Two-spotted spider mite (Tetranychus urticae)
Pest description and crop damage Two-spotted spider mite (TSSM) (Acari: Tetranychidae) is a polyphagous piercing-sucking mite pest with over 300 host plants. TSSM is small in size (about 1/50 inch), oval in shape and normally pale yellow to bright green in color with two large characteristic dark spots on the either side of the body. TSSM adults have four pairs of legs, and male adults are smaller than the female adults. TSSM can be observed with the naked eye, but microscopy can further assist in proper identification.
TSSM is mainly a concerning pest in indoor hemp production as it is usually managed by naturally occurring biological control agents (natural enemies) in outdoor production. TSSM feeds on plant cell contents and can be found on the underside of hemp leaves. TSSM feeding causes white stippling marks (look like pin pricks) on the hemp plant leaves. Additionally, black spots on the leaves consisting of mite feces are often visible. In extreme situations, TSSM causes webbing that can cover hemp flower buds or, in extreme cases, whole plants.
Biology and life history TSSM can infest hemp plants in both indoor and outdoor production in the PNW. Populations can build quickly when there is high temperature and low humidity; and in such ideal conditions, TSSM can potentially complete a lifecycle in just over a week. In indoor hemp production systems, TSSM may complete a lifecycle in 1-2 weeks. TSSM females usually deposit eggs on the leaf surface, typically on the underside of leaves, and the size of an egg is about half the size of an adult female. In about 2 days, a minute 6-legged larva emerges from the egg and starts to feed. The larva then molts to an 8-legged nymph and goes through one more nymphal stage before turning into an adult. Immediately after emergence, adults' mate and start to produce eggs. In outdoor conditions, TSSM populations overwinter as adult females in leaf litter and other debris on the soil surface, and they are orange red in color compared to pale yellow to bright green color during summer.
Scouting and thresholds As discussed above, TSSM populations are usually controlled by natural enemies in outdoor hemp production, and scouting is usually not critical. Recent research from Colorado State University and Virginia Tech University suggest that even if TSSM infests hemp plants grown outdoors, the injury is unlikely to be significant. On the other hand, mite scouting is important in indoor hemp production. It is important to inspect the plants and quarantine as needed prior to introducing plants to the greenhouse.
TSSM populations can be effectively controlled by biological control agents. Specifically, the predatory mite Phytoseiulus persimilis is reported to be an effective biological control agent against TSSM, and this agent is commercially available in the US. Other biological control agents for TSSM include green lacewing larvae and minute pirate bugs.
Removing TSSM infested hemp plants and isolating them from clean plants can help to minimize the spread of TSSM in indoor production.