Northern Prairie Wildlife Research Center
However, grasshoppers are the principal above-ground insect consumers; therefore, it is no surprise that the effect of fire on grasshopper populations has been studied more than for other species.
Knutson and Campbell (1976) found that early spring burning caused grasshoppers to emerge 3 weeks earlier than normal and grasshoppers were higher in number the second year following an early burn. Midspring burning produced fewer grasshoppers than early burning, and late spring burning produced fewer grasshoppers than mid- or early spring burning.
Nagel (1973) quantitatively measured the effect of a single spring burn on the biomass and density of arthropods in the native True Prairie near Manhattan, Kan. He measured herbivorous, non-herbivorous, and total arthropods both at night and during the day once every 2 weeks from June 6 to August 26 on formerly grazed, burned, and unburned pastures. Grazing was controlled on the burned pasture to prevent overgrazing of the burned areas. Both pastures were on a similar upland range site.
He found that the burned area contained significantly greater numbers of arthropods and greater biomass than the unburned area.
The numbers of non-herbivorous insect species were equal on both areas, but non-herbivorous insect biomass was higher on the burned than on the unburned areas. Greater numbers and less biomass were collected during the day than at night, mostly due to higher numbers of Diptera (flies) collected during the day.
Arnett (1960) found that areas burned in late March produced greater grasshopper populations than heavily grazed areas in this same general area (Nagel 1973).
Timing is a major factor in insect fire ecology. Early spring burning results in earlier emergence and higher numbers of grasshoppers than a late spring burn, especially if coupled with heavy grazing pressure (Knutson and Campbell 1976; Arnett 1960).
Cancelado and Yonke (1970) also found greater population differences "from the beginning to the middle of the growing season than later in the year, where they are reduced or are not apparent...." Too much or too little litter decreases grasshopper populations. Late spring burning reduces litter and kills many of the grasshopper nymphs directly.
The general consensus seems to be that late spring burning reduces insect populations more than early spring burning. This is probably due to mortality of newly hatched insect nymphs during the burn, which reduces the population potential of subsequent generations.
Cancelado and Yonke (1970) tested the effect of spring burning on Hemiptera (sucking insects) and Homoptera (cicadas, hoppers, aphids, and scale insects) at the Tucker Prairie Research Station in east-central Missouri, burning 30 acres (12 ha) of 145-acre (59 ha) site on March 23, 1968.
Significantly higher numbers of Cicadellidae (Homoptera), Miridae, and Lygaeidae (Hemiptera) occurred on the burned area than on the unburned area.
In the Waubun Prairie of northwestern Minnesota, Tester and Marshall (1961) conducted a fall burn (October 28, 1857), early spring burn (March 15, 1958), and a late spring burn (April 11, 1958) on upland sites hayed since 1921. The early spring site burned very hot due to high winds and then was grazed during the study period. The late spring site was not grazed.
Most grasshoppers came from areas having light to moderate amounts of litter. Optimum grasshopper habitat consisted of vegetation that was recovering from burning rather than that freshly burned or long unburned.
There was no significant change in Coleoptera numbers during the first year, but in the second year both the spring and the fall burns contained high numbers of Coleoptera. This was correlated to the sparse littler found on both sites.
Lussenhop (1976) studied the effect of fire on soil arthropods at the Curtis Prairie near Madison, Wis. Burning of re-established tallgrass prairie that had been burned biannually since 1950 was continued for two more burns on one area and discontinued on the other. As a control, a third area was raked to remove the litter. Soil microarthropods were counted about 2 weeks after the burns and again about 7 months after the burns.
First-year readings showed no significant difference in soil microarthropods, but by the fourth year the unburned areas had significantly fewer herbivore and carnivore species than the burned and control (raked) areas.
Lussenhop concluded that the unburned area was less productive in vegetation and roots, therefore soil microarthropods decreased in specific numbers.
Rice (1932) studied the effect of fire on prairie animal communities following spring burning. At least two burns were studied, one of which occurred March 12, 1932, near Seymour, Ill. She found "10 charred spider cocoons/sq m, dead tenebrionid beetles, carabid beetles and cut-worm larvae at 3/sq m, and living under a 3 by 8-inch piece of charred wood, 19 chinch bugs, 2 cut-worm larvae, 1 ground beetle, 1 slug and 2 centipedes. Aulax larvae in stems of Silphium and Lactuca, Eurosta and Lepidoptera larvae in goldenrod stems were killed."
Mortality was severe, compared to eight living larvae/stem in unburned areas. Ants increased in numbers, and earthworms came to the surface earlier but then decreased with decreasing soil moisture the first month after the burn.
Rice (1932) found that insects (minus ants) were reduced 35%, not all insects were killed outright by the spring burn, those insects not killed by the fire fled to adjacent unburned areas shortly after the burn, soil temperatures were not high enough to kill all animals in hibernation, gall insects suffered very high mortality rates, and ants and other underground organisms were not immediately affected by the fire.
In the southern mixed prairie, fire will top-kill mesquite and leave the stems in a state where wood borers will attack and aerate them to such an extent that they will easily be consumed by a reburn (Wright and Bailey 1982).
Coleopterans and Hemipterans appear to increase following burning (Cancelado and Yonke 1970; Winter 1984). This may also be due to increased productivity following burning (Lussenhop 1976). Insect exoskeletons contain nitrogen which becomes available to plants after fire (Kirchner 1977).
Gall insects suffer high mortalities from fire. However, not all are killed; many flee or escape by hiding under rocks or other objects, and ants actually increased in one study (Rice 1932).
Winter (1984) in a Butte County, Idaho, study of sage sparrows (Amphispiza belli) and Brewer's sparrows (Spizella breweri) found that a September 5, 1982, head-fire burn of sagebrush-grassland at 81 F (27 C) air temperature, 29% relative humidity, and a 8.5 ft/second (2.6 m/second) wind had increased flixweed (Descurainia sophia) in the burned areas. Lepidopteran larvae were very abundant on the plants.
Sage sparrows spent less time foraging after the burn. The author attributed this decrease primarily to greater arthropod abundance. Greater numbers of arthropods occurred on the burned plots than on unburned areas during July of the year after the fire. This increase consisted largely of larvae.
It seems possible that a shorter foraging time of certain bird species may be a good indicator of greater insect densities. In a study of this same area in Idaho, Petersen and Best (1986) found that, although fire changed the vegetation, it did not affect the composition of nestling diets or food size of the sage or Brewer's sparrow.
In the southeastern U.S., bobwhite quail (Colinus virginianus) are called "fire birds" because they can be found at the edges of burns before the fire even stops smoking. They fill their crops in a matter of minutes because dead insects and seeds are so readily abundant (Stoddard 1963).
Fire to control insects and diseases in crop residue is age-old. But the effect of fire on grassland insect populations is not as well understood or documented.
Insects, especially grasshoppers, are an important herbivorous component of grasslands. Nematodes represent an even larger potential with a biomass of at least 10 times greater than above ground invertebrates (Risser et al 1981).
Fire causes an immediate decrease in insect populations (except ants and other underground species), followed by a gradual increase in numbers as the vegetation recovers. The insects eventually reach a population level higher than adjacent areas, then decline to near preburn levels as vegetation and soil litter stabilize.