Research Paper: Relationship Between Diet and Growth Weight of Tobacco Hornworm

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Tobacco Hornworm

An Analysis of Growth Rates of the Tobacco Hornworm and Various Food Supplies: A Controlled Experiment

This paper provides the details of an experiment conducted with tobacco hornworm (Manduca sexta) larvae and various food sources found both in nature and laboratory-created. Manduca sexta has long been observed to favor the leaves of the tobacco and tomato plant, as well as other species of plant in the Solonaceae family, with direct destructive consequences for the tobacco and tomato industry as well as effects on other agricultural products. Studies regarding food sources for the Manduca sexta and other similar species of moth larvae (especially the tomato hornworm, or Manduca quinquemaculata) have attempted to determine alternative food sources that might be more attractive to the insects in agricultural settings, thus diverting their consumption away from agricultural plants and products and mitigating damages to crops caused by larval eating habits. The research described herein failed to find a source of food that was preferential to the Manduca sexta larvae when compared to tobacco and tomato plant leaves, yet analysis of growth rates found that a laboratory-created solid food, when the only food source presented to the moth larvae, led to a markedly greater growth rate for the larvae. The findings suggest that alternative food sources are a viable means of affecting tobacco hornworm populations, though further research is needed to establish methods of making such food sources preferential to the natural food preferences exhibited by the species.

Introduction

The Solanaceae family of plants includes a wide variety of species, many of which are edible by humans and/or other animal species and others of which are highly poisonous. Nightshade, a plant with semi-mythical evil powers, is a member of this family of plant, as are the tomato and tobacco plants as well as potatoes, eggplants, and many others that are less recognizable by name and the appearance of their fruits. Given the popularity of many species in the Solanacae family with human beings in terms of consumption both for taste and nutritional properties, it is perhaps unsurprising that many other species also find these plants a good source of the energy and nutrients needed for growth and the maintenance of proper functions and life cycles. Of course, the fact that other species find certain parts of these plants especially edible and show a clear preference for these plants over other offered alternatives also poses a problem for human consumption of these plants in terms of possible crop degradation brought on by large populations of these species.

The tobacco hornworm moth, Manduca sexta, or more accurately the larval caterpillars of the species, have shown a clear preference in nature for the leaves of tobacco plants and tomato plants as a food source, granting the species its common name (as well as the common name of its close cousin the tomato hookworm, Manduca quinquemaculata). Despite the fact that these species eat the leaves of the plants, which are not palatable or even edible (in the case of the tobacco plant) by human beings, their consumption of these leaves can have detrimental effects on the overall plants, destroying food sources when it comes to the edible plants and creating economic problems for the tobacco industry, which specifically utilizes the leaves of the plant in its agriculturally-derived products.

There has been a fair amount of research into the digestive preferences of the Manduca sexta as well as the effects that food sources and other variables have on the growth and development of the species, much of it with the purpose of determining alternative food sources or methods for disrupting the larval development cycle in order to reduce levels of crop destruction and effects on agribusiness. Temperature and protein levels have been shown to have huge effects on the growth rates, development cycles, and even the mortality of Manduca sexta larvae, for instance, suggesting that dietary adjustments are indeed an effective way of affecting the species and its agriculturally destructive feeding behaviors (Petersen et al. 2000). This has definite implications for both the reasons behind the research at hand and the expectation of certain results in this research.

Specifically, this research attempts to determine the effects of different food sources on the growth rates of Manduca sexta larvae, and previous research into the area suggests that there will be potentially large differences in these growth rates based on the composition of the different food sources (Petersen et al. 2000; Kingsolver 2007). Leaves from the tomato and tobacco plants, the natural food preferences of the species, will be compared to leaves from another plant (the common radish or Raphanus sativus) and a laboratory-created solid nutritional compound composed of Mulberry leaf powder, one-percent soy flour, corn meal, agar, and beet powder as food sources both for preference and for effects on growth rate. Preference will be tested by simply providing all food sources to a group of larvae, whereas controlled access to different sources will allow for a measurement of comparative growth rates.

The Manduca sexta has long been a subject of special interest for researchers investigating the metabolism and growth and life cycles of insect larvae generally (Kingsolver 2007). This in and of itself demonstrates the scientific importance of the research being undertaken, as the response of the species to the different food sources will shed yet further light on