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The bigheaded grasshopper, Aulocara elliotti (Thomas), distributed widely in western North America, inhabits a variety of grasslands from southern Canada to central Mexico. Large populations develop in the desert, mixedgrass, shortgrass, and bunchgrass prairies.
This grasshopper not only reduces grass forage by consuming it but also by cutting it down. The cut grass may become litter but it may also be used for food by grasshoppers including the bigheaded grasshopper and by other insects that feed on the ground. Field cage tests run in Montana and in Wyoming have demonstrated that the feeding activity of one bigheaded grasshopper per square yard reduces grass forage equal to 20 pounds dry weight per acre. In this area of the mixedgrass prairie, annual forage production averages 600 pounds dry weight per acre.
Microscopic examinations of crop contents of older nymphs and adults show that the majority of crops contain fragments of more than one plant species (average 2.2). Although these determinations indicate that the bigheaded grasshopper is obtaining a mixed diet, one species of plant is most abundant in a crop. Depending on availability, the bigheaded grasshopper grazes heavily on blue grama, western wheatgrass, needleandthread, thread-leaf sedge, and needleleaf sedge. Crested wheatgrass, an introduced forage plant, is a preferred and nutritious host. From direct observations and crop examinations, this grasshopper is known to feed on two species of sedges and 22 species of grasses.
Although no specific investigations of dispersal and migration of the bigheaded grasshopper have been made, evidence of long distance movements has been found incidental to other studies. At two locations, one in Arizona and one in Wyoming, sites that were found with no or few bigheaded grasshoppers at one sampling date had high densities of adults a few days later. Migrants in Arizona have been recorded as traveling distances of one to seven miles. Evidence of mass migration by the bigheaded grasshopper is provided by the recent discovery of large numbers of adults preserved on the ice of Grasshopper Glacier in the Crazy Mountains of Montana. Presumably swarms of the bigheaded grasshopper arose from areas of the mixedgrass prairie lying northeast of the mountains at lower altitudes, where outbreaks occur repeatedly.
The nymphs (Fig. 1-5) are identifiable by their color patterns, structures, and shape:
(1) Head with lateral foveolae triangular and visible in dorsal view; antennae filiform but flattened; face moderately slanted.
(2) Pronotum with disk patterns like the adults, chiefly with light lines in form of an "X."
(3) Hind femur with two dark bars on upper part of medial area and four to seven dark spots on lower carinula. Hind tibia blue with three dark annuli.
(4) Color drab gray and tan with fuscous markings.
A. elliotti nymphs can be separated from A. femoratum nymphs mainly through differences in coloration. Adult females of A. elliotti and A. femoratum are distinguishable by the shape of the posterior margin of the eighth abdominal sternum. In A. elliotti the posterior margin is without deep clefts (Fig. 9); in A. femoratum the posterior margin has two deep clefts.
Exposed to low soil temperatures, eggs break diapause during winter. In the laboratory eggs held at 37° to 41°F break diapause in 80 days. With warming spring temperatures, the soil temperature rising to 50°F and above, the eggs resume embryonic development. They complete the process after exposure to 450 day-degrees (base 50°F) of heat and are ready to hatch. Emergence of the first instars occurs in mid-spring mainly during morning hours, and especially when the temperature is rising rapidly and the soil is moist. Eggs of a particular pod hatch in succession within seconds of each other. On the surface of the soil the young grasshopper squirms to free itself from the embryonic membrane. It usually takes six to eight minutes to complete this process and crawl away. During this time the young grasshopper is vulnerable to predation by ants.
Female adults become receptive to mating when they are six to eight days old. Pair formation and courtship consists primarily of visual cues. Normally the male makes a quick approach to the female and silently dis-plays himself by tipping the hind femora and waving the antennae. Once the male mounts and succeeds in making genital contact, copulation lasts 40 to 70 minutes.
Females deposit their first group of eggs when they are 12 to 20 days old (average 15 days). When ready to lay eggs a female will select one of the many bare areas in her habitat and work her ovipositor into the soil. She then deposits in the top one-half inch of soil seven to nine eggs, which become enclosed in a tough pod. Immediately after ovipositing a female spends a minute actively sweeping soil particles over the hole left by the extraction of her ovipositor. She performs this final act of maternal care with her hindlegs using the tarsi as brushes.
Fecundity of the bigheaded grasshopper is less than that of the migratory grasshopper. When pairs were reared individually in field cages and fed leaves of western wheatgrass, females lived an average of 72 days and produced 76 eggs per female. When they were fed wheat leaves, females lived an average of 87 days and produced 116 eggs. The greatest reproduction - 161 eggs - was achieved by a female fed wheat leaves. Unprotected from predators in their natural habitat, individuals have a shorter life and lower fecundity. Under natural conditions, research suggests an average adult longevity of approximately 20 days and a fecundity of 15 eggs per female. There is one generation annually.
The pod is slightly curved, one-half to five-eighths inch long and three-sixteenths inch in diameter (Fig. 10). The pod cap has a short nipple in the center. Eggs are pale yellow and 5.2 to 5.5 mm long.
Once the bigheaded grasshopper has reached an outbreak condition, the population may continue at high densities for five or more years. For generally unknown reasons, populations eventually decline or crash. In a few cases, however, causes are apparent. Naturally occurring diseases such as Nosema may decimate populations, and insecticidal control can reduce densities much below the economic level.
Hewitt, G. B. 1978. Reduction of western wheatgrass by the feeding of two rangeland grasshoppers, Aulocara elliotti and Melanoplus infantilis. J. Econ. Entomol. 71: 419-421.
Hussain, N. and R. E. Pfadt. 1976. The utilization of food by bigheaded grasshopper, Aulocara elliotti (Thomas). Pakistan J. Forestry 26: 171-176.
Kemp, W. P. and N. E. Sanchez. 1987. Differences in post-diapause thermal requirements for eggs of two rangeland grasshoppers. Can. Entomol. 119: 653-661.
Onsager, J. A. and G. B. Hewitt. 1982. Rangeland grasshoppers: average longevity and daily rate of mortality among six species in nature. Environ. Entomol. 11: 127-133.
Pfadt, R. E. 1949. Food-plants, distribution, and abundance of the big-headed grasshopper, Aulocara elliotti (Thos.). J. Kansas Entomol. Soc. 22: 69-74.
Van Horn, S. Neumann. 1966. Studies on the embryogenesis of Aulocara elliotti (Thomas) (Orthoptera, Acrididae). I. External morphogenesis. J. Morph. 120: 83-114.
Next Species in Subfamily: Aulocara femoratum