There are three major groups of mites that attack woody ornamental plants. These are the spider mites, the false spider or flat mites, and the gall or eriophyid mites. Mites are not insects, but are more closely related to spiders and ticks. The body of a spider mite or false spider mite is separated into two distinct parts: (1) the gnathosoma and (2) the idiosoma. The gnathosoma includes only the mouthparts. The idiosoma is the remainder of the body and parallels the head, thorax and abdomen of insects. After hatching from the egg, the first immature stage (larva) has three pair of legs. The following nymphal stages and the adult have four pairs of legs.
Spider mites are the most common mites attacking woody plants. False spider mites and eriophyid mites are less common. Eriophyid mites exhibit great modification of body structure. They have only two pair of legs, as the four rear legs are absent. The eriophyid mites are microscopic, elongate, spindle-shaped, translucent, and the abdomen usually has transverse rings present.
Spider mite development differs somewhat between species but a typical life cycle is as follows. The eggs are attached to fine silk webbing and hatch in approximately three days. The life cycle is composed of the egg, the larva, two nymphal stages (protonymph and deutonymph) and the adult. The length of time from egg to adult varies greatly depending on temperature. Under optimum conditions (approximately 80 degrees F), spider mites complete their development in five to twenty days. There are many over-lapping generations per year. The adult female is capable of laying several hundred eggs during her life.
The majority of eriophyid mite species go through four stages of development – the egg, two nymphal instars and the adult. The length of life cycle is variable depending on the species, but it is usually approximately seven days.
Some of the more common woody plants attacked by mites include azalea, camellia, citrus, silver thorn, hibiscus, ligustrum, photinia, pyracantha, rose, viburnum, juniper, arborvitae, holly, pittosporum, wax myrtle, and croton.
Eriophyid mites attack a wide range of plants including black olive, camellia, juniper, podocarpus, boxwood, maple and citrus.
All mites have needle-like piercing-sucking mouthparts. Spider mites feed by penetrating the plant tissue with their mouthparts and are found primarily on the underside of the leaf. All spider mites spin fine strands of webbing on the host plant – hence their name.
When twospotted spider mites remove the sap, the mesophyll tissue collapses and a small chlorotic spot forms at each feeding site. It is estimated that 18 to 22 cells are destroyed per minute. Continued feeding causes a stippled-bleached effect and later, the leaves turn yellow, gray or bronze. Complete defoliation may occur if the mites are not controlled.
Southern red mites first attack the lower leaf surface. As the population increases, the mites move to the upper surface. Injured leaves appear gray.
Sixspotted mites feed along the midrib on the underside of the leaf. The upper surface has yellow spots. When heavy infestations occur, the entire leaf becomes yellow, distorted and drops prematurely.
Spruce spider mite feeding causes the plants to appear off-color and eventually turn completely brown when high numbers are present.
False spider mites produce no webbing. Damage from these mites varies considerably, ranging from faint brown flecks to large chlorotic areas on the upper leaf surface to brown areas on the lower leaf surface, depending on the host.
Eriophyid mite feeding results in the following damage symptoms: (1) russeting of leaf and fruit (citrus); (2) leaf galls (juniper); (3) leaf blistering on top with hairy growth underneath (black olive); (4) discolored and stunted terminal growth (podocarpus and wax myrtle); and (5) discolored bud scales, floral parts and leaves (camellia).