Two-spotted spider mites (Tetranychus urticae) have been observed recently, especially on plants in hanging baskets hung high in hot dry areas in the greenhouse. Two-spotted spider mites (TSSM) are a major pest of many greenhouse crops. They feed primarily on the undersides of leaves where they pierce the cells with their stylet-like mouthparts and suck out the contents. The feeding injury results in a faint chlorosis or stippling on upper leaf surfaces. The destruction of cells results in reduced photosynthesis and, as damage increases, whole leaves turn yellow. Leaves may drop and eventually lead to death of the plant. In severe infestations, plants may be covered with webbing, which is why they are referred to as spider mites.
Look on leaf undersides for the mites and their eggs. Magnification, such as a hand lens, is needed to see them. TSSM are small and have eight legs. Adult females are about 0.5mm long and have an oval body that is rounded at the rear end. Males are slightly smaller with a body that is narrower and more pointed at the rear end. The color can vary from orange, to light yellow, to yellow-brown, to dark-green, with two dark spots on both sides of the body. TSSM lays more eggs and develops faster at high temperatures and lower humidity. Development starts around 54º F, increases with temperature, starts to decline after 86º F and stops at 104º F. TSSM can be a problem during dry and hot weather conditions. Water-stressed plants are more susceptible to spider mites.
Biological management: The most effective biological control agent for TSSM is the predatory mite Phytoseiulus persimilis. P. persimilis feeds only on two-spotted spider mites and feeds on all life stages, but has preference for eggs. Phytoseiulus persimilis develops faster than TSSM, as long as temperatures are not too high. At 68°F the average generation time of P. persimilis is shorter than that of the mites. Under dry warmer conditions P. persimilis has difficulty keeping up. The predatory mite Neoseiulus (Amblyseius) californicus is more tolerant of higher temperatures and lower humidity. Large concentrations of TSSM (hotspots) can also be controlled using gall midge Feltiella acarisuga. The larvae of Feltiella feeds on all stages of two-spotted spider mites.
Chemical management: For chemical management of TSSM, contact or translaminar miticides can be used. Translaminar miticides are more effective because they penetrate leaf tissues and form a reservoir of active ingredient within the leaf which provides extended residual activity. Treatments require good coverage to the undersides of the leaves and may need repeat applications during warm weather. Miticides with translaminar activity include abamectin (Avid, Lucid, Minx2), abamectin & bifenazate (Sirocco), chlorfenapyr (Pylon), and spiromesifen (Savate). Contact miticides include fenpyroximate (Akari), bifenazate (Floramite SC, Actaute SC), hexythiazox (Hexygon, Hexcel 50 DF), Clofentazine (Ovation), and pyridaben (Sanmite).
Do not use Savate on New Guinea impatiens or geranium. Always read pesticide labels for plant safety information and make sure to follow all label recommendations and restrictions. Also be sure to rotate across different modes of action (MOAs) to prevent resistance development in the pest population.
For more information on TSSM refer to the following links:
Geoffrey Njue, Extension Educator, UMass Extension's Greenhouse Crops & Floriculture Program