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Spider Mite
Control Biologically |
15
By Ramana G. Colfer, Jay A Rosenheim, and Larry D. Godfrey
Information for this article was taken from
a paper presented by UC Davis Researchers, Ramana G. Colfer,
Jay Rosenheim, and Larry D. Godfrey. Research for this project
was conducted at UC Davis. The article was published in
full in California-Arizona-Texas Cotton Magazine, Fall 2001.
Spider mites, Tetranychus spp. are foliar
feeders in cotton which destroy photosynthetic cells as
they feed on the plant. High populations of mites can lead
to defoliation and are known to cause economic damage to
cotton in the San Joaquin Valley. “While spider mites
can be important primary pests, they are especially known
as secondary pests. They display rapid rates of population
growth following the application of broad spectrum insecticides
(carbamates, organophosphates, and pyrethroids) for control
of Lygus bugs or cotton aphids.”
Cultural practices can also play a role in
mite outbreaks as well. Dusty roadways and dry dusty fields
can contribute to the problem. Watering roads or using ground
covers will help reduce outbreaks. Plants stressed for water
will be less tolerant of spider mite damage as well. Spider
mite populations reproduce rapidly during hot weather and
in most parts of California may feed and reproduce all year.
Natural Predators
Knowing that spider mites have many natural enemies, UC
Davis Researchers, Ramana G. Colfer, Jay Rosenheim, and
Larry D. Godfrey set out to look at controlling spider mites
biologically. Their study was designed to evaluate the naturally
occurring biological control of spider mites. The study
focused on two groups: omnivorous western flower thrips,
Frankliniella occidentalis and generalist predatory bugs,
Big eyed bugs (Geocoris spp.), and (O. tristicolor) minute
pirate bugs.
“Researchers found that western flower
thrips were successful in substantially reducing spider
mite population growth during the early season, and this,
led to greater seed cotton yields at the end of the season.”
They also did not observe any yield losses from early-season
plant feeding by the western flower thrips.
The generalist predators, especially big eyed
bugs and minute pirate bugs were very effective at suppressing
mites. “For three different field seasons, we found
that predators suppressed spider mite populations to levels
76 to 99 percent below those observed where generalist predator
abundance has been reduced.”
The study observed that generalist predators
suppressed mites over a wide range of initial mite densities.
This again points out the importance of preserving natural
enemies in controlling spider mites. Whenever possible,
replace broad spectrum insecticides with more biological
alternatives. Insecticides applied during hot weather usually
appear to have the great effect on mites, causing dramatic
outbreaks within a few days.
Releases of predatory mites can be very useful
if naturally occurring predators are not abundant. Regular,
careful monitoring can help growers keep track of field
conditions .
“Cotton is generally inhabited by a diverse and abundant
complex of generalist predators. If the populations of these
predators are conserved, there is the potential to partially
or completely control many pests of cotton.”
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