Melanoplus differentialis Thomas Orthoptera Acrididae

Natural History

Distribution. This native grasshopper occurs widely in the central and western regions of the United States, and in northern Mexico; in Canada it occurs in southern Saskatchewan and British Columbia. Within the United States it is absent from the Atlantic and Gulf Coast region, except that it occurs in the Pennsylvania, New Jersey, and Maryland area. It is infrequent in the Pacific Northwest area. Also, within the large geographic area generally inhabited by differential grasshopper, it is rare in arid environments.

Host Plants. The host plants preferred by differential grasshopper are tall broadleaf plants such as those typically associated with fence rows, irrigation ditches, and fallow fields. It prefers plants in the family Compositae such as ragweed, Ambrosia spp.; sowthistle, Sonchus asper; sunflower, Helianthus annuus; and prickly lettuce, Lactuca scariola; though it will feed on other broadleaf plants such as kochia, Kochia scoparia; and smartweed, Polygonum sp.; and on such grasses as bermudagrass, Cynodon dactylon; slender oat, Avena barbata; barley, Hordeum sp.; and Johnsongrass, Sorghum halepense. In North Dakota alfalfa fields, differential grasshopper reportedly ate kochia, Kochia scoparia; quackgrass, Agropyron repens; squirreltail grass, Hordeum jubatum; bristly foxtail, Setaria spp.; and field bindweed, Convolvulus repens, in addition to alfalfa (Mulkern et al., 1962). On prairie, they ate mostly stickseed, Lappula echinata; wavyleaf thistle, Cirsium undulatum; quackgrass, Agropyron repens; and pepperweed, Lepidium densiflorum (Mulkern et al., 1964).

Crops sometimes injured include alfalfa, clover, corn, cotton, soybean, sugarbeet, timothy, and small grains such as barley and wheat. Differential grasshopper is not normally an important vegetable pest. It occurs among vegetables if weeds are present within, or adjacent to, crops. However, during periods of great abundance all vegetables are at risk, because under such conditions virtually all green vegetation may be consumed. (See color figure 29.)

Natural Enemies. The natural enemies of the crop-feeding Melanoplus spp. are quite similar. For information on natural enemies of differential grasshopper, see the section on natural enemies of migratory grasshopper, Melanoplus sanguinipes (Fabricius).

Weather. Weather affects the distribution of differential grasshopper, but in a manner somewhat different from some other grasshopper species. Differential grasshopper is associated with dense vegetation, so it follows that it would thrive in areas with adequate moisture to support lush growth of plants. It is a common, and damaging, species on the eastern edge of the Great Plains, where rainfall is plentiful, and relatively infrequent along the drier western edge of this region. Wakeland (1961) documented the expansion of differential grasshopper populations into areas of the northern Great Plains dominated by migratory grasshopper, Melanoplus sanguinipes (Fabricius), a species that is better adapted to dry conditions. Migratory grasshopper was supplanted by differential grasshopper, as long-term mean precipitation levels increased and temperatures decreased in this area. When weather returned to normal, however, migratory grasshopper resumed its status as the dominant species.

High levels of precipitation are not entirely advantageous for differential grasshopper. Precipitation during the warm months leads to outbreak of disease in differential grasshopper populations. This is a short-term response, and disease outbreaks occur only when grasshoppers are abundant. Differential grasshopper seems to be more susceptible to disease than some other species, including migratory grasshopper (Wakeland, 1961). Precipitation accompanied by cool weather during the hatching period is also detrimental to differential grasshopper, as with all grasshoppers, largely because it disrupts feeding during the critical early life of the grasshopper. The late onset of winter can favor grasshopper population increase, because it allows adults additional time to produce eggs.

Life Cycle and Description. A single generation occurs annually, with the egg stage overwintering. This is a late-season species, with eggs hatching about three weeks after those of twostriped grasshopper, Melanoplus bivittatus (Say), and two weeks after Melanoplus sanguinipes (Fabricius). In Colorado, eggs hatch in June, usually within a two-week period. Nymphs complete their development in July-August; adults are present from August to October.

