Understanding how developmental processes evolve is a fundamental challenge for evolutionary biology. The Tobacco Hornworm, Manduca sexta, a major agricultural pest of tobacco and tomato in the Southeast US, is an important model system for understanding the growth, development and metamorphosis of insects, and laboratory colonies of M. sexta have been maintained for over 40 years. Recent studies have revealed surprising genetic variation and developmental plasticity in the number of larval stages (instars) in field populations of M. sexta. The proposed research will study the nutritional and developmental mechanisms that cause variation in instar number, and document the consequences of instar number for caterpillar growth and feeding, adult size, and female fecundity (number of eggs). This project will document how variation in development is maintained within insect populations in nature, and how developmental variation influences feeding, growth and fitness of this important agricultural pest.
This project will include the training of undergraduate and graduate students in Biology at the University of North Carolina and Duke University as well training high school teachers.
The size of an adult insect is determined by how much it grows during its immature stages. In most insects, including most agricultural and medical pests, the immature larva grows in a series of discrete steps, called instars, before transforming into a pupa and then an adult. What determines the number of larval instars, and how is this influenced by environmental factors? How do larval growth and instar number determine adult size and development time—the time from hatching to adulthood? In the field, how do predators, parasites and other natural enemies of insect larvae cause natural selection on larval growth and development, and adult size and development time? Answering these questions is important both for our general understanding of growth, development and adult size, and for predicting feeding, survival and success of insect pests. This research project studied the Tobacco Hornworm (Manduca sexta) to address these questions. Hornworms are an important ‘model’ organism for understanding physiology and development of insects, and are an agricultural pest on tobacco, tomatoes, peppers and related plants. Our studies show that natural populations of Hornworms typically have 5 larval instars, but may express 6 or more instars depending on environmental conditions. Lower rearing temperatures and poor food (hostplant) quality increase the likelihood that a caterpillar will have additional instars. Having additional instars results in a longer development time, but large adult size and more eggs. As a result, producing additional instars may be an adaptive response to poor nutritional conditions that prolongs development but increases adult size and reproduction. Variation in instar number may be an important factor in evolutionary changes in the number of instars in different species. By contrast, some domesticated lab populations of Hornworms that have been maintained on high-quality food for hundreds of generations have largely lost their capacity for additional instars except under extreme conditions. In most organisms except birds and mammals, higher rearing temperatures typically result in smaller adult body size, a widespread pattern of phenotypic plasticity called the temperature-size rule. This pattern also occurs when Hornworms are reared on tobacco or other typical hostplants. However, when reared on a novel, lower-quality hostplant, Devil’s Claw, the opposite occurs: higher temperatures result in larger body size. This suggests that common forms of phenotypic plasticity can be altered by simple changes in nutrition and perhaps other environmental factors. Given that Devil’s Claw is a low-quality hostplant, why do Hornworms in the field lay their eggs on it, and caterpillars eat it? Field experiments show that natural enemies play a key role here. On tobacco and other typical hostplants, Hornworm caterpillars are often parasitized and eventually killed by an insect parasitoid, Cotesia congregata: In North Carolina more than 60-80% of Hornworms in the field can be parasitized. However, Hornworms feeding on Devil’s Claw completely escape parasitism. As a result, even though Devil’s Claw is less nutritious to Hornworms than Tobacco, the overall fitness of individuals is similar on the two hostplants. This illustrates one way that insects can expand the range of hostplants they that use. Field experiments also show that on typical hostplants, Hornworn caterpillars that develop rapidly have greater survival because they escape parasitism, and adult females that are larger produce more eggs. This means that there is natural selection for both rapid larval development and large adult size; but large size and rapid development are not correlated. This may be one reason why Hornworms are voracious eaters and among the largest insects: a last-instar Hornworm can weigh 12-15g—more than a pocket mouse-- and can completely defoliate a tomato plant in a matter of days.
