Distribution. This native insect is found widely in North and Central America. It is abundant enough to be damaging everywhere in the United States, and in Canada it damaged crops in Ontario and Quebec. It is most serious as a pest in the southern United States, particularly the southwest.
Host Plants. Saltmarsh caterpillar's peculiar common name is derived from initial description as a pest of salt-grass hay grown in the vicinity of Boston. This is an anomaly, and despite the wide host range of this insect, grasses are not particularly preferred. Broadleaf weeds are the normal host plants, but larvae commonly disperse from these late in the growing season to damage vegetable and field crops. Vegetables injured include asparagus, bean, beet, cabbage, carrot, celery, corn, lettuce, onion, pea, tomato, turnip, and probably others. Field crops damaged are alfalfa, clover, cotton, soybean, sugarbeet, and tobacco. The favored weed host seems to be pigweed, Amaranthus spp., but many others may be consumed, including anglepod, Gonolobus sp.; sicklepod, Cassia tora; dog fennel, Eupatorium capillifolium; ground cherry, Physa-lis spp.; and mallow, Anoda sp.
Natural Enemies. Saltmarsh caterpillar larvae frequently are parasitized, particularly by tachinids (Diptera: Tachinidae). In Arizona, the most common parasitoids were Exorista mellea (Walker) and Lesche-naultia adusta (Loew), but Gymnocarcelia ricinorum Townsend and Lespesia archippivora (Riley) were also observed (Taylor, 1954). Jackson et al. (1970) documented the biology and importance of L. adusta. Arnaud (1978) reported additional species of tachinids associated with saltmarsh caterpillar. Hymenopteran para-sitoids are known from both the larval and egg stages (Taylor, 1954; Taylor and Stern, 1971), and include Apanteles diacrisiae Gahan (Braconidae); Therion fuscip-enne (Norton), T. morio (Fabricius), Casinaria genuina (Norton), Hyposoter rivalis (Cresson) (all Ichneumoni-dae); Psychophagus omnivorus (Walker), Tritneptis hemerocampae Vierick (both Pteromalidae); Anastatus reduvii (Howard) (Eupelmidae); and Trichogramma semifumatum (Perkins) (Trichogrammatidae). A cyto-plasmic polyhedrosis virus is known (Langridge, 1983), but there are little data on importance. General predators such as lady beetles (Coleoptera: Coccinelli-dae), softwinged flower beetles (Coleoptera: Melydri-dae), and assassin bugs (Hemiptera: Reduviidae) prey on these caterpillars, but are not thought to be very important in population regulation (Young and Sifuentes, 1959).
Life Cycle and Description. Total generation time requires 35-40 days under ideal conditions, but most reports from the field suggest about six weeks between generations. The number of generations per year is estimated at one in the northern states to 3-4 in the south. Overwintering reportedly occurs in the mature larval stage, with pupation early in the spring. Salt-marsh caterpillars usually are infrequent early in the season, but may attain high numbers by autumn.
Larvae are active dispersers, a habit that is relatively uncommon among caterpillars. Most commonly, late instar larvae are found individually or in large numbers ambling over the soil, searching for suitable food. Damage to margins of crop fields often occurs as such larvae desert drying weeds for irrigated crops. Stracener (1931) reported that young larvae drop readily from plants when disturbed, spin a strand of silk, and are blown considerable distances by wind. Frequency of distribution by wind is unknown. (See color figure 87.)
Pupa. Pupation occurs on the soil among leaf debris, in a thin cocoon formed from silken hairs interwoven with caterpillar body hairs. The dark brown
pupa measures about 30 mm long. Duration of the pupal stage is about 12-14 days.
Adult. Adults are fairly large moths, measuring 3.5-4.5 cm in wingspan, and are distinctive in appearance. They are predominantly white, though generally the wings bear numerous, small, irregular black spots. The hind wings of the male are yellow; those of the female are white. The underside of the male's front wings may also be tinted yellowish. Most of the abdominal segments are yellow, and bear a series of large black spots dorsally. Mating occurs the evening following emergence, and egg deposition the next evening. Females usually live only 4-5 days, but may produce more than one cluster of eggs. (See color figure 216.)
Accounts of saltmarsh caterpillar biology were provided by Hinds (1904), Stracener (1931), and Young and Sifuentes (1959). Rearing methods were given by Dunn et al. (1964) and Vail et al. (1967a).
Larvae are defoliators. Young larvae feed gregariously and skeletonize foliage. Older larvae are solitary and eat large holes in leaf tissue. On celery, most feeding is restricted to leaf tissue, unlike many other lepidopterous species, which may also feed readily on petioles (stalks) (Jones and Granett, 1982). Older larvae may disperse long distances in search of food, sometimes moving in large numbers. Commonly this is associated with maturation of cotton or weeds in the autumn. Thus, these caterpillars tend to be damaging to fall-planted vegetable crops. Foliage consumption at least doubles with each succeeding instar, and mature larvae can consume over 13 sq cm of thick-leaved foliage, such as sugarbeet, daily (Capi-nera, 1978b). Capinera et al. (1987) measured bean foliage consumption by each instar, and recorded over 400 sq cm of foliage consumed during the life of a caterpillar. Further, they estimated that 1.0-1.5 mature
caterpillars per plant could inflict 20% defoliation, a level adequate to cause yield loss.
Insecticides are commonly used to suppress salt-marsh caterpillars if they become abundant in vegetable crops. Baits are not effective. Most damage occurs at field margins as larvae disperse into crops from nearby senescent vegetation. Both chemical insecticides and Bacillus thuringiensis are recommended. Physical barriers, including ditches or trenches with steep sides, can be used to interrupt invasion of crops by caterpillars.
Was this article helpful?