Controlling Broad Mites In The Greenhouse

Broad mites might be extremely small, but the damage they inflict sure isn’t. This pest feeds on a variety of vegetable crops in the greenhouse including cucumber, eggplant, sweet pepper, and tomato. When temperatures are between 60ºF and 70ºF and the relative humidity is 60% to 80%, broad mites can thrive.

Females have been known to lay up to 40 eggs during their lifespan; however, this is dependent on temperature and relative humidity. Photo courtesy of Raymond Cloyd.

Females have been known to lay up to 40 eggs during their lifespan; however, this is dependent on temperature and relative humidity. Photo courtesy of Raymond Cloyd.

All life stages including the egg, larva, nymph, and adult may be present simultaneously during the growing season. Broad mites primarily feed in groups on the underside of young leaves, and they tend to avoid light. They reside and feed on the meristematic tissues of plants, requiring tender living tissue that provides an ideal food source for development. Growers will know this pest is in their greenhouses when plant damage becomes noticeable, as the mites are rarely detected.

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Identification And Biology
Adults are approximately 0.0009 inches (0.25 mm) in length, shiny, amber to dark green in color, and oval. Males are usually smaller than females.

There are four distinct life stages: egg, larva, nymph, and adult. Development from egg to adult takes five to six days at 70ºF to 80ºF, and seven to 10 days at 50ºF to 65ºF. Females can lay up to 40 eggs during their lifespan; however, this is dependent on temperature and relative humidity.

Broad mites have a reproductive system in which unmated females only lay eggs that produce male offspring. Eggs are oval, white, and covered with bumps or protrusions. Six-legged larvae emerge from eggs, which transition into eight-legged nymphs, and then eventually adults.

Detect Feeding Damage
Broad mites feed in groups, primarily on the underside of young leaves where females lay eggs. They are cell-feeders using their piercing-sucking mouthparts to feed on the epidermis of leaves. This causes leaf margins to curl downward, and leaves may become hardened, brittle, puckered, and/or shriveled.

Feeding may also cause leaves to appear darker green than normal. They may have distorted veins, and some leaf damage may resemble exposure to a phenoxy-based herbicide such as
2,4-D, a virus, or nutritional imbalances (e.g., magnesium deficiency).

In addition, fruit can be misshapen and/or cracked. Broad mites may inject toxins during feeding. On cucumber, shoots may be distorted with leaves curling downward, and fruit cracked, thus reducing marketability. Severely infested plants may be stunted and even die. Damage symptoms may still be expressed even after treating with a miticide.

When mite populations are extensive, they will move and feed on the upper leaf surface resulting in severe deformation. Lower leaf surfaces may appear bronzed.

Studies have indicated that five broad mites or fewer on young pepper plants may cause substantial plant damage resulting in less fruit production. In general, tomatoes tend to be less susceptible to broad mite infestations than both cucumber and sweet pepper. This may be associated with either nutritional quality or the production of defensive chemicals in tomato leaves.

Broad mites can spread among greenhouse-grown crops via air currents, leaves of adjacent plants contacting each other, and by workers handling infested plant material and then touching non-infested plants. Females have been known to attach to the legs and antennae of adult greenhouse whitefly (Trialeurodes vaporariorum) and/or sweet potato whitefly B-biotype (Bemisia tabaci); resulting in another means of dispersal. Furthermore, males may transport female nymphs, eggs, and adult females to new leaves.

Click on the next page to read about management options…

Management Options
As a food source is necessary for survival, growers should implement sanitation practices, such as removing any weeds before introducing new plant material and disinfecting benches. Miticides labeled for use against broad mite on vegetable crops may be used; however, broad mite populations may be difficult to suppress with contact miticides because the mites are in the meristematic tissues.

When using contact miticides, thorough coverage of all plant parts and repeat applications are critical in order to obtain high mortality of the pest. Vegetable plants with hairs or trichomes may inhibit suppression of broad mite populations when using miticides because the hairs prevent spray droplets from reaching the leaf surface where mites are located.

A more effective option may be miticides with translaminar activity that are labeled for use on vegetables. Translaminar means that after a foliar application, the material penetrates leaf tissues and new terminal growth, forming a reservoir of active ingredient within the leaf or new growing points. Therefore, these miticides are more likely to come into contact with broad mites feeding in the tissues.

Preventive applications may be required, particularly on highly susceptible vegetable crops, because once damage is evident, it is too late to initiate practices that will suppress broad mite populations. As such, it is recommended to remove and immediately dispose of plants exhibiting symptoms, and even those adjacent to symptomatic plants, in order to prevent broad mite populations from spreading.

Biological control of broad mites on greenhouse-grown vegetable crops is a management option that involves using commercially available predatory mites such as Neoseiulus californicus, N. cucumeris, or Amblyseius swirskii. It has been reported that releases of N. cucumeris successfully suppressed broad mite populations on greenhouse-grown sweet pepper. When using biological control, it is important to apply predatory mites early in the crop production cycle before broad mites become established.