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Insect control with spiders

  • To: Multiple recipients of list <iris-l@rt66.com>
  • Subject: Insect control with spiders
  • From: bills@tiger.hsc.edu (Bill Shear)
  • Date: Thu, 27 Feb 1997 13:30:13 -0700 (MST)

I've spent the last 35 years or so studying spiders, and every other summer
teach a course in the Biology of Spiders at the Highlands Biological
Station in North Carolina (next offering: Summer 1998).  I thought the
listers might be interested in some work done at the U. of Tennessee on
insect control using spiders.

Several garden plots were set up under very similar conditions.  The
control plots were not mulched and neither insect nor spider control was
exerted.   Another set of plots was mulched (to provide cover for the
spiders); no other insect control.  Yet another plot was mulched, insects
were not controlled, but all spiders seen were removed daily.  Finally,
another set of plots was mulched, no insect or spider control, and flowers
were added (to attract additional insects to feed the spiders).  The least
amount of insect damage--significantly less--was found on the vegetables in
the plots that were mulched and from which no spiders were removed.  The
flowers didn't seem to help.

The experimenters concluded that modifying the habitat (mulching) to
provide cover for spiders encouraged them to control insects.  This result
was surprising because spiders are generalist predators on insects (they
eat any and all insects of an appropriate size), and most biological
control agents are specific to one or a few species of pests.  It was
received wisdom that generalist predators had little impact on insect
pests, since they do not focus on one or a few species.  However, I must
add that the level of damage even in the good spider plot was close to what
many gardeners would consider unacceptable.

In my own vegetable garden, I mulch heavily with autumn leaves collected
the previous fall.  I now, after ten years of this, rarely use any
pesticides at all and see very little damage.  It may be that spider
populations and other beneficials build up over the years to higher
levels--I do see a lot of spiders in my garden (and the house, too, but
that's another story).  Or it may be that mulched plants in a highly
organic soil are simply healthier and more pest-resistant.

In any case, don't kill spiders!

Best wishes, Bill
William A. Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943 USA
phone (804) 223-6172
FAX (804) 223-6374

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