Spider Mite Control
Description: Common across North America, many species of the spider mite (family: Tetranychidae) attack both indoor and outdoor gardens and can be very destructive in greenhouses. They live in colonies, mostly on the underside of leaves and feed by piercing leaf tissue and sucking up the plant fluids. Feeding marks show up as light dots on the leaves; as feeding continues, the leaves turn yellow, and may dry up and drop off. Spider mites are most common in hot, dry conditions and when their natural enemies have been killed off by insecticide use. They are also very prolific, which is why heavy infestations often build up unnoticed before plants begin to show damage. Large populations may be accompanied by fine webbing. Host plants are many and include strawberries, melons, beans, tomatoes, eggplant, ornamental flowers, trees and most houseplants.
Spider mites are not true insects, but are classed as a type of arachnid, relatives of insects that also includes spiders, ticks, and scorpions. Adults are reddish brown or pale in color, oval-shaped, and very small (1/50 inch long) – about the size of the period at the end of this sentence. Immature stages resemble the adults except only smaller.
Life Cycle: Most spider mite species overwinter as eggs on the leaves and bark of host plants. In early spring, as temperatures warm, tiny six-legged larvae begin hatching and feed for a few days before seeking shelter where they molt into the first nymphal stage. Nymphs have eight-legs and pass through two more molts before becoming mature adults. After mating, females are capable of producing as many as 300 eggs over a couple of weeks. Hot, dry weather favors rapid development of these pests. During such conditions the time it takes to pass from egg to adult may occur in as little as 5 days. There are several overlapping generations per year.
Note: Dispersal over a wide area occurs when spider mites are carried on their webbing by the wind.
Control: If pests are found, pinch or prune off infested leaves or other plant parts. Use the Bug Blaster or wash plants with a strong stream of water to reduce pest numbers. Commercially available beneficial insects, such as ladybugs, lacewing, and predatory mites are important natural enemies. For best results, make releases when pest levels are low to medium. If populations are high, use a least-toxic, short-lived natural pesticide to establish control, then release predatory insects to maintain control. Insecticidal soap or botanical insecticides can be used to spot treat heavily infested areas. Horticultural oils should be applied early in the season or late in the fall to destroy overwintering eggs.
Tip: Control strategies must take into account the fast development time of this pest, especially during warm weather when eggs are laid continuously. Just targeting the adults will do little good – repeat treatments are almost always necessary.
Photo Credit: University of Maryland Extension