Weed & Pest
UW entomologist explains biology, control of grasshoppers
Casper – Year in, year out, the grasshopper is the major above ground herbivore in a grassland biome in North America.
“They’re always out there consuming – they consume a lot more in proportion to their bodyweight than anything else – and they’re an invisible force consuming forage, but usually at levels that leave enough to go around,” says UW Extension Entomologist Scott Schell.
“If there aren’t that many out there, there are some good things they should do for us. It’s thought that their activity, at lower levels, is like mowing a lawn, which stimulates grass growth and encourages nutrient cycling,” says Schell. “The activity and movement is important in the ecology of the system, and they’re also food for many prey animals.”
Schell adds they’re also important for species of concern like the sage grouse. “Grasshoppers are important in the early diet of sage grouse chicks – they need them for protein and proper health,” he says.
However, sometimes grasshoppers can be a detriment to other wildlife. He gives as an example silver sage, which grasshoppers will eat when everything else they prefer is gone.
There are over 500 species of grasshoppers in North America, and 121 of those are found in Wyoming.
“They can be found just about any time of year,” says Schell. “On warm days in the winter you can find a few species, while other species overwinter as eggs laid in the top layer of soil, an inch to an inch-and-a-half deep.”
Schell says grasshoppers usually occur in assemblages, in groups of anywhere from five to 20 species, and, depending on habitat, one species or a few will be predominate.
“In most years, in most areas, the grasshopper density is usually less than eight per square yard,” he adds.
However, densities can reach much higher than that, and it can be hard to see outbreaks coming on.
“Most grasshoppers live and die in the vicinity where they hatch, and many species are the size of a grain of rice when they hatch, so they’re hard to notice,” says Schell. “That’s not good, in that it’s a lot harder to deal with them when they’re big.”
“We’re lucky in North America that our grasshoppers have one generation per year, unlike other parts of the world,” says Schell. “In the U.S., eggs are laid mainly in soil, some in the crowns of plants, at one to four egg pods per female, on average, depending on the species. Five to 40 eggs per pod is average, and some crop and garden pest species can produce 50 to 100 per pod, and that’s one of the reasons why there are problems in the crop areas.”
A grasshopper can eat vegetation equaling its weight per day, and can waste six times more.
“A lot of grasshoppers will clip a blade of grass, eat what they want and leave the rest. Ten adult twostriped grasshoppers per square yard can defoliate a corn crop,” says Schell. “If you figure out the poundage, even though you can’t see them on the field, with 30 per square yard, that’s 200 pounds of grasshoppers per acre, which is the equivalent of putting out a very hungry sheep that can eat its body weight every day. They’re small, but they make up for it.”
Schell says the reasons for grasshopper outbreaks are yet to be determined. They can be tied to a natural cycle, weather, sun spots and egg pod production, to name a few.
As for the 2011 grasshopper forecast, Schell adopts a weather forecasting system and calls for a 50 percent chance of grasshoppers in the spring.
Of the factors that favor another large hatch, Schell says, “Many eggs are in the ground, and even with unfavorable weather in the spring, the higher number of eggs increases the hatch. If there are 1,000 eggs in the ground, and 90 percent are killed, that’s still 100 per square yard. The long, mild fall probably allowed all the eggs to develop to the stage where they can successfully overwinter.”
Schell said Wyoming is almost to the point where there’s a problem with predator saturation.
“Grasshoppers are food for many different animals, but they can swamp their predators,” he says. However, he adds that there may be a buildup in the population of grasshopper predators and parasites.
And, Schell says grasshopper overpopulation eventually means less quality food, with means lower quality eggs and less viable nymphs.
“There was an area in Montana that had 100 grasshoppers per square yard in 2009 on the range, and in 2010 the population crashed,” he explains. “There was a problem with protein deficiency, so in the fall and spring of 2010 they crashed, because the eggs that were laid were not viable.”
However, he says he doesn’t expect that kind of an extreme in Wyoming, as there was enough precipitation and vegetation to go around in 2010.
Looking to the future and at research projects, Schell said UW is cooperating in a biopesticidal trial with researchers in the Montana Agricultural Research Service lab.
“We’re looking at the use of pathogens, and trying to find a North American strain of fungal pathogen to control grasshoppers,” says Schell.
In addition to the search for a fungal pathogen to control populations, the researchers are working on a project that would define the economics of control and establish a remote sensing survey. Schell says the question they’re trying to answer is whether grasshopper control equals more forage.
“They eat a lot, but we need to show concrete evidence – because of the grasshopper programs funded through public money – as justification that there is an economic benefit that goes back to public coffers,” says Schell.