MITES

General Description

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.

Important Species

The most common spider mites found in Florida on woody plants are:

sixspotted miteEotetranychus sexmaculatus
spruce spider miteOligonychus ununguis
southern red miteOligonychus ilicis
twospotted spider miteTetranychus urticae

The predominant false spider mites are:

privet miteBrevipalpus obovatus
(no common name)Brevipalpus phoenicis

There are hundreds of species of eriophyid mites, but some of the most common are:

Acaphylla steinwedenion camellia
Eriophyes bucerason black olive
Paracalacarus podocarpion podocarpus
Phytoptus canestriniion boxwood
Trisetacus quadrisetuson juniper

Biology

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.

Host Plants

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.

Seasonal Abundance

The twospotted spider mite prefers the hot, dry weather of the summer and fall months, but may occur anytime during the year. Southern red mite populations are at their peak during the winter months under mild, humid conditions. These mites can be found year round in the landscape, especially on shaded or stressed pyracantha. Populations in the nursery usually disappear when the new spring growth develops. The sixspotted spider mite is most abundant during winter and spring on azalea, especially after a cold winter. Spruce spider mites occur during the hot, summer months, especially on plants under water stress.

False spider mites and eriophyid mites occur throughout the year, but predominantly during the summer and fall months.

Damage

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).

Eriophyid mites are thought to possess chemicals in their salivary secretions that act as growth regulators. When the mites feed, these chemicals are injected into the plant. Leaves may become discolored or plant growth patterns may be changed. On foliage growth, modifications are initially more readily found on embryonic plant tissue. Discoloration (russeting) occurs on mature leaves and fruits.

Eriophyid mites induce plant galls. These galls develop from epidermal cells that are infected by growth regulators injected by the mites. Each species of mite has particular chemicals that cause galls to form which are of specific benefit to the mite. After the induced change has altered the behavior of the affected cell or cells, the mite does not have to remain on the site to insure continuation of gall growth. Eriophyid galls occur on soft plant parts, usually on green tissue that was infested when the plant was young. Galls occur in many different shapes. These include pouch or purse, bladder galls, nail galls, finger galls and head galls.

Sampling

For detection of spider and false spider mites, a 10X to 15X magnifying glass is a necessity. Examine the undersides of the leaves closely for mites, cast skins and webbing. A more efficient technique is to place a sheet of white typing paper beneath the leaves and strike the foliage sharply. The mites will fall onto the paper and can be more easily observed and identified than on the green foliage.

Eriophyid mites are so small that they are virtually impossible to see without a microscope and a trained eye. If damage symptoms indicate a possible infestation, take the affected plant parts to your Cooperative Extension Service Office.

Biological Control

Spider mites are commonly attacked by predator mites. Five species are commercially available in the U.S.: Phytoseiulus persimilis, Mesoseiulus longipes, Neoseiulus californicus, Galendromus occidentalis and Amblyseius fallicus. Predator mites can be distinguished from spider mites because of their longer legs. The front pair of legs is often extended forward. They are more active and move about at a fast pace. They are often red or orange in color. P. persimilis is the most common predator and preys on all stages of mites. It can consume 20 eggs or 5 adults daily.

Predatory Mite

Much research has been conducted concerning biological control, but there is very little information on how to employ beneficials in the nursery.

Chemical Control

Insecticidal soaps and oils should be carefully considered when a pesticide is required. They are effective against mites and the least toxic to people, other non-target organisms and the environment.

Most miticides are not effective on eggs. Therefore two or more applications of the miticide will be required at five-day intervals during the summer or seven-day intervals during the winter.

For the most current insecticide recommendations to control these pests, please contact the local county office of the Cooperative Extension Service. If you have access to the World Wide Web you can consult the University of Florida's Insect Management Guide at:

http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide

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