Scapteriscus vicinus Scudder Orthoptera Gryllotalpidae

Natural History

Distribution. These mole crickets were inadvertently introduced to the southeastern United States in about 1900. Shortwinged mole cricket, Scapteriscus abbreviatus Scudder, was first observed at Tampa, Florida in 1899, but separate introductions were discovered near Miami in 1902 and Brunswick, Georgia in 1904. Southern mole cricket, Scapteriscus borellii Giglio-Tos (known until recently as S. acletus Rehn and Hebard), was similarly introduced to major seaports, beginning with Brunswick in 1904, and followed by Charleston, South Carolina in 1915, then Mobile, Alabama in 1919, and finally Port Arthur, Texas in 1925. Tawny mole cricket, Scapteriscus vicinus Scudder, was first observed at Brunswick, Georgia in 1899. The origin of these crickets is uncertain, but Argentina and Uruguay are likely sources, because they occur in these areas of southern South America.

In the years since introduction to the United States, the Scapteriscus spp. have expanded their ranges, but they differ considerably in their current distribution. Shortwinged mole cricket, which is flightless, remains fairly confined to the southern Florida and southern Georgia-northeast Florida introduction sites, though it also occurs in Puerto Rico and the Virgin Islands. It has been redistributed in southern Florida, but is largely found in coastal areas. In contrast, southern mole cricket is now found from North Carolina to eastern Texas, including the northern regions of Georgia and Alabama and the entire peninsula of Florida, and recently was detected in Yuma, Arizona. Tawny mole cricket is somewhat intermediate in its spread; it occurs from North Carolina to Louisiana, and throughout Florida, but thus far remains restricted to the southern coastal plain.

These are not the only mole crickets found in North America, but they are most damaging. For example, a native species, the northern mole cricket, Neocurtilla hexadactyla (Perty), is widely distributed in the eastern states west to about South Dakota and Texas, and including southern Ontario, but is not a pest. European mole cricket, Gyllotalpa gryllotalpa (Linnaeus), has been introduced from Europe into the northeastern states, but is of minor significance. Changa, Scap-teriscus didactylus (Latreille), invaded Puerto Rico from South America before 1800, and has caused considerable damage to crops on this island.

Host Plants. Mole crickets are omnivorous, feeding on animal as well as plant material. Several studies have indicated that when provided with grass or collected from grass-dominated habitats, southern mole cricket is less damaging than tawny mole cricket. Southern mole cricket feeds mostly on other insects, whereas tawny mole cricket is principally herbivorous (Matheny, 1981; Matheny et al., 1981; Walker and Ngo, 1982). Both species are associated with tomato and strawberry fields (Schuster and Price, 1992). Among vegetable crops reported to be injured are beet, cabbage, cantaloupe, carrot, cauliflower, collards, eggplant, kale, lettuce, onion, pepper, potato, spinach, sweet potato, tomato, and turnip. Other plants injured include chufa, turf, and pasture grasses, peanut, strawberries, sugar cane, tobacco, and such flowers as coleus, chrysanthemum, and gypsophila. Among the turf grasses, bahiagrass and Bermudagrass are commonly injured by tawny mole cricket, whereas St. Augustine grass and Bermudagrass are favored by shortwinged mole cricket. Mole crickets also feed on weeds such as pigweed, Amaranthus spp.

Natural Enemies. Few natural enemies of Scapter-iscus mole crickets exist naturally in North America. Among the natural enemies are amphibians such as toads, Bufo spp.; birds such as sandhill cranes, Grus canadensis; and mammals such as armadillos, Dasypus novemcinctus. They, and the few predatory insects that attack crickets such as tiger beetles (Coleoptera: Cicin-delidae), are not effective. Therefore, several natural enemies have been introduced from South America (Parkman et al., 1996). The most effective introduced beneficial insect is the parasitoid Ormia depleta (Wiede-mann) (Diptera: Tachinidae), which was imported from Brazil. This fly is attracted to the calls of male mole crickets. Its release has resulted in reduced mole cricket injury in southern Florida (Frank et al., 1996). A less effective parasitoid is Larra bicolor Fabricius (Hymenoptera: Sphecidae), which was imported from Bolivia but seems to be constrained by availability of suitable adult food sources in Florida (Frank et al., 1995). An entomopathogenic nematode, Steinernema scapterisci, was introduced from Uruguay (Nguyen and Smart, 1992). It is fairly specific to mole crickets, persists readily under Florida's environmental conditions, and is dispersed by crickets. Field collections consistently show infection levels of 10% or greater (Parkman et al., 1993a,b; Parkman and Smart, 1996), and infected crickets die within 10-12 days.

