Spiders the Ultimate Predator?
Researchers are just beginning to evaluate the potential for native insect
predatorsincluding spiders such as this long-jawed orb weaverto
hold agricultural pests in check.
Researchers investigate a broad range of biological tools for control of
insect pests. These include use of parasites, pathogens, growth regulators,
bio-insecticides, and foreign predators.
Now, Agricultural Research Service
scientists are beginning to exploit a vast reserve of untapped potential
--native predators, including spiders, a neglected group of biocontrol
For example, when scientists checked out a Georgia cottonfield last year,
they determined that about one-fourth of the specimens of one spider species
had dined on eggs of two cotton pests. In Colorado, they conducted the first
North American survey for spiders that kill cereal aphids and other wheat
These predators belong to the animal phylum known as arthropods --creatures
with jointed legs, a segmented body, and a hard exoskeleton.
"Arthropods make up the vast majority of all known animal
species,"says ARS entomologist Matthew H. Greenstone. "They include
arachnids --the class that includes scorpions, mites, and spiders --as well as
insects, centipedes, and millipedes."Unlike insects that have three pairs
of legs, arachnids have four pairs.
"In most agricultural ecosystems,"says Greenstone, "arthropod
predators are the most consistently present and abundant natural enemies of
insect pests. But they have been largely neglected, mainly because their
effectiveness has been difficult to demonstrate."
Greenstone works today at ARS' Plant Sciences and Water Conservation
Research Laboratory in Stillwater, Oklahoma. But he started his investigations
of native predators in 1982 in Columbia, Missouri, at the agency's Biological
Control of Insects Research Laboratory. There he pioneered new biochemical
tests to help scientists include these predators in their biocontrol research.
Using a gasoline-powered insect vacuum, technician Brian Jones samples the
number of spiders at various points in an Oklahoma wheat field. The tubular
extension prevents crushing of wheat stems and enables the airflow to be
maintained for efficient sampling.
He believes that the most efficient and direct approach to gathering
long-term data on arthropod predation is serological analysis. This means using
antibodies --molecules produced by white blood cells that he grows in a special
medium in his laboratory incubator --in assays to identify the remains of prey
in a predator's gut.
"These assays,"he says, "must be specific, because the gut
may contain not only target antigens --molecules recognized by the antibodies
--but also antigens from other related species. The assays have to be highly
sensitive, because the antigens often occur in exceedingly small amounts,
sometimes less than a few millionths of a gram.
Gut Assays While You Wait
Greenstone pioneered use of monoclonal antibodies to analyze the predator
guts. Working with ARS biological technician Clyde E. Morgan at Columbia,
Greenstone used the spined soldier bug as a laboratory model to develop a fast
ELISA (enzyme-linked immunosorbent assay) test. In just 2-1/2 hours, it could
detect remains of cotton bollworm larvae in the soldier bug's gut.
"This test, developed in 1982, was the first to use a monoclonal
antibody to study predation,"says Greenstone. "It was also the first
to distinguish different growth stages of a single caterpillar species, as well
as related caterpillar species."
The test distinguishes cotton bollworms from their cousins, the tobacco
budworm and groundcherry fruitworm. It also recognizes three other caterpillar
pests in the same family --the fall armyworm, cabbage looper, and velvetbean
caterpillar --along with the imported cabbageworm.
In 1990, Greenstone, working with ARS microbiologist Melissa K. Stuart at
Columbia, perfected and simplified the monoclonal antibody-based predator gut
content assays. They developed a simple assay --called the immunodot --that was
less expensive and faster than ELISA.
Long-jawed orb weavers are abundant spiders in Colorado and Oklahoma wheat
fields. They feed on greenbugs, which are serious pests of winter
"The immunodot assay does not require the elaborate and expensive
equipment needed for interpreting ELISA assays,"Greenstone says.
A few years later, Greenstone, Stuart, and entomologist James H. Hunt at the
University of Missouri at St. Louis proved that the immunodot assay worked with
jumping spiders and paper wasps, as well as with soldier bugs.
"Such rapid, easily interpreted assays make immunoassay technology more
accessible to arthropod ecologists. That, in turn, is helping to increase our
knowledge of arthropod predators' role in biocontrol,"says Greenstone.
Since 1995, he has continued this work in Stillwater.
Last year, Greenstone and University of Georgia entomologist John R.
Ruberson collaborated on research that will alert growers as to which natural
enemies are attacking budworms and bollworms in cottonfields.
"Bollworms and budworms alone cost southeastern cotton growers several
hundred million dollars a year in damage and chemical controls,"says
Greenstone. "To make intelligent pest management decisions, growers need
to know which natural enemies are attacking these two pests."
Ruberson collected more than 3,000 predatory insects and spiders from a
30-acre cottonfield. The most important predators were big-eyed bugs, red
imported fire ants, Scymnus lady beetles, and a native winter spider
species, Chiracanthium inclusum.
Ruberson analyzed the creatures with the ELISA test, using Greenstone's
monoclonal antibody. He found that 6.5 percent were positive for budworm and
bollworm egg remains.
"This 6.5 percent appears low,"Greenstone says, "but the egg
numbers themselves were fairly low during most of the season. Some predators
had high rates of egg feeding, including 25 percent of the winter
The assay and insect census data will be combined to determine the relative
importance of each predator in suppressing budworms and bollworms.
Entomologist Matthew Greenstone replenishes growth medium for hybridomas, the
cultured white blood cells he uses to produce monoclonal antibodies to analyze
the gut contents of spiders and other arthropod predators.
In Stillwater, Greenstone's new focus is biological control of cereal
aphids. He and ARS biological technician Brian G. Jones recently completed the
first quantitative survey of spiders in winter wheat in North America.
"Very good data from Great Britain and northern Europe show that
spiders can help to control aphids in wheat and barley fields in those
countries,"says Greenstone. "Surprisingly, nobody had looked at
spiders in these crops on this continent.
"We surveyed winter wheat fields in southeastern Colorado and uncovered
a spider fauna much more diverse than those reported in Great Britain, Northern
Europe, and New Zealand. There, one family of spiders --Linyphiidae, the
line- or sheet-web weavers --dominates.
In Colorado, Linyphiidae constituted just one-fifth of the spider
individuals. Six other families each made up 10 percent or more,"says
"Now that we know the spiders, we need to develop the tools to perform
gut analysis to detect Russian wheat aphids, greenbugs, and other important
aphids in wheat and barley. Then we'll be able to combine gut and census data
and determine which spiders and other arthropod predators are most important in
wheat and barley fields."
Today, in collaboration with an independent antibody-producing facility at
Oklahoma State University, Greenstone is developing monoclonal antibodies to
detect the remains of three major cereal aphids --the greenbug, Russian wheat
aphid, and corn leaf aphid.
Another collaborator, William O.C. Symondson of the University of Wales in
Cardiff, is developing monoclonals for the English grain aphid, bird cherry oat
aphid, and rose grass aphid.
"With these six antibodies,"Greenstone says, "we will be able
to study the role of predators in suppressing cereal aphids in many parts of
the world." --By Hank
Becker, Agricultural Research Service Information Staff.
Matthew H. Greenstone is at the USDA-ARS
Plant Sciences and
Water Conservation Research Laboratory, 1301 N. Western St., Stillwater, OK
74075; phone (405) 624-4119, fax (405) 372-1398.
"Spiders the Ultimate Predator?" was published in the
August 1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.