Variation in growth and instar number in field and laboratoryManduca sexta

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Abstract
The tobacco hornwormManduca sextahas been an important model system for understanding physiological control of growth, development and metamorphosis of insects for more than half a century. Like allManduca,M. sextatypically has five larval instars, with developmental commitment to metamorphosis occurring early in the 5th (final) instar. Here we show thatM. sextafrom a field population in North Carolina (USA) shows substantial intraspecific variation in the number of larval instars when feeding on a modified artificial diet. Individuals with six instars consistently exhibited slower growth rates during early larval development than individuals with five instars. The frequency of individuals with six instars decreased with increased rearing temperature. In contrast,M. sextafrom a laboratory colony consistently had five instars, and had more rapid larval growth rates thanM. sextafrom the field. We identify a threshold body size at the start of the 5th instar that predicts whether an individual will have five (greater than 600 mg) or six instars (less than 600 mg). Variation in field populations inManducaprovides an important resource for understanding physiological control, developmental plasticity and evolution of growth rate, body size and instar number.

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