Spider Mites

Problem type: Insect

Name of problem: Spider Mites

Plant name(s): All plant types are susceptible to spider mites including spruce, juniper, rose, daylily, perennials, African violet, apple trees and other various deciduous trees and shrubs

Symptoms / Characteristics:
Spotting, mottling or flecking of foliage showing discoloration or yellowing. Where large populations are feeding, chlorotic areas will coalesce making large portions of the leaf and often, entire leaves become bleached looking or bronzed and slightly scorched. Injury can cause leaf and needle drop and will ultimately cause severe stress on the plant, or kill the plant.

Spider mites are not insects but more closely related to spiders, having four pairs of legs and a single oval body. They are generally orange-red in the adult stage and lighter to white when young. The two-spotted spider mite is greenish-yellow with two dark spots on its back. Mites are very small, ranging from 0.25-1 mm long, making them difficult to see without a hand lens. Their mouthparts allow them to produce webbing or silk strands onto leaves, needles and branches. This webbing gives the mites and eggs protection against natural enemies and environmental conditions. Silk production varies among spider mite species, and some will make very little webbing.

Spider mites feed by sucking fluids from plant tissue, causing the stippling and yellowing of foliage. Mites and webbing will usually be found on the underside of leaves. They do not travel far and many mites and eggs can be found on one leaf. They can be blown by wind to other hosts by hanging on their webbing. Females can lay a dozen eggs daily for several weeks and the mites develop from eggs to adult in just a few days. This can lead to large and damaging populations in a short period of time. Spider mites thrive under hot, dry, windy conditions whereas wet weather slows reproductive rates.

Mites can overwinter on host plants, in the soil, debris, or in grass as either adults or eggs, becoming active again in the spring when they seek a suitable host.

Control / Preventions:
Spider mites are usually not killed by regular insecticides, leading to one reason why spider mites have become a problem. By applying the wrong chemical, natural predators of the spider mite are killed, contributing to spider mite outbreaks. Spider mites have many natural enemies, including the lady beetle and lacewings that can provide effective control if present. Commonly, predatory mites are purchased and used to control spider mites. If biological predators are being used, do not apply pesticides that will kill them.

Spraying foliage with a forceful stream of water can remove and kill many mites as well as removing webbing that collects dust and protects mites against predators. This method will also conserve the natural predators. Plants can be sprayed with horticultural oil, dormant oil and insecticidal soap for effective control. Thorough coverage of plant is necessary for control.

There are few chemical controls that will effectively control spider mites and are usually not recommended. There are chemicals made for spider mite control and are called miticides or acaricides. When purchasing a pesticide for spider mite control, be sure that one of those names are present on the label. Products claiming spider mite control might have only weak miticides in them and will not be very effective. Repeating chemical applications at 10-14 day intervals will almost always be needed, as eggs are usually not affected.

Inspect plants before purchasing, looking closely at the underside of leaves for any sign of mite activity or webbing. To observe mites, shake a branch over a piece of white paper. Moving specks could indicate spider mites.