Overview
Two spotted spider mites (Tetranuchus urticae), also known as greenhouse red spider mites or carmine mites, 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.
Description
Adult two spotted spider mites are minute, 0.5 mm long, pale tan in color with two greenish black spots, one on each side of the abdomen. They have 8 legs and two red eye spots can be seen with magnification. Male mites 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.
Damage
Two spotted spider mites 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 plants.
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.
Life Cycle
A complete two spotted spider mite life cycle takes about 14 days at 70°F and 33 days at 59°F and only 7 days at 86°F. Females lay eggs on the lower leaf surface and 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 temperatures rise 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 a deteriorating food supply. 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.
Monitoring Tips
Inspect leaves using a 10x 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.
Biological Controls
Two spotted spider mites can reproduce very rapidly on greenhouse cucumber so it is important to apply biological control agents as soon as mite damage is detected. A combined attack using the three biological controls listed below will provide the best results.
Phytoseiulus persimilis is a predator mite that is very effective control for TSSM on most plants. The life cycle of persimilis is similar to that of TSSM, but the predators develop twice as fast as the pest at moderate greenhouse temperatures. When TSSM are first seen, introduce persimilis onto all infested leaves. On greenhouse peppers and woody ornamental plants, introduce fallacis predatory mites as well as persimilis for control over a longer period.
Stethorus punctillum is a lady beetle that feeds on all stages of TSSM and can be used with other biocontrol agents. Stethorus is active over a wide temperature and humidity range (60-90°F and 20-90% rH). They should be released in mite infested sites as soon as TSSM are detected.
Feltiella acarisuga is a predatory midge that may also be used to control TSSM on cucumber and other crops. Feltiella feeds on all stages of TSSM. Adults fly and locate mite colonies from a distance. Feltiella do best at humidities over 50% rH and at high mite densities. They should be released early in the season once TSSM becomes established.
Using Chemicals
Chemical control in conjunction with biological control is recommended if mites are clustering in balls or 'stringing' down from the plant, or if they are detached in high numbers without predators present. Check compatibility lists before using any pesticide with biological controls.
It is important to finish each crop with low population of two spotted spider mites. High numbers at the end of the year result in the survival of many diapausing (overwintering) spider mites that will re-infest the next crop early in the season. If mites are still a problem in late July, a miticide should be applied in August to prevent diapausing mites from over wintering. Pesticides are less effective against the diapausing form of TSSM present in the fall.
Other Measures
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, spots and other mite hiding spots will reduce the number of diapausing mites in the greenhouse. Also place a small amount of Stratiolaelaps in these areas to reduce TSSM populations. Do not maintain ornamental plants in vegetable greenhouses and remove weeds. Keep grass or vegetation mowed adjacent to the greenhouse.
Misting two spotted spider mite infested sites with fine sprays of water will slow TSSM reproduction and increase feeding and reproduction of predatory mites.