Spider Mites

Spider Mites Often found on the undersides of plant leaves, these tiny sucking pests can wreak-havoc on indoor and outdoor gardens. Learn methods for organic spider mite control here. Description

Many species of the spider mite (family: Tetranychidae), so common in North America, attack both indoor and outdoor plants. They can be especially destructive in greenhouses.

Spider mites are not true insects, but are classed as a type of arachnid, relatives of 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.

Mites 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, especially where their natural enemies have been killed off by insecticide use. Some of the many species common in North America are predators of the plant-feeding mites, which make up the vast majority. They are also very prolific, which is why heavy infestations often build up unnoticed before plants begin to show damage.

Large populations are often accompanied by fine webbing. Host plants are many and include strawberries, melons, beans, tomatoes, eggplant, ornamental flowers, trees and most houseplants.

Life Cycle

Most 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 continuously produce 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:  Spider mites are wind surfers. They disperse over wide areas riding their webbing on the breezes. Careful containment and disposal of infested plants is crucial.

Damage

Spider mites, almost too small to be seen, pass into our gardens without notice. No matter how few, each survives by sucking material from plant cells. Large infestations cause visible damage. Leaves first show patterns of tiny spots or stipplings. They may change color, curl, and fall off. The mites activity is visible in the tight webs that are formed under leaves and along stems.

The University of California Agricultural and Natural Resources division’,s Integrated Pest Management website  says the following about the damage mites cause:

• On annual vegetable crops — such as squash, melons, and watermelons — loss of leaves can have a significant impact on yield and lead to sunburning.
• On crops such as sugar peas and beans, where pods are attacked, spider mites can cause direct damage.
• On ornamentals, mites are primarily an aesthetic concern, but they can kill plants if populations become very high on annual plants. Spider mites are also important pests of field-grown roses.

Control

Chemical pesticide use actually encourages the spread of spider mites by killing the beneficial insects that prey on them. Mites are also known to develop quick resistance to various pesticides. For these reasons, it’,s important to control mites with effective, natural, organic methods.

• Prune leaves, stems, and other infested parts of plants well past any webbing and discard in trash (and not in compost piles). Don’,t be hesitant to pull entire plants to prevent the mites spreading to its neighbors.
• Use the Bug Blaster  to wash plants with a strong stream of water and 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 pesticide (Take Down Spray ) to reduce infestations, then release predatory mites to maintain control.
• Insecticidal soap. neem oil. or botanical insecticides can be used to spot treat heavily infested areas.
• On fruit trees, horticultural oil should be applied early in the season or late in the fall to destroy overwintering eggs.
• Dust on leaves, branches, and fruit encourages mites. A mid-season hosing (or two!) to remove dust from trees is a worthwhile preventative.
• Water stress makes both trees and garden plants more susceptible to mite infestations. Make sure your plants are properly watered.

Tip:  Management 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 if eggs and larvae survive. Repeat treatments are almost always necessary. The use of leaf shines and washes  helps control and prevent further infestations.

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Photo Credit: University of Maryland Extension

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