Tetranychus urticae (twospotted
spider mite) causes an infestation
of houseplants and of cultivated
plants, and is known as one cause
of contact urticaria.
The cultivated plants which tend
to get infested include the leaves
of lima beans, common dry beans,
and blackeyed peas.(1)
Tetranychus urticae, also known as the twospotted spider mite, is commonly encountered on house plants. Spider mites are pests of cultivated plants, and are most likely encountered under dry and warm conditions. When mites are very numerous, they produce a silky webbing covering infested areas and extending from leaf to leaf to cover the entire plant. These mites suck out plant juices. The loss of chlorophyll first results in whitish or yellowish speckled areas on the upper surfaces of leaves, and eventually in a more uniformly bronzed or yellowed discoloration, defoliation, and studting or even death of the plant.(2)
Mites are members of the arachnid class along with spiders and ticks. Hard to visualize well with the naked eye, they are easily viewed with 10Xmagnification. The largest forms, adult females, are about one millimeter long. Adults have 8 legs and an oval body, with 2 red eyespots near the head end of the body. Females usually have a large dark blotch on each side of the body and numerous bristles coveing the legs and body.(3)
They live in colonies under surface of leaves, each colony may contain hundreds of individuals. The term "spider mites" is derived from the tendency to produce a silk webbing on infested leaves. This webbing is an easy way to distinguish them from all other types of mites.(3)
Spider mites are a known source of occupational asthma, hay fever, and urticaria.(4) The immediate type sensitivity may have cross reaction with the house mite, Dermatophagoides pteronyssinus.(5) Of interest to dermatologists is that this common pest may be responsible for the development of contact urticaria.(6) Most likely individuals to develop this sensitivity are those who work in greenhouses with the affected plants.
(1) UC pest Management Guidelines. Beans Spider Mites. University of Californai Satatewide Integrated Pest Management projet. http://
(2) Ebeling, Walter. Urban Entomology. Division of Agricultural Sciences University of California Berkeley, 1975. pp 548-549.
(3) UC Pest Management Guidelines. Spider Mites Home & Landscape. University of California Statewide Integreated Pest Managment Project.: http://
(4) Delgado J; Gomez E; Palma JL; Gonzalez J; Monteseirin FJ; Martinez A; Martinez J; Conde J. Occupational rhinoconjunctivitis and asthma caused by Tetranychus urticae (red spider mite). A case report. Clinical and Experimental Allergy, 1994 May, 24(5):477-80.
(5) Kroidl R; Maasch HJ; Wahl R. Respiratory allergies (bronchial asthma and rhinitis) due to sensitization of type I allergy to red spider mite (Panonychus ulmi KOCH). Clinical and Experimental Allergy, 1992 Oct, 22(10):958-62.
(6) Reunala T; Bjorksten F; Forstrom L; Kanerva L. IgE-mediated occupational allergy to a spider mite. Clinical Allergy, 1983 Jul, 13(4):383-8.
© 1997 Dermatology Online Journal