The salt marsh moth, scientifically known as Estigmene acrea, is a common and widespread tiger moth found throughout the United States, southern Canada, and extending south into Central America. Its prevalence in diverse habitats, coupled with its broad diet, makes it an interesting subject for study. This article delves into the life cycle and dietary habits of this moth, providing a comprehensive overview for both amateur enthusiasts and seasoned professionals.
Identification of the Adult Salt Marsh Moth
Adult salt marsh moths are fairly large, measuring 3.5 to 4.5 cm in wingspan, and are distinctive in appearance. The head and thorax are white. The abdomen is orangish-yellow with black spots in both sexes. The tip of the abdomen is white on females. Forewings of adults are white with black spots. The amount and size of the black spots varies, but the spots are configured in a way that, if you connected them, they would form lines and bands. There are usually five fairly prominent spots evenly spaced along the costa (leading edge) of the forewing. Some individuals lack spots completely. On males, the underside of the forewing is sometimes tinted yellow or orangish-yellow, and the fringe is sometimes that color. On females the underside is white, sometimes with pale yellow tinting along the costal margin and the main veins. Hindwings are different in the two sexes: on males, they are yellowish-orange, and on females, they are white. The bodies of adults are white on the head and thorax, with the abdomen orangish-yellow. On males, the underside of the thorax is orange. The abdomen is mostly orange above and below, white on the sides. The tip of the abdomen is white. On most abdominal segments there is a black spot above in the middle (middorsal), two black spots on each side, and a black spot on the underside. Males have a pair of internal, glandular structures (coremata) that can be extended (everted) and inflated. The third segment (femur) of each leg is mostly yellowish-orange, black just at the base.
Distinguishing the salt marsh moth from similar species can be challenging. The Virginian tiger moth's caterpillar looks similar, too. The salt marsh caterpillar, however, has noticeably longer hairs at the front and end of the body, especially at the rear, while the Virginian's hairs are long, soft, and vary considerably in length over the whole body. Other similar moths in our region include the agreeable tiger moth (Spilosoma congrua), which has a pure white abdomen, and the fall webworm moth (Hyphantria cunea). The color patterns on the abdomen should help separate these from salt marsh moths. After that, however, you need to look at color patterns of the legs and other arcane characteristics. Estigmene albida has smaller and fewer black spots on the wings, and a more southern distribution.
Salt Marsh Moth Caterpillar Identification
Caterpillars are mottled with yellow and brown and are hairy like a woolly worm. The long, bristly hairs arise from orange or black, raised tubercles. The colors of the hairs can vary quite a bit. The hairs can be all grayish-white, yellowish, reddish brown, or blackish. One common coloration has grayish brown hairs along the top side and reddish brown hairs on the sides. The caterpillar is up to 2 3⁄16″ in length and is extremely variable in appearance. Mature caterpillars are usually dark, often black and mottled or striped with yellow. Sometimes they are yellowish or straw-colored. The breathing pores (spiracles) are white. Each thoracic and abdominal segment has six orange or black warts that bear a dense tuft of long, erect hairs (setae). The setae are mostly soft, not stiff or bristly. They may be cream-colored or grayish to yellowish or dark brown. Larvae are densely-haired “woolly bear” caterpillars that range in coloration from yellow to orange to black and potentially with striping.
Life Cycle of the Salt Marsh Moth
The salt marsh moth develops through four distinct life stages: egg, larva (caterpillar), pupa, and adult. Overwintering is as mature larvae (prepupae) that pupate into adults in early spring. Adults are found in open areas, including open woodlands, prairies, meadows, agricultural fields, abandoned fields, and all kinds of marshes, including salt marshes. Adults are active from May to September in Minnesota. There are 2 broods, May - August.
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Egg Stage
After emerging and mating, adult females lay their eggs mostly on the lower surface of leaves. The eggs are nearly spherical in shape, and measure about 0.6 mm in diameter. Initially they are yellow, but soon become grayish in color. Females commonly produce 400 to 1000 eggs in one or more clusters. It is not unusual to find a single egg cluster containing 1200 eggs.
