Estigmene acrea (Drury)
Description. The adult saltmarsh caterpillar is a large, light-colored moth. The abdomen usually is orange to yellow and usually has black spots on the top. Male and female moths differ in the way their wings are colored. In the male the fore wing is white and the hind wing is orange to yellowish. In the female both wings are snowy white on the top and yellow below. The wing surfaces of both sexes are marked with variable black spots. The wing span usually is 1 ½ to 2 inches.
The female may lay 400 to 1,000 spherically shaped eggs in one or more clusters on the underside of host plant leaves. The eggs are about 0.06-mm in diameter and light yellow in color, but turn grayish as they age. Eggs require 4 to 5 days to hatch. Newly hatched saltmarsh caterpillar larvae are about 2-mm in length and feed gregariously on the bottom side of leaves. As the larvae mature they disperse and feed singly. They typically pass through 5 instars, but may produce as many as 7 instars. The larval stage of the saltmarsh caterpillar is sometimes called a “wooly bear,” because the body is covered with long hairs giving it a wooly appearance. These larvae may be yellowish brown or straw colored, but are usually gray to black to reddish brown with a dark head. The full sized larva reaches a length of 2 1/4 inches. The larval stage requires 20 to 45 days for completion. Saltmarsh caterpillar larvae are very active, particularly the late instars, and are often seen dispersing in very large numbers after their initial food source has been depleted, matured or sprayed with herbicide. They have a very wide host range feeding on some grasses, but primarily feed on broadleaved plants.
They pupate on the soil under plant debris in a fragile, silken cocoon heavily covered with interwoven hairs from the caterpillar’s body. Pupation requires 12 to 14 days when active or through the winter. Populations are usually at their greatest late season, but large numbers are sometimes encountered in the Spring following warm, wet winters.
Damage. Saltmarsh caterpillars can be severe defoliators when present in large numbers. Saltmarsh caterpillars may migrate in mass from weedy areas in search of food. They are general defoliators and rarely feed on fruiting structures. Early instars tend to skelentonize leaves, while the feeding of larger larvae results in a ragging appearance. Dispersing spring populations are more problematic than late season infestations. These larvae will move into seedling cotton from weedy habitats and can quickly reduce the stand. On seedling cotton, a single late instar will consume approximately 3 sq-inch of leaf tissue in a 7 day period. Although they have been observed infesting entire fields, damage is usually relegated to field edges adjacent to the weedy source.
Management and decision making.
Cultural management. The most effective method forcontrolling saltmarsh caterpillars is to grow cotton containing genes from the bacteria, Bacillus thuringiensis (Bt). These genes produce proteins that are toxic to most leaf and fruit feeding caterpillars after ingestion. Although the single gene varieties were considered moderately effective towards saltmarsh caterpillars, the new multiple toxin cotton varieties have proven highly effective to small and large larvae.
Since most saltmarsh caterpillar populations originate in weedy areas, controlling these weeds before the insect population develops, or avoiding killing these weeds and dispersing the larvae are effective means for preventing problems.
Biological control. Saltmarsh caterpillars are preyed upon by a vast array of larval and egg predators and parasitoids. Tachinid flies tend to be the most common larval parasitoid encountered and Trichogamma frequently parasitize the eggs. Common predators include big-eyed bugs, minute pirate bugs, damsel bug, lacewing larvae, assassin bugs, spiders, lady beetles and collops beetles. The most important biological control occurs in wild habitats, but can also be important in crop. In Arizona, late season insecticide applications have resulted in large outbreaks of saltmarsh caterpillars originating in cotton due to destruction of natural enemies.
Scouting. Saltmarsh caterpillars should be scouted for using general field observation scouting procedures. Late season population will be detected by the appearance of noticeable defoliation. When defoliation is detected, the affected areas of the field should be thoroughly scouted accessing the level of defoliation and along with the presence of actively feeding larvae. Early season populations are more difficult to detect and need to be detected in a more timely fashion. These populations are often first detected as larvae crossing roads. It is important to check the entire field perimeter for dispersing populations, paying particular attention to areas adjacent to weedy habitats. Scouting weedy habitats at planting will often provide valuable insight into infestation potential following crop emergence.
Chemical Control and Action Thresholds. Late season populations of saltmarsh caterpillars rarely warrant control and should only be treated when harvestable bolls are still being filled and defoliation is severe.
|Suggested Insecticides and Rates for Managing Saltmarsh Caterpillars in Cotton
|Amount of formulated
|Acres treated per gal or lb of
|Mode of Action Group (IRAC)|
|Cry 1Ac, Cry1F
(Baythroid XL 1)
|0.013-0.02||1.6-2.6 fl oz||80-49.2||3A|
(Brigade 2, generics)
|0.04-0.10||2.6-6.4 fl oz||49.2-20||3A|
(Ammo 2.5, generics)
|0.04-0.1||2.0-5.0 fl oz||64-26||3A|
(Asana XL 0.66E)
|0.03-0.05||5.8-9.6 fl oz||22-13.3||3A|
(Declare 1.25, generics)
|0.01-0.015||1.02-1.54 fl oz||125.5-83.1||3A|
(Karate 1 EC, generics)
(Mustang Max 0.8 E)
|0.018-0.025||2.8-4 fl oz||45.7-32||3A|
(Hero 1.24 EC)
|—||10.3 fl oz||12.4||3A|
1rates vary depending on formulation.