Camille Espada, Howard Community College
Amanda Wheland, Howard Community College
Mentored by: Will Gretes, Ph.D.
The purpose of this study is to better understand how feeding in the tobacco hornworm, Manduca sexta, affects future food preferences. Previous research composed by Yamamoto (1974) and deBoer and Hanson (1984) attempted to understand this behavior. Yamamoto (1974) suggested that larvae acquire their preference as they grow, which he refers to as “induction of oligophagy”. Alternatively, deBoer and Hanson (1984) looked at their feeding behavior from a different view suggesting that their preference is innate and it can be modified to what they can eat, calling it an “induction of preference”. We suggest that future experiments using less-palatable plant species outside of the hornworm’s normal host range can provide some insight into this dilemma.
The tobacco hornworm, larval Manduca sexta, is a very picky eater (see Figure 1 for a comparison of the various larval stages). Of the nearly 400,000 species of plant, hornworm larvae appear to feed exclusively in nature on those plants in the family Solanaceae, which includes familiar plants like tomato, tobacco, eggplant and potato. How does this species of caterpillar maintain such a level of diet restriction? Some studies have suggested that the tobacco hornworm acquires its host-plant specificity as it feeds on solanaceous plants [1, 2]. Alternatively, other studies have demonstrated that this feeding preference is innate and does not require previous feeding experience . Both studies observed that the feeding behavior of these larvae is somewhat malleable, however they disagree with respect to how far these modifications extend.
Figure 1: Manduca sexta life cycle. © The Worm Lady .
Experiments performed by Yamamoto  demonstrated that larvae that had not been reared on a solanaceous diet were willing to feed on several disparate plants in a choice test, which included mullein, collard, cowpea (all non-solanaceous plants), as well as tomato. Since larvae were willing to feed on foods outside of their normal host range, Yamamoto categorized them as polyphagous (see Figure 2 below).
Figure 2: Feeding of M. sexta larvae in a four-choice test conducted by Yamamoto . Larvae were tested as unfed 1st instars or reared to the 2nd, 3rd, or 4th instar on tomato, cowpea, or an artificial diet (n = 40 for each test).
Similarly, deBoer and Hanson  observed that the feeding preferences of these caterpillars were influenced by their rearing diet. This time, changes in feeding thresholds were evaluated using a two-choice test where larvae almost always displayed an increased preference for the rearing food. For example, in a test between tomato and jimson weed (both highly-palatable solanaceous plants), fifth instar larvae reared on tomato preferred tomato over jimson weed, while larvae reared on jimson weed preferred it over tomato (see Figure 3 below).
Figure 3: Feeding behavior of M. sexta larvae in a two-choice test conducted by deBoer and Hanson (1984). Larvae were tested as 5th instars reared on tomato (TM) or jimsonweed (JW), or as unfed 1st instars (n= 21, 25, and 42 respectively).
These authors described this change as an “induction of preference”. When comparing solanaceous plants to non-solanaceous plants, rearing on the non-solanaceous plant increased its acceptance relative to the other plant, but this never exceeded that of the solanaceous plant (see Figure 4). Additionally, first instar larvae universally demonstrated a strong preference for solanaceous plants over non-solanaceous foods. deBoer and Hanson (1984) used this observation as evidence that the hornworm larvae are innately oligophagous.
Figure 4: Feeding behavior of M. sexta larvae in a two-choice test conducted by deBoer and Hanson (1984). Larvae were tested as 5th instars reared on tomato (TM) or cowpea (CP), or as unfed 1st instars (n= 35, 21, and 26 respectively).
It could be argued that the common fault between the studies is that the rearing plant and the testing plant were always the same. Yamamoto reared larvae on tomato and tested them on tomato, and therefore cannot rule out an induction of preference for just tomato. deBoer and Hanson also always used the rearing plant as one of the choice options in their experiments. Therefore these authors failed to test for and cannot rule out an induction of oligophagy.
Gretes et. al  attempted to demonstrate both inductions of preference and inductions of oligophagy using a no-choice test (with only one food being offered during an experiment) instead of a choice test. Unfortunately, the no-choice tests used by these researchers could not reveal any increases of the acceptability of the already highly-palatable solanaceous plants (see Figure 5). Larvae reared on both solanaceous and non-solanaceous diets appeared to find potato, tobacco, and groundcherry (all solanaceous plants) highly palatable, however no increases in acceptability were observed.
Figure 5: Consumption rates of M. sexta larvae in no-choice tests conducted by Gretes et al. (2016). Larvae were tested as 5th instars given potato, tobacco, or grouncherry after rearing on one of several solanaceous or non-solanaceous foods. Total consumption of each food was measured over time.
These experiments did reveal an induction of preference for a non-solanaceous plant (see Figure 6), and reported increased acceptability for a species closely related to cowpea, the corkscrew vine (data not shown).
Figure 6: Consumption rates of M. sexta larvae in a no-choice test conducted by Gretes et al. (2016). Larvae were tested as 5th instars given cowpea after rearing on one of several solanaceous or non-solanaceous foods. Cowpea appeared most acceptable to those larvae that were reared on it.
Less palatable, non-solanaceous plants can be used to test for inductions of preference and inductions of oligophagy in M. sexta larvae, as many of these plants are only mildly acceptable and increases in feeding can be significant. We propose testing non-solanaceous plants from two major non-solanaceous families: Brassica and Fabaceae. Using a no-choice feeding test we will attempt to determine if rearing on a plant increases the acceptability of that plant (an induction of preference), and if rearing on a plant increases the acceptability of other related plants (an induction of oligophagy).
This material is based upon work supported by the National Science Foundation under Grant No. 1458149.
 Yamamoto, R. (1974). Induction of host plant specificity in the tobacco hornworm, Manduca sexta. Journal of Insect Physiology, 20(4), 641-650.
 del Campo, M.L. & Renwick, J.A. (1999). Dependence on host constituents controlling food acceptance by Manduca sexta larvae. Entomologia Experimentalis et Applicata, 93, 209-215.
 deBoer, G. & Hanson, F. E. (1984). Foodplant selection and induction of feeding preference among host and non-host plants in larvae of the tobacco hornworm Manduca sexta. Entomologia Experimentalis et Applicata, 35(2), 177-193.
 The Worm Lady. 2013. http://thewormlady.ca/hornworm-life-cycle.php.
 Gretes, W. C., Stanwyck, E. A., & Hanson, F. E. (2016). Innate and acquired components of oligophagy in the herbivorous lepidopteran, Manduca sexta. Entomologia Experimentalis et Applicata, 160(3), 259-271.