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Battle of the Bugs: Spider Mites of Summer

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Spider Mites of Summer

By George Giltner, Adv. Master Gardener, MS Biology

In the hot and humid Summer months, a common pest problem is spider mites which infect tomatoes, snap beans, roses, lantana, maple, redbud, blackberries, strawberries, blueberries, squash, cucumber, and about 200 other species of plants.  The most common spider mite throughout the U.S. is the two spotted spider mite, Tetranychus urticas . 

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Spider mites at various life stages. Photo by LSU AgCenter.

Symptoms appear similar to a graying of the leaves which is caused by destruction of chlorophyll containing cells by the piercing mouth parts of the mites.  One may think that the plant is undergoing some type of heat stress or fungal disease, but close observation of the leaves reveals small webs that all spider mites produce.  A magnifying lens with 10x power will reveal the presence of a minute (1/50th inch) spider mite with 8 legs.

The best chance of control is early in the infestation. Miticides are available in garden stores, however most are not effective on eggs.  Therefore plan on multiple applications. After continuous use, most acaricides  become ineffective as the spider mites built resistance to chemical sprays.  If no control measures are taken, populations can spread rapidly to completely defoliate the leaves or to impair plant flowering and fruiting.

Insecticidal soaps, commercially available natural predators, and oils are options that are friendly to nature and less toxic to humans.  Note that oils and soaps may burn plants, especially at higher rates.  Natural predators can be purchased from internet sites, but they are delicate forms of life that can be destroyed during shipping, especially in summer.

Important natural predators include lady bugs, predatory mites (Metaseiulus, Amblyseius, and Phytoseiulus), minute pirate bugs, lacewing larvae, big eyed bugs, and thrips (Leptothrips).  Therefore use of broad spectrum insectides can destroy these predators, making your problems much worse.

Spider mites are difficult to control.  For home gardeners persistence of treatment is the most important aspect of control.  If the infection becomes too difficult, consider eliminating affected plants by bagging and burning during calm periods of the day.  Spider mites are like spiders.  They use their webs to spread in the wind.

The entire life cycle (eggs to larva to two nymphal stages to adult) can be completed within five days, but can be extended with lower temperatures to 20 days.  Adult females can live 4 weeks while producing several hundred progeny. Continuous generations are produced though summer.

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