Twospotted spider mite, Tetranychus urticae
Order: Acari
Family: Tetranychidae
Description
Adult females are very small, about 1.3 mm (1/20 inch), and adult males are even smaller. The mites are usually light yellow or green with two dark dots on both sides of their bodies but change color to orange-red when overwintering. In indoor hemp production under controlled conditions, spider mites can remain active year-round. The eggs are relatively large, about half the size of a female mite.
Quick Facts
• Twospotted spider mites are significant pests of indoor grown hemp but are rarely a pest in outdoor hemp production. They are highly polypgahous and feed on wide variety of other plants as well, including corn, pears, raspberry, eggplant, and ornamental plants, to name a few.
• Many spider mite species produce webbing for protection from natural enemies, adverse environmental conditions, and to aid in dispersion to new hosts. When mite densities are high, webbing can cover the entire hemp plant.
• There are many management options available for controlling twospotted spider mite; however, the misuse of miticides can lead to the development of resistant mite populations. When possible cultural and biological control should be incorporated into management programs.
Twospotted spider mite with webbing. These mites are significant pests of indoor grown hemp but are rarely a pest in outdoor hemp production. They feed on a wide variety of other plants, including corn, pears, raspberry, eggplant, and many ornamental plants. Twospotted spider mites are small; however, visual indicators such as the presence of webbing, feeding injury, and plants with a dirty or rusty appearance, can help determine whether an infestation is present. Image credit: David Cappaert, Bugwood.org
Twospotted spider mite eggs, exuviae, and adult. Image credit: David Cappaert, Bugwood.org
Adults of twospotted spider mite with orange-red appearance. Image credit: David Cappaert, Bugwood.org
Infestation of twospotted spider mite on hemp. Note the abundant webbing. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
After emerging from an egg, the six-legged larva begins feeding on the host plant until molting. After the first molt, the mite is a slightly larger 8-legged nymph. The nymph molts two more times and develops into an adult. A single female can lay several hundred eggs over a period of two to four weeks, with multiple generations produced over a 2–3-month period. Outdoors, females overwinter in leaf litter or other protective covers such as tree bark.
Injury
Spider mites have piercing/sucking mouthparts and their feeding causes speckling, stippling, and wilting of leaves. The mites feed on the underside of leaves. When densities are high, the leaves desiccate, turn yellow or bronze, and may drop prematurely. Feeding injury of twospotted spider mite can also cause reduced flower growth. Discoloration of stems and the presence of black dots (mite feces) on leaves can also be observed in severe infestations. These mites produce webbing that can cause cosmetic damage and cover the whole hemp plant, causing it to appear rusty or dirty.
Monitoring
During hot, dry conditions, plants should be inspected for symptoms of feeding injury such as stippling and leaf wilting. The presence of webbing over plants indicates that a severe infestation is present. Using a hand lens, the undersides of leaves should be examined for slow-moving mites and the presence of debris appearing as tiny specks. To sample mites, dislodge mites by shaking hemp stems over a piece of white paper or cloth tray and count the number of mites on the white surface.
Biological control
There are several natural enemies of twospotted spider mites that can be incorporated into management programs. The predatory mite, Phtoseiulus persimilis, is commercially available in the U.S. and provides effective control against twospotted spider mites in hemp. Other insect predators like green lacewing larvae and minute pirate bugs can also be used as biological control agents of the mites.
Cultural control
In indoor hemp production, plants infested with twospotted spider mites should be isolated from clean plants. Movement of personnel and equipment between infested and clean areas should be minimized. Removing overwintering host plants near the cultivated field can help reduce mite numbers, and it is recommended that weed removal continue throughout the growing season. Proper watering is extremely important since drought-stressed plants are more susceptible to outbreaks of spider mites.
Chemical control
Prolonged and repeated use of insecticides and acaricides will lead to resistance. When acaricides are used, multiple applications are required at differing intervals depending on the season.
Various soaps and oils can reduce twospotted spider mite densities temporarily and have low non-target effects. For maximum effectiveness, all parts of the plant need to be covered completely and this is best accomplished through high spray volume with high pressure. Soaps and oils do not kill eggs and need frequent reapplications since direct contact with mites is necessary. For more specific information on chemical control of twospotted spider mite, consult the Utah State University fact sheet and the Pacific Northwest Pest Management Handbook.
References
Colorado State University. (n.d.). Twospotted Spider Mite. Available https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Twospotted%20spider%20mite%20with%20photos.pdf
Fasulo, T. 2009. Twospotted Spider Mite. University of Florida. Available https://entnemdept.ufl.edu/creatures/orn/twospotted_mite.htm#:~:text=The%20twospotted%20spider%20mite%20is,12%20pairs%20of%20dorsal%20setae.
Oregon State University. (n.d.). Hemp-Mite (Two-Spotted spider). Pacific Northwest Pest Management Handbooks. Available https://pnwhandbooks.org/insect/agronomic/hemp/hemp-mite-two-spotted-spider
Pulkoski, M., & H. Burrack. 2020. Twospotted Spider Mite in Industrial Hemp. NC State: Extension. Available https://content.ces.ncsu.edu/twospotted-spider-mite-in-industrial-hemp#:~:text=The%20twospotted%20spider%20mite%20(TSSM,most%20vegetables%20and%20field%20crops.
Utah State University. (n.d.). Two-Spotted Spider Mite. Utah State University: Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/hemp-spider-mites