Larval Stage
After hatching the larvae develop through five to seven increasingly larger instars. There are five to seven instars. Upon hatching the larvae are about 2 mm in length, brown in color, and bear numerous long setae over the entire length of the body. During this stage, and the subsequent instar, larvae feed gregariously on the lower leaf surface, usually failing to eat entirely through the leaf. Larvae attain a length of about 10 mm during the first instar. Second instars display longitudinal stripes, usually brown, yellowish, and white, and the body hairs become darker. Larvae attain a length of about 15 mm. During the third instar, larvae become darker, but a consistent color pattern is not apparent. Larvae attain a length of about 30 mm. In the fourth and fifth instars, larvae maintain the same general appearance as earlier stages, but grow to a length of about 45 and 55 mm, respectively. Larvae usually are dark, but sometimes are yellowish brown or straw colored. The larvae are marked by long body hairs, and these also vary in color from cream or grayish to yellowish brown to dark brown. Although they are decidedly hairy, the hairs are not as dense or as stiff as those found in woollybear larvae. Duration of larval development was 24 to 37 days. In contrast, Young and Sifuentes (1959) and Capinera (1978b) reported six instars in Mexico and Colorado, respectively. Development time of the six instars was about 3, 2, 2, 2, 3, and 8 days, respectively, for a larval period of 20 to 22 days, depending on diet.
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.
A generation can be completed in 35 to 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 three to four in the south.
Pupal Stage
Pupation occurs on the soil among leaf debris, in a thin cocoon formed from silken hairs interwoven with caterpillar body hairs. It appears that they incorporate their own hair into the cocoon. The dark brown pupae measures about 30 mm in length.
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Adult Stage
Mating occurs the evening following emergence, and egg deposition the next evening.
Dietary Habits of the Salt Marsh Moth
The salt marsh caterpillar has a broad diet, consuming a wide variety of plants. This contributes to its status as a pest in various agricultural settings.
Larval Diet
Larvae (Salt Marsh Caterpillars) are pests on many plants. The larvae feed mostly on a wide variety of herbaceous plants, but also on the leaves of trees and shrubs. They are agricultural pests of many crops, including cabbage, cotton, walnuts, apple trees, tobacco, pea, potato, clovers, and maize. Larvae feed on a wide variety of mainly weedy plants including anglepod (Gonolobus), dog fennel (Eupatorium capillifolium), ground cherry (Physalis), mallow (Anoda), milkweed (Asclepias), pigweed (Amaranthus), and sicklepod (Cassia tora), plus crops such as alfalfa, asparagus, bean, beet, cabbage, carrot, celery, clover, corn, cotton, lettuce, onion, pea, potato, soybean, sugarbeet, tobacco, tomato, and turnip. 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.
Young larvae feed in groups that scrape and chew on the underside of leaves; leaf veins and the leaf upper surface remain intact. Older larvae feed individually and chew entirely through leaves, leaving the foliage ragged. Young larvae feed gregariously and skeletonize foliage. Older larvae are solitary and eat large holes in leaf tissue. 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 (Capinera 1978). 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.
Adult Diet
Adults do not feed extensively, focusing primarily on reproduction.
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Ecological Role and Pest Status
The salt marsh moth is a native insect found throughout the United States. Its distribution extends to Central America, and in Canada it has damaged crops in Ontario and Quebec. 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.
The caterpillars are relatively uncommon in the spring, but the fall generation is more numerous and causes the most foliage damage. Commonly this is associated with maturation of cotton or weeds in the autumn. Thus, these caterpillars tend to be damaging to fall-planted crops. Saltmarsh caterpillar is a sporadic pest that uncommonly warrants control.
Natural Enemies and Control
The saltmarsh caterpillar has many natural enemies. These include the maggotlike larvae of tachinid flies that feed inside the caterpillars and an Ophion species of parasitoid (parasitic) wasp that is orangish as an adult. As a larva Ophion kills the host during its pupal stage and mummifies the pupal case.
If the larvae are intolerably abundant, the underside of infested foliage can be thoroughly sprayed once or more with Bacillus thuringiensis (Bt) or spinosad. Spinosad is toxic to bees and certain beneficial predatory insects for at least several hours after the application so if applying it to plants that are blooming wait until the evening when honey bees are no longer active. Insecticides are commonly used to suppress saltmarsh 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.
Conservation Status
Not usually required. NCGR: G5 - Demonstrably secure globally, though it may be quite rare in parts of its range, especially at the periphery.