Controlled burning has long been touted as a tool to revive grassland productivity. New research indicates fire may also be a good prescription for ridding rangelands of grasshopper pests.
Lance Vermeire and David Branson, with USDA's Agricultural Research Service in Miles City and Sidney, MT, respectively, have been studying the effect of fire on grasshopper abundance. They've found heat from the fire can cause mortality of certain grasshopper eggs in the soil.
Vermeire, a rangeland ecologist, began his work with grasshoppers and the effect of fire more than five years ago in Oklahoma. He says different grasshopper species have different egg-laying strategies. Several species bury their eggs vertically in the soil .25 to 1.5 in. deep. These eggs are typically safe because the duration of most rangeland fires isn't long enough to heat the soil to that depth.
However, some grasshopper species — including the white-whiskered grasshopper, which is among the top five grasshopper pests — lay their eggs horizontally in the soil and close to the surface. Vermeire says it's these eggs that fire can have an impact in reducing.
“We're showing shifts in species composition of grasshoppers,” he says of their eastern Montana field trials.
Heat and egg survival
In a lab setting, the duo tested their theory to evaluate how heat from fire impacts survival rates of grasshopper eggs. Eggs laid near the surface from white-whiskered grasshoppers were compared to eggs of a migratory grasshopper laid vertically and deeper in the soil. Heat treatments were devised to mimic different grassland settings and applied to the cups of eggs.
Vermeire says their lab work showed fires that burned with a heat intensity comparable to about 4,000 lbs./acre of fuel had virtually no survival of the white-whiskered grasshopper eggs. When the fire intensity was equivalent to about 2,800 lbs./acre of fuel, mortality was still about 65%. In the trials, the grasshopper eggs that were laid deeper in the soil were unaffected.
From those findings, Vermeire says grassland fires should produce, on average, 60-80% mortality rates on white-whiskered grasshopper eggs. Thus, Vermeire suggests land managers consider incorporating fire into their management system as a means to help control grasshoppers.
“We know that, on average, the boom cycle for grasshoppers is about every seven years. By incorporating fire into rangeland management systems, landowners can help break the grasshopper cycles so it doesn't reach those outbreak levels,” Vermeire says.
When should you burn?
The timing of the burn can be flexible, Vermeire adds, “as long it's done when the eggs are in the ground.”
Since most grasshopper eggs hatch in the spring, burning in summer, fall or early spring works best. Vermeire says fires at each of these times have produced similar mortality rates.
In addition, Vermeire says burning annually or every other year is probably unnecessary. He suggests burning every 5-7 years as sufficient to keep high grasshopper populations at bay. In fact, burning different portions of land rotationally through the years would likely be most effective.
Kindra Gordon is a freelance writer and former managing editor of BEEF. She is based in Whitewood, SD.