Life Cycle and Description. Southern and tawny mole cricket are quite similar in appearance and biology. Shortwinged mole cricket differs in appearance owing to the short wings but also in behavior, because it has no calling song and the short wings render it incapable of flight. Typically, the eggs of these three species are deposited in April-June, and nymphs predominate through August. Beginning in August or September some adults are found, but overwintering occurs in both the nymphal and adult stages. Maturity is attained by the overwintering nymphs in April, and eggs are produced at about this time. A single generation per year is normal, though in southern Florida there are two generations in southern mole crickets and an extra peak of adult flight activity in the summer, resulting in spring, summer, and autumn flights from the two generations (Walker et al., 1983).

  1. The eggs are deposited in a chamber in the soil adjacent to tunnels. The chamber is usually constructed at a depth of 5-30 cm below the soil surface. It typically measures 3-4 cm in length, width, and height. The eggs are oval to bean-shaped, and initially measure about 3 mm long and 1.7 mm wide. The eggs increase in size as they absorb water, eventually attaining a length of about 3.9 mm and a width of 2.8 mm. The color varies from grey or brownish. They are deposited in a loose cluster, often numbering about 25-60 eggs. Duration of the egg stage is about 10-40 days. Total fecundity is not certain, but over 100 eggs have been obtained from a single female, and the mean number of egg clutches produced per female was reported to be 4.8 (Hayslip, 1943).
  2. Hatchlings are whitish initially but turn dark within 24 hours. They may consume the egg shell or cannibalize siblings, but soon dig to the soil surface. The juvenile stages resemble the adults, but nymphs have poorly developed wings. The number of instars is variable, probably 8-10 (Hudson, 1987). Nymphs and adults create extensive below-ground tunnel systems, usually within the upper 20-25 cm of soil. When the soil is moist and warm they tunnel just beneath the surface, but crickets tunnel deeper if the weather becomes cooler or the soil dries. They come to the surface to forage during the evening, usually appearing shortly after dusk if the weather is favorable.
  3. Mole crickets have peculiar enlarged forelegs that are used for digging in the soil. The foretibiae have large blade-like projections, called dactyls, and the number and arrangement of dactyls are diagnostic. These crickets also bear antennae which are shorter than the body. Females lack a distinct ovipositor. Both sexes have elongate cerci at the tip of the abdomen. The male produces a courtship song that is attractive to females; they normally call during the night. Except for the shortwinged mole cricket, the male enlarges the entrance to his burrow, forming a horn-shaped opening, in preparation for calling. This increases the volume of the call, and allows flying females to locate males. Mating occurs within the male's burrow in the soil, and apparently the female may usurp the burrow after mating.

Shortwinged mole cricket bears front wings that are shorter than the pronotum. The front wings cover the hind wings, which are minute. The body is mostly whitish or tan in color, though the pronotum is brown mottled with darker spots. Also, the abdomen is marked with a row of large spots dorsally, and smaller spots dorsolaterally. These crickets measure 22-29 mm long. The two dactyls on the foretibiae are slightly divergent, and separated at the base by a space equal to at least half the basal width of a dactyl. Shortwinged mole cricket makes no calling song, producing only a weak 1-5 pulse chirp during courtship. (See color figures 159 and 171.)

Southern mole cricket has long hind wings that extend beyond the tip of the abdomen. The front wings are longer than the pronotum, about two-thirds the length of the abdomen. They are broad and rounded at the tips. This cricket is brown, with the dorsal surface of the pronotum often quite dark. As with short-winged mole cricket, in southern mole cricket the two dactyls on the foretibiae are separated at the base by a space equal to at least half the basal width of a dactyl. Thus, these two species can be distinguished by the wing length. Southern mole cricket produces a calling song that consists of a low-pitched ringing trill at about 50 pulses per second. It usually is emitted during the first two hours after sunset. (See color figure 161.)

Tawny mole cricket is quite similar to southern mole cricket in general appearance, with moderately long front wings and long hind wings, a yellowish brown body, and a dark pronotum. It can be distinguished from southern mole cricket by dactyl form. The tibial dactyls are nearly touching at the base, separated by less than half the basal width of a dactyl. Tawny mole cricket produces a loud, nasal trill at about 130 pulses per second during the first 90 minutes after sunset. (See color figure 160.)

Summaries of mole cricket life history were given by Worsham and Reed (1912), Thomas (1928), Hayslip (1943), and Walker (1984), though biology of short-winged mole cricket is poorly documented. Keys to North American and Caribbean area mole crickets were provided by Nickle and Castner (1984).

Damage

The crickets usually damage seedlings, feeding above-ground on foliage or stem tissue, and below-ground on roots and tubers. Girdling of the stems of seedling plants at the soil surface is a common form of injury, though young plants are sometimes severed and pulled below-ground to be consumed. Additional

Mole Cricket
Adult tawny mole cricket.
tawny (lower).
Mole Patterns
Pronotal patterns of mole crickets: southern mole cricket (top left and right), tawny mole cricket (lower left), short-winged mole cricket (lower right).

injury to small plants is caused by soil surface tunneling, which may dislodge seedlings. Southern mole cricket does much more tunneling injury than tawny mole cricket.

Management

  1. Various approaches to population estimation have been developed. A commonly used, but less reliable technique, is the assessment of population density by the frequency of soil surface tunneling. Tunneling is affected by soil moisture levels, and is most appropriate for assessing nymphs. A more consistent, but labor intensive, approach for estimation of nymph and adult abundance is flushing with about a 0.5% aqueous solution of dishwashing soap. Soil flushing is affected by soil moisture conditions, with greater cricket extraction efficiency as the soil approaches field capacity (Hudson, 1989). Flushing with synergized pyrethrin insecticide solution is equally effective (Hudson, 1988). A soil washing apparatus also has been developed to separate crickets from soil (Fritz, 1983). Adults can be captured with sound traps that use electronic sound synthesizers to lure crickets to a catching device, usually a large funnel.
  2. Liquid and granular formulations of insecticides are commonly applied to the soil to suppress mole crickets. Insecticide application should be followed by irrigation, because the insecticide needs to enter the root zone of the plants to be most effective. Bait formulations are also useful. Various baits have proven effective, but most contain wheat bran, cottonseed meal, or some other grain product plus 2-5% toxicant. Also, addition of 5-15% water and 2-5% molasses to the grain-toxicant mixture are sometimes recommended (Thomas, 1928; Walker, 1984).

Cultural Practices. Most injury to vegetable transplants occurs on small plants, so placement of larger plants is suggested as a strategy to avoid injury (Schuster and Price, 1992). Crickets can quickly invade crop land that has been fumigated or otherwise cleared of crickets, so isolation from sources of crickets, or planting in large blocks of land with proportionally little edge, is desirable (Poe, 1976).

Host-Plant Resistance. Efforts have been made to find turf and pasture grass varieties that are resistant to attack by mole crickets. If grass varieties contain antibiotic properties, or otherwise limit the reproductive abilities of mole crickets, this can translate into fewer crickets seeking food within vegetable crops, because grass-containing fields are a principal source of mole crickets. Thus far, strains have been identified which are fairly tolerant of feeding or which are not preferred by mole crickets, primarily the finer textured grass selections, but considerable improvement in these grasses is needed before they can affect cricket population biology.

Biological Control. Biological control of mole crickets can be enhanced by the application of the entomopathogenic nematode Steinernema carpocapsae. This nematode can be cultured and applied in the same manner as an insecticide, and is fairly persistent. It is more effective when applied to adults than applied to nymphs.

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