  1. The eggs are creamy white, yellowish, or light brown. They are elongate-cylindrical, and taper to a blunt point at each end. The eggs measure about 4-5 mm long and 0.85 mm in diameter. They are clustered within an elongate, cylindrical pod consisting of about 40-200 eggs arranged in four columns and held together by frothy material. They are deposited in the soil, and the upper portion of the pod is plugged with additional froth. They are normally deposited among the roots of grasses and weeds, especially along edges of fields, and in moist soil. Embryonic development occurs in the autumn after eggs are deposited, but the embryo enters diapause at about the point of 50% development, and must endure a period of cold before commencing growth.
  2. The nymphs normally develop during the warmest period of the summer, and complete their development in about 30 days, though development times are extended by cool weather. There are six instars, but the nymphs are not distinctive in appearance. First instars are greenish, yellowish or brownish with an indication of a black stripe on the outer face of the hind femora. Instars 2-6 are similar but possess a curved-dark stripe extending from the back of the eye across the pronotum, and bordered below by a narrower white stripe. The black stripes on the femora are pronounced. The hind tibiae are light green or gray. The nymphs increase in body length from 5.36 mm in the first instar, to 5.2-6.8, 9.4-12.6, 12-14, 18-21.5, and 22-32 mm in instars 2-6, respectively. The number of antennal segments is 12,14-17,19-20, 21-22, 25-26, and 26 in instars 1-6, respectively.
  3. The adults are large grasshoppers, the males measuring 28-34 mm long, the females 3244 mm. They display more color variability than most grasshoppers. They may be principally brownish-green, or yellow, or almost entirely black, though the yellow form is most abundant. The most distinctive feature of these grasshoppers is the row of black marks, arranged in herring-bone fashion on the outer face of the hind femora. These markings are not so evident in the infrequent black form, which instead bears four white blotches on its otherwise black hind femora. The front wings in all except the black form are uniform grayish or brownish; the hind wings in all forms are colorless. The males bear large and boot-shaped cerci. (See color figure 164.)

Like many other grasshoppers, differential grasshopper tends to roost on elevated locations at night. This allows them to bask in the morning sun, and to assume activity early in the day. Bushes and other tall vegetation are favorite perches.

Aspects of the biology of differential grasshopper were treated by many authors, including Parker and Shotwell (1932), Parker (1939), and Shotwell (1941). Kaufmann (1968) gave some interesting biological information, but due to low rearing temperature the relevance of this study to field biology is questionable. An excellent synopsis was presented by Pfadt (1994e), who also pictured all stages of development. Melano-plus differential is included in many grasshopper keys, including those by Blatchley (1920), Dakin and Hays (1970), Helfer (1972), Capinera and Sechrist (1982), and Richman et al. (1993). This species was also included in a key to grasshopper eggs by Onsager and Mulkern (1963). Rearing of Melanoplus species was described by Henry (1985).

Damage

This species seemingly has benefited from agricultural practices more than most grasshoppers, with grasshopper survival increased by the abundance of weeds associated with crops, and the irrigation practices of western farms. It also readily exploits disturbed sites in cities and towns. Unlike some of the arid environment-loving species, its numbers and damage may increase following long-term increases in precipitation. The damage caused by differential grasshopper principally takes the form of leaf removal. Plants may be completely defoliated, or left ragged. As this grasshopper tends to roost in elevated locations at night, where they may nibble while rest-

Adult male differential grasshopper.

Male cercus from differential grasshopper.

ing, trees and shrubs outside the normal dietary range are sometimes severely injured.

Management

Management of the various Melanoplus spp. grasshoppers is substantially the same. For information on differential grasshopper management, see the section on management under migratory grasshopper, Mela-noplus sanguinipes (Fabricius).

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