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Pest Status
Background
Releases and Recoveries
Chapter 22
Tetranychus urticae Koch, Twospotted
Spider mite (Acarina: Tetranychidae)
R.J. McCLANAHAN
The twospotted spider mite. Tetranychus urticae Koch, continues to be a major problem
in greenhouse crops, particularly cucumbers, chrysanthemums, and roses. The floral
crops are fairly well protected by granular applications of the systemic insecticide
aldicarb, but cucumber growers are having a difficult time keeping mites under control.
Resistance to dicofol and Pentac® (Zoecon Industries Ltd.) has developed in some
greenhouses. The effective material oxythioquinox was withdrawn from use on cucumbers
in 1974.
The predaceous mite Phytoseiulus persimilis Athias-Henriot has been the only biological
control agent seriously considered for control of T. urticae in greenhouses,
although other mite predators are effective in orchards. The European workers have had
encouraging results with P. persimilis and mass rearing is carried out at a number of
government and commercial establishments. Russian sources report a production of 83
million P. persimilis per year from one rearing unit (Askretkov & Tolmacheva 1980).
Probably the most extensive use of P. persimilis on greenhouse crops is in the Netherlands
where some 400 ha (60% of total production) of cucumbers is protected by the
predator (Koppert 1980).
Research on the interaction of prey mites and P.persimilis has mostly been reported
from Europe. Methods of introduction have been worked out for tomatoes (French et
al. 1976) and cucumbers (Gould 1977), while the optimum conditions of temperature and
humidity were defined by Stenseth (1979b) in Norway. Many references pertain to the
compatibility of various insecticides and fungicides and the use of P.persimilis (e.g.
Jeppson et al. 1975), but these results were superseded by investigations using a strain of
P. persimiiis resistant to phosphate insecticides (Stenseth 1979a).
At Harrow, laboratory studies proved that some acaricides were selective in favor of
the predator (McClanahan 1971). However, the best materials were experimental or not
widely accepted by growers for twospotted mite control. Since then oxythioquinox has
been withdrawn from use on greenhouse cucumbers, and the twospotted mites have
developed varying degrees of resistance to acaricides (McClanahan 1980).
It is difficult to detail releases of P. persimilis in Canada, since there are several sources,
including commercial enterprises, and only part of the production is used in Canada. A
colony has been maintained at the Harrow Research Station for many years, and limited
numbers are available on request by research establishments or educational projects.
Acknowledgements of such shipments usually record successful establishment. P.
persimilis are reared and sold by Better Yield Insects in Ontario and by Applied Bio­
Nomics Ltd. in British Columbia. The number of predators available at the release point
is dependent on the number of prey mites in the shipment and on the interval between
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