Webspinning Spider Mites

To the naked eye, spider mites appear as tiny moving dots. Adult females are about 0.01 inch (0.3 mm) long. Spider mites live in colonies, mostly on the lower surfaces of leaves, and produce a webbing that can cover much of the undersurface of the leaf. Adults have eight legs and an oval body, usually with two red eyespots near the head. Eggs are spherical and translucent. Immatures resemble adults and feed on leaves in the same way.

These four species of spider mites are similar in appearance; it is important, though, to be able to distinguish early season infestations of strawberry mites because this species is more damaging to cotton and because it is more susceptible to miticides. Strawberry mites form compact colonies on the undersurface of cotyledons and early true leaves. The infested part of the leaf puckers upward and later turns red, then brown; this mite produces only light webbing. Infested plants may lose most of their lower leaves by first bloom. Pacific and twospotted mites produce dense webbing and can cause reddening and abscission of leaves, but only do so at higher densities.

Pacific mites spread out from the base of the leaf along the main veins, and will colonize the upper leaf surface to a greater degree than is commonly observed with the other species. Carmine and twospotted mites start in a leaf fold or at the base of leaf blades near the petiole. They gradually spread to the edges of the leaf. All three species produce considerable webbing and are usually much less abundant than strawberry mites early in the season. Adult female carmine mites are red, while adult females of the other three species are green or straw yellow with dark blotches on the side.

Spider mites can cause leaves or parts of leaves to turn yellow or red and to drop. Loss of leaf surface reduces energy available to maturing fruit, so squares and bolls may fail to develop and may eventually drop. Entire plants in heavily infested areas of the field may be defoliated.

Managing spider mites requires preserving natural enemies as long as possible each season and anticipating outbreaks following insecticide applications. When treating for mites, follow resistance management guidelines.

Preserve natural enemies of mites by avoiding early season, broad-spectrum insecticide applications. The most important predator early in the season is the western flower thrips. Later, bigeyed bugs, minute pirate bugs, predaceous mites, and other predators are also important. Releases of the western predatory mite, Galendromus occidentalis, may help control populations of pest mites, but more research is needed in this area.

Water-stressed plants stimulate spider mite outbreaks; be sure to keep the crop properly irrigated. In addition, sprinkler irrigation has been observed to suppress spider mites. Pima cotton is less susceptible to spider mites than upland cotton varieties.

Biological control as releases of predatory mites and sprays of insecticidal soap, some oils, and sulfur are acceptable to use on organically grown cotton.

Research has confirmed that populations of the twospotted and Pacific mites in some areas have developed resistance to propargite (Comite), abamectin (Agri-Mek), or any combination of these. However, resistance can change, even during the field season, so it is important that you monitor for resistance immediately before making a decision about which miticide to use.

Rotation of abamectin (Agri-Mek), etoxazole (Zeal), spiromesifen (Oberon), hexythiazox (Onager), or other recently registered miticides with the older miticides may help to reduce resistance to any one of them and slow the development of resistance in areas where it is not yet a problem. Growers are urged to use a miticide only once per season, and, if a second application is needed, switch to another miticide. Growers should also rotate to a different miticide the following season.

While it is important to rotate miticides with a different mode of action, each miticide has characteristics that make it more or less useful at different times of the year and under different circumstances. Phorate (Thimet) applied at planting remain effective for about 6 weeks and are useful for early season infestations. Base a decision to use these miticides on a history of early season mite infestations and potential benefits derived from controlling other early season pests. Mite populations that typically move in 6 weeks or more after planting are best controlled with a foliar miticide applied when the populations appear.

If infestations occur when plants are small and/or V-shaped seed lines are prominent, complete coverage is hard to achieve. In this instance use a miticide such as abamectin, which has the ability to move through the leaf tissue.

Plants with more than four true leaves will allow adequate coverage for the use of foliar miticides such as etoxazole (Zeal), spiromesifin (Oberon), and hexythiazox (Onager). Because all three of these miticides work by regulating mite growth, their best use is when mite populations are low and just starting to build.

Propargite (Comite) is phytotoxic to cotton cotyledons and must be applied later in the season. Sulfur only kills strawberry mite. Abamectin (Agri-Mek) is effective against mites anytime during the season but works best early to mid-season before the leaf ages and "hardens off." Fenproximate (Fujimite) is a contact miticide that can only be applied by ground and is, therefore, best if used early in the season. Zeal and Oberon have proven to be the best options for mites after layby where applications must go on by air. Oberon also has the added benefit of providing control of whitefly. Any of these products can be used in areas where propargite resistance is a problem.

In most field situations, strawberry mite is the first species present during the growing season and it is susceptible to all of the early season miticides (sulfur, hexythiazox, abamectin, and fenproximate). If possible, it is important to save abamectin for just before layby. Following the first miticide application, mite populations are likely to consist of either twospotted or Pacific spider mites. At this point it is important to determine their level of resistance to propargite and abamectin when determining the best miticide to use.

In all situations, early season use of pyrethroids for aphids, lygus bugs, or whiteflies can aggravate spider mite populations because they destroy natural enemies so avoid them when possible. On the other hand, most miticides are specific for mites and should not cause disruptions of insect pests.

The critical time for monitoring spider mites is between crop emergence and first open boll. To improve efficiency of your monitoring program, combine sampling of spider mites with other pests. From crop emergence to seedling growth, sample mites, aphids, and thrips together as described in MONITORING SPIDER MITES, APHIDS, AND THRIPS. From early squaring to boll development, combine sampling for spider mites, aphids, and whitefly as described in MONITORING SPIDER MITES, APHIDS, AND WHITEFLY. Record your results .

Generally treatment of seedling cotton is required if defoliation is occurring and the mite populations are high. From early squaring to first open boll, treatment can be considered if 30 to 50% of leaves have spider mites following the monitoring procedures outlined above.

Spot Treatments

Sometimes field margins are much more severely infested than the remainder of the field, particularly when another host crop, such as alfalfa, beans, sugarbeet, or safflower, is grown next to the cotton. In such cases, treatment of a field margin may be justified. Monitor field margins separately from the remainder of the field.

• Example pest monitoring forms