation
Leaves with a mottled appearance of the upper surface of leaves are usually the first sign (Fig 4). Webbing covering leaves as well as entire shoots are noticed in heavy infestations (Fig 4 & 5). Spider mites can be detected by removing a leaf and looking at the underside of the leaf with a hand lens with a minimum magnification of 10x.
Spider Mite Damage
Two-spotted spider mites are found feeding on the underside of leaves. They feed by sucking and removing chlorophyll from individual plant cells with their needle-like mouthparts called chelicerae. This empties the cells of their contents, giving the leaf a mottled appearance. Peach trees can tolerate some damage, but heavy infestations can result in severe leaf drop. This will cause fruit not to size properly, and the reduce leaf cover will result in overexposure to sunlight and cause sunburn.
The adult female overwinters in weeds and decaying leaves on the floor of the orchard. In early spring, the mites become active in weeds in the orchard floor and lower parts of peach trees. As conditions become hot and dry, the mites become more active and increase in number. Spider mites begin to inhabit higher elevations in the tree until the entire tree is covered as the warm and dry weather persists.
The female two-spotted lays eggs on the underside of leaves. As temperatures become warmer, eggs hatch sooner. During the hottest part of the summer, two-spotted spider mites can be hatched from eggs and become adults in as few as 10 days. Spider mites can have as many as 18 generations during a season.
To control two-spotted spider mites, monitor the peach orchard regularly from May through August. Maintain proper cultural practices, including irrigation, fertilization, and proper pruning, in efforts to minimize stress. Pyrethroid use will reduce populations of predators of spider mites and cause spider mite populations to increase. Use miticides such as bifenazate that target spider mites specifically. For more information on the management of spider mites, refer to the current Southeastern Peach, Nectarine, and Plum Pest Management and Culture Guide.