Distribution. The tobacco and tomato hornworms are very similar in appearance, biology, and distribution. However, the tobacco hornworm, Manduca sexta (Linnaeus), is more common in the southern United States, especially the Gulf Coast states. Its range extends northward to New York, and even sometimes into southern Ontario. The tomato horn-worm, Manduca quinquemaculata (Haworth), in contrast, is uncommon along the Gulf Coast, but relative to tobacco hornworm it is more likely to be encountered in northern states and southern Canada. These are native species, occurring from the Atlantic to the Pacific Oceans, though for tobacco hornworm the range extends south into Central and South America, and the Caribbean.
Host Plants. These insects feed only on solanac-eous plants, particularly tomato and tobacco. They are recorded from other vegetables such as eggplant, pepper, and potato, but such feeding is unusual. Several solanaceous weeds are reported to serve as hosts, including groundcherry, Physalis spp.; horsenettle, Solanum carolinense; jimsonweed, Datura stramonium; and nightshade, Solanum spp.; but wild hosts are unimportant larval food sources relative to crops. Adults imbibe nectar from flowers of several plants such as catalpa, Catalpa speciosa; daylily, Hemerocallis sp.; hollyhock, Althaea rosea; jimsonweed; four o'clock, Mirabilis jalapa; mallow, Hibiscus lasiocarpus; mimosa, Albissia julibrissin; and tobacco.
Natural Enemies. Many natural enemies are known from tobacco and tomato hornworm. Lawson (1959) provided a good summary of their importance in North Carolina. Natural enemies of the egg stage include Trichogramma sp. (Hymenoptera: Trichogram-matidae) and Telenomus sp. (Hymenoptera: Scelioni-dae), but these are thought to be of little importance. Of greater importance is the stilt bug Jalysus spinosus (Say) (Hemiptera: Berytidae) and other general predators such as big-eyed bugs (Hemiptera: Lygaeidae) and lacewings (Neuroptera: Chrysopidae).
The larvae often are parasitized by Cotesia congrega-tus (Say) (Hymenoptera: Braconidae), and they sometimes are seen bearing clusters of white pupal cases formed by this wasp, attached to their bodies. In North Carolina, 30-40% of hornworms may be parasitized by this wasp. In contrast to this one species of wasp accounting for most of the parasitism by Hymenop-tera, numerous insects in the order Diptera, particularly the Tachinidae, attack tobacco and tomato hornworms. Records of tachinid parasitism from tomato hornworm include Compsilura concinnata (Mei-gen), Drino incompta (Wulp), D. rhoeo (Walker), Lespesia frenchii (Williston), Winthemia leucanae (Kirkpatrick), and W. quadripustulata (Fabricius). Records of tachi-nids from tobacco hornworm include Carcelia spp., Drino incompta, D. rhoeo, Lespesia spp., Metavoria orientalis Townsend, and Winthemia quadripustulata. In North Carolina, the proportion of hornworm larvae parasitized by tachinids increases during the season, sometimes attaining 100%. The number of tachinid eggs per hornworm larva also increases seasonally, indicating a greater abundance of flies. Also of great significance as a mortality agent of larvae is Polistes spp. (Hymenoptera: Vespidae). These wasps kill and consume, or sometimes carry away to provision their nest, a very high proportion of larvae. Apparently, Polistes spp. is the most important larval mortality factor in North Carolina.
Pathogens sometimes affect hornworms. A bacterial disease which causes the larva to blacken and to droop is sometimes reported to kill larvae, but this does not occur regularly, and is observed mostly at very high larval densities. Larvae also are susceptible to infection by the fungus Entomophaga aulicae, but this disease also does not occur widely (Morrow et al., 1987).
Vertebrates may prey on the pupae. Skunks and moles are particularly important. Tachinids emerge from the prepupal and pupal stages, but attack only the larvae.
Life Cycle and Description. Although these insects are common, they are not serious pests except in home gardens. Thus, there is surprisingly little information about these insects, and particularly about tomato hornworm. The following information applies specifically to tobacco hornworm except where noted, but apparently is equally applicable to tomato hornworm.
The number of annual generations ranges from 1 to 2 in Canada to 3-4 in northern Florida, but two generations per year is common over most of the ranges of these species. The proportion of insect that enter diapause increases from about 5% in June to 95% in mid-August, as day length decreases. In northern Florida the insects are active from April to November, but they are abundant only for the first two generations because many pupae enter diapause. In North Carolina they occur from mid-May to October, and evidence of a third generation is poor. Throughout their range, hornworms overwinter in the pupal stage, and in the northernmost parts of their range they may not overwinter successfully, occurring only after dispersal northward during the summer. The life cycle can be completed in 30-50 days, but often is considerably protracted.
white but during the second instar become green with white lines laterally. The tobacco hornworm develops seven straight oblique whitish lines laterally. The white lines are edged with black on the upper borders, and the "horn" is usually red. The tomato hornworm is superficially similar, but instead of the seven oblique lateral bands it bears eight whitish or yellowish "V"-shaped marks laterally, and pointing anteriorly. The "V''-shaped marks are not edged in black. Also, in tomato hornworm the "horn" tends to be black. The body of both species is usually light-green, but occasionally dark-brown or blackish forms occur. There normally are five instars, but occasionally six are observed. The mean head capsule width is 0.8, 1.2, 2.0, 3.0, and 5.0 mm for instars 1-5, respectively. Corresponding mean larval body lengths are 6.7, 11.2, 23.4, 49.0, and 81.3 mm, respectively. Larval development time averages about 20 days, but ranges from 13-44 days depending on temperature. Mean development time for larvae reared under insectary conditions in northern Florida is 3.4, 2.9, 3.0, 3.9, and 6.6 days for instars 1-5, respectively. (See color figures 82 and 83.)
Pupa. Mature larvae drop to the soil at maturity and burrow to a depth of 10-15 cm. There they form a pupal cell measuring about 7 cm long and 4 cm wide, and pupate within. The interval between entering the soil and pupation is usually 4-8 days. The pupa is large and elongate-oval, but pointed at the posterior end. It measures 45-60 mm long and 13-14 mm wide. The pupa bears a pronounced maxillary loop, a structure which encases the mouthparts. The maxillary loop in tobacco hornworm extends back about one-fourth the length of the body, whereas in tomato horn-worm it is longer, usually extending for about one-third the length of the body. The color of the pupa is brown or reddish-brown. Duration of the pupal stage is protracted and variable. Pupal duration often exceeds 100 days, even in the summer generations, but may be as short as 15 days, and 21 days is about average. Overwintering pupae do not emerge synchronously, with emergence spread from May to early
August; a few even diapause through two winters. (See color figure 268.)
Adult. The adults of both species are large moths with stout, narrow wings, and a wingspan of 80130 mm. The front wings are much longer than the hind wings. The females are larger than the males and can be differentiated by the narrower antennae. Both species are dull-grayish or grayish-brown, though the sides of the abdomen usually are marked with six orange-yellow spots in tobacco hornworm and five spots in tomato hornworm. The hind wings of both species bear alternating light and dark bands. Adults become active at sunset, when they can be observed feeding at flowers. Although some adults remain active throughout the night, most activity occurs early in the evening and just before dawn. Both hornworm species likely produce sex pheromone, but only that of the tobacco hornworm has been identified. The pre-oviposition period of moths is about two days, and eggs are deposited for about 4-8 days. (See color figures 217 and 218.)
A detailed description of tobacco hornworm biology was given by Madden and Chamberlain (1945), and a more abbreviated but useful description of both species was established by Gilmore (1938). Diapause development was reported by Rabb (1966). Rearing was described by several authors, including Yama-moto (1968, 1969), Stewart and Baker (1970), and Bell and Joachim (1976). Keys to the adults were included in Hodges (1971).
Larvae are defoliators, and usually attack the upper portion of plants initially. Rather than chewing holes in the leaf, they usually consume the entire leaf. Sometimes they attack green fruit. As the larvae of hornworms attain such a large size, they are capable of high levels of defoliation. Madden and Chamberlain (1945) calculated the consumption of tobacco foliage by larvae and reported that 5.2, 9.7, 33.5, 175.4, and 1941.4sqcm of foliage were eaten by instars 1-5, respectively, for a total of 2165 sq cm. Notice that about 90% of the foliage consumption occurred during the
final instar, and though the exact value of foliage consumption would differ on tomato, the pattern would be the same. Larvae blend in with the foliage and are not easy to detect. Thus, it is not surprising that they often were not observed until they caused considerable damage at the end of the larval period.
Cultural Practices. The pupae are large and are not buried very deeply in the soil, so greater than 90% mortality is caused by normal soil tillage practices. Hand picking and destruction of larvae is often practical in the home garden.
To take advantage of the preference of Polistes wasps for hornworm larvae, wasp shelters or nesting boxes were placed in tobacco fields to encourage the wasps, and wasp colonies were relocated into tobacco (Lawson et al., 1961). Although wasp predation was inadequate to prevent damage to tobacco, this approach might be satisfactory for tomato.
Tobacco and tomato hornworms thrive on tobacco plants that are allowed to revegetate after harvest of the leaves, leading to high populations during the next year. Destruction of tobacco stalks, or inhibition of sprouting by application of plant growth regulators, greatly reduces hornworm populations in subsequent seasons (Rabb, 1969). Although not documented, timely destruction of tomato crop residue likely would have similar beneficial effects.
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