A complex matrix of factors affects whether or not grazing is the right
solution to grasshopper problems. Weather conditions, grasshopper densities,
and the type and composition of plant communities must all be considered.
Branson is studying different grazing strategies over large areasfrom
100 to 1,000 acresto see which grazing systems reduce grasshopper
Fire on the Range
Sidney researchers have turned to another force that can alter grasshopper
One such fire that occurred near the Montana laboratory wasn't planned.
It rolled through the U.S. Forest Service's Little Missouri National
Grassland in western North Dakota in the late fall of 2001. Fortunately,
it swept through an area where the researchers had already established
sampling sites for another experiment.
"A year after the fire, we saw a significant reduction in grasshopper
densities," says Branson. Different grasshopper species lay their
egg pods at different depths in the soil. "Pods at shallow depths
died, as did grasshoppers that spend the winter as nymphs. But pods
laid deeper appeared less affected by the fire."
Branson and colleague Lance T. Vermeire, an ARS rangeland ecologist
at Fort Keogh in Miles City, are further investigating how fire affects
individual species of grasshoppers and their eggs as well as the plant
communities on which they depend.
In a prescribed fire conducted in the Little Missouri National Grassland,
Branson again observed a short-term reduction in grasshopper densities
after the burn. But only a few of the dozen species that can contribute
to an outbreak appeared to be affected.
"If a few particular species of grasshopper are dominating an
area, fire could be beneficial for reducing hopper numbers," Branson
says. "But we'd also have to consider the fire's effects on rangeland
conditions, plant production, and ground cover."
For all the trouble grasshoppers seem to cause, researchers are finding
that they could be fulfilling an important function on managed rangelands,
native grasslands, and prairies.
Grasshoppers digest only about 12 to 20 percent of the plant material
they eat. The rest is excreted as "frass," which acts as fertilizer
for the surrounding vegetation. "We're finding that in some instances,
grasshoppers can improve rangeland productivity," says Branson.
"It depends on what they're eating: When they feed on plants that
decompose more slowly, they are able to cycle those nutrients back to
the environment faster."
One long-term goal is to figure out which prevailing ecological conditions
allow grasshoppers to have this desired effect. "Grasshoppers actually
have very specific plant preferences, so the particular combination
of plants and hoppers on rangeland plays a big role. It could be that
if a rancher is facing moderate grasshopper densitiesand the right
rangeland conditions existit might be a bad idea to try to control
the hoppers. In the long run, they could be doing more good than bad,
especially when you consider what it costs to control them."
Branson adds that during an outbreak, however, control might prove
beneficial to long-term rangeland productivity.
Stefan T. Jaronski, an insect pathologist at the Sidney laboratory,
studies naturally occurring microbes that can be used to suppress outbreaks
of hoppers as well as Mormon crickets. The fungus Beauveria bassiana
appears to be a strong candidate.
"Beauveria is like a fatal case of athlete's foot for these
insects," Jaronski explains. "Grasshoppers often pick up the
fungal spores on their feetand because of the insects' open circulatory
system, the fungus grows very quickly inside their bodies."
He's been investigating the efficacy of Beauveria in a base
of canola oil, a known grasshopper attractant. But Jaronski recently
found a new twist to the oil carrier.
"It turns out that raw, unprocessed canola oil, which is black
and rather strong-smelling, is even more effective as an attractant
than the refined, store-bought oil we've previously used," he says.
Very close to the stuff that is extracted from the seed, raw oil contains
higher concentrations of the fatty acids hoppers find so irresistible.
Greenhouse tests so far indicate that the oil can increase the effectiveness
of the fungus. Large-scale field trials to verify these observations
are being conducted by ARS along with USDA's Animal and Plant Health
Inspection Service and the University of Wyoming.
Strains of another fungus, Metarhizium anisopliae var. acridum,
are also getting Jaronski's attention. "It's far more virulent
and specific than Beauveria," he says. "It affects
only insects of the group Orthoptera, like grasshoppers and locusts,
and not honey bees, beetles, or other potentially beneficial species."
Safety data have been generated by scientists in Africa and Australia
who are using Metarhizium on locusts. Jaronski's lab tests have
shown it to be highly infectious for most American grasshoppers. He's
eager to study the fungal strains' effect on hoppers and is working
with international scientists to bring the two promising strains into
the United States for field trials.By Erin
K. Peabody, Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS
National Program (#304) described on the World Wide Web at www.nps.ars.usda.gov.
To reach scientists featured in this article, contact Erin
Peabody, USDA-ARS Information
Staff, 5601 Sunnyside Ave., Beltsville, MD 20705-5129; phone (301)
504-1624, fax (301) 504-1641.