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The larvae are mainly nocturnal and prefer younger leaves, often stripping growing shoots, particularly in the final instar. Two or more healthy larvae can completely defoliate a tobacco plant, leaving only midribs and stem. During major infestations entire plantations may be defoliated. Severe damage most commonly occurs during late July and August in the USA. With heavy infestations of tomatoes, caterpillars may even feed on developing fruit, leaving large, open scars.
Malathion, diazinon, carbaryl and fenitrothion have been used with great effect. In the southern USA it is recommended that hornworms on tobacco be treated with insecticides when infestation levels exceed the economic threshold of five or more large (2.5 cm or longer), unparasitized larvae, per 50 plants (North Carolina State University, 1998).
Cultural practices are very important. Early planted tobacco, correct (not excessive) nitrogen fertilization, sucker control, stalk destruction, and autumn ploughing all help to reduce overwintering populations (Reagan et al., 1978). Additionally, certain cultivars of tobacco have been found to be resistant to M. sexta in the USA (Johnson, 1996).
Field evaluation in North Carolina of a transgenic tobacco cultivar containing a Bacillus thuringiensis insecticidal protein gene demonstrated that damage by M. sexta remained below the economic threshold for the duration of the trial (Warren et al., 1992).
There are a number of natural enemies that help control hornworm populations, with two having been used in augmentative biological control programmes - the braconid Cotesia congregata, and the bacterium Bacillus sphingidis.
Early generations are potentially damaging to marketable tobacco in the southern USA, sometimes stripping entire plantations. Severe damage most commonly occurs during late July and August. Later generations feed after harvest on non-commercial suckers. Nevertheless, these later generations are commercially important as they produce overwintering pupae. With heavy infestations of tomatoes, caterpillars may even feed on developing fruit, leaving large, open scars.
Field studies were conducted during 1981 in Maryland, USA, to determine the relationship between pest density and yield loss from artificial infestations of fourth and fifth instar larvae. At several growth stages of the tobacco cultivar Md. 872, yields declined as the number of larvae per plant, or the percentage of infested plants increased. The data showed that yield loss varied according to date of infestation and plant phenology; tobacco plants became more tolerant of infestations as they aged. Based on control costs, and the market value of Maryland tobacco at the time of the study, larval population levels capable of causing 1.8% yield loss justified treatment. Economic injury levels varied considerably with plant growth stage and distribution patterns of infestations (Kolodny-Hirsch and Harrison, 1986).