How To Control Spider Mites

Don’t let spider mite infestations get out of control along with summer’s heat. Here’s how to keep their populations in check.

by Jessica Walliser
PHOTO: Jessica Walliser

As summer’s heat arrives so too do the spider mites. These little critters are barely visible to the human eye. They aren’t insects, but rather relatives of spiders and ticks. They have eight legs, though you’ll need a magnifying glass to count them, and even at full maturity, they measure less than 1/20 inch.

Spider mites live in large colonies and collectively they spin fine webbing to shelter themselves. Often the webbing is spotted far before the mites themselves. The webs are commonly found on the undersides of infested leaves and are far finer than spider webs are. The best way to see the mites themselves is to shake an infested leaf over a sheet of white paper and look for small moving specks on the paper.

Spider mites are found all across North America, and they produce several generations per year. Each mite lives only a few days, so females lay many eggs and populations can quickly spiral out of control if there are no predators to keep them in check.

Because these mites feed by sucking out plant juices, they cause mottled, yellow foliage. Often the mottling is more noticeable on the top leaf surface and the webs are more noticeable below. Severely damaged foliage will often dry up and drop off the plant. Hot, dry weather exacerbates the damage, as the plants are already stressed and the mites are easily transported from one plant to another on summer breezes.

Spider mites are often found on garden phlox, dwarf Alberta spruce, azaleas, melons, grapes, strawberries and other plants. Removing affected leaves helps, but it isn’t enough to control infestations. Millions of mites are already present before gardeners even notice damage to their plants.

Spider mite populations can explode after pesticide applications. This is because these pesticides also kill off the predatory beneficial insects naturally keeping mite numbers in check. Some chemical pesticides actually stimulate mite reproduction. Encourage beneficial predatory insects, like minute pirate bugs, big eyed bugs, ladybugs, lacewings and damsel bugs by interplanting susceptible plants with lots of flowering herbs, sweet alyssum, members of the aster family and other plants with small flowers.

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Also, be sure to keep your garden well-watered, and mulch it well to keep the soil moist. If it still remains necessary for you to use a product to control the mites, turn to lightweight horticultural oils or insecticidal soaps, as they are quite effective. Be sure to follow label instructions carefully and apply to the undersides of the leaves in addition to the tops and stems.

Predatory mite species that feed on spider mites can also be purchased from insectaries and released into the garden. They are a huge help in keeping pest mite species in check.

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