Bollworm Complex
Egg - The small, dome-shaped eggs are white when first deposited, but develop reddish-brown bands before hatching. Color plate.
Larva - These moderately hairy larvae may be cream-colored (first instar), pale green (second or later instars), rose (late instars), or brown (late instars). As they mature, they develop pale longitudinal stripes and scattered black spots (chalazae). From hatch to maturity, they vary from 1.5 to 44 mm in length. Color plate.
Pupa - The pupae are about 31.5 mm long and 6 mm wide. Initially, a shiny reddish-brown, they become dark brown near the time of adult emergence.
Host plants - H. zea occurs on at least 14 cultivated plants: alfalfa, bean, corn, cotton, okra, peanut, pea, sorghum, soybean, strawberry, sweet pepper, sweet potato, tobacco, and tomato. It is also found occasionally on such wild hosts as toadflax. Although corn is the preferred host, this pest is very destructive to cotton.
The major agronomic hosts of the tobacco budworm are tobacco and cotton, the former being the preferred host. Budworms have been collected from soybeans but economic loss has not been documented. Species of deergrass (Rhexia spp.) are the most common wild hosts, but toadflax, beggarweed, groundcherry, geranium, and ageratum are also commonly infested.
Damage - Bollworms (Heliothis zea) and tobacco budworms (H. virescens) comprise the "bollworm complex" on cotton. Damage by third-generation larvae usually causes the greatest economic loss; however, second-generation bollworms may reach threshold levels. First instars feed on the epidermis of tender plant terminals. Later instars, however, move into squares, blooms, buds, and bolls. They not only feed superficially but also burrow into young bolls and squares, often hollowing them and facilitating the introduction of pathogens. Each of these larvae is capable of destroying several fruiting forms; therefore, low populations can cause economic damage.
Life history - In North Carolina, bollworms overwinter as pupae in the soil at a depth of 5 to 10 cm (2 to 4 inches). Diapausing pupae occur in August or September. Adult moths emerge from late April to mid-May to mate and lay eggs (oviposit). Each female lays an average of 1,000 eggs, which usually hatch within 2 to 5 days.
In spring and early summer when the overwintering generation moths emerge, no single species of plant is abundant or desirable enough to receive the majority of eggs. Therefore, eggs and larvae occur in moderate numbers on corn (in the whorl), tobacco, cotton, and various weedy hosts. When the many acres of field corn start silking, almost 100 percent of the H. zea eggs are deposited on corn. The same is true concerning flowering tobacco and H. virescens eggs. Later in the season when corn silks dry up, third-generation bollworm moths oviposit on a variety of hosts: cotton, soybeans, peanuts, and others. Therefore, most of the H. zea population on cotton occurs in August and early September. Tobacco budworm moths, however, continue to prefer tobacco for oviposition, only sporadically laying eggs on cotton. Therefore, cotton is not normally attacked as readily by the tobacco budworm as it is by the bollworm in North Carolina.
On cotton, larvae of the "bollworm complex" require 17 to 25 days to feed and mature. They then pupate in the soil for about 16 days. A new generation of moths emerges from the pupae, mates, and lays eggs. Up to four generations occur each year in North Carolina.
Insecticide applications are most effective when the bollworms are very small. For specific information concerning insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
