 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.
REFERENCES
(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.
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