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Spider Mite Management

Two-spotted spider mite (TSSM), also known as greenhouse red spider mite or carmine mite, is one of the most common pests of protected crops in greenhouses and interior plantscapes. It also occurs on berry and fruit crops and many woody ornamentals grown outdoors.

Adult TSSM are minute, 0.5 mm (1/50 inch) long, and a pale tan color with two greenish-black spots, one on each side of the abdomen. They have 8 legs and, under magnification, 2 red eyespots can be seen. Male TSSM are half the size of the females. Diapausing adults are orange-red with dark side spots. The carmine strain of TSSM found on tomatoes is a brick-red color in the adult stage. Immature and larval mites have 6 legs and the spots are less visible.

TSSM damage plants by piercing and sucking the contents of cells, which results in speckling on leaves as the cells turn yellow and die. Although most mites are on the undersides of leaves, the damage is visible on both leaf surfaces. As damage increases, the whole leaf may turn yellow and wither. The carmine strain of TSSM causes more serious damage to tomato. Yield losses start to occur in greenhouse cucumbers and tomato crops when about 30% of the leaf surface area is damaged. Ornamental plants attacked by TSSM show leaf damage and reduced growth.

A complete TSSM life cycle takes about 14 days at 21C (70F), 33 days at 15C (59F) and only 7 days at 30C (86F). Females lay eggs on the lower leaf surface; larvae hatch from eggs in about 3 days. Nymphs pass through two more stages before becoming adults. Feeding and number of eggs laid by TSSM females increases as temperature rises and humidity drops. Under hot, dry conditions, TSSM populations can cause extensive damage and quickly get out of control. When TSSM populations are high, they disperse easily throughout the crop on air currents and are also carried along on workers’ clothing as they handle plants. TSSM diapause in response to short days, lower temperatures, or deteriorating food supplies. They travel up or down the plant, away from light, searching for protected places to hibernate, such as crevices in the greenhouse structure or at the soil line at the base of posts. This phase doesn’t feed and is very difficult to control with either chemicals or biological controls.

Inspect leaves under 10-15 X magnification for signs of TSSM infestation. Some growers use bean seedlings or climbing runner beans as trap plants to detect the first appearance of TSSM on new crops. TSSM damage is easy to see on bean leaves, which alerts growers that there may be mites present in the crop (the bean plants also serve another purpose as they are good nursery plants for predatory mites.)

Thoroughly clean up the crop at the end of the season. Wash the greenhouse structure with a strong detergent. Dormant oil sprays applied to cracks in concrete, posts and other mite hiding spots will reduce the number of diapausing mites in the greenhouse. Do not maintain ornamental plants in vegetable greenhouses and remove weeds. Keep grass or vegetation mowed adjacent to the greenhouse. Misting TSSM infested sites with fine sprays of water will slow TSSM reproduction and increase the feeding and reproduction of predatory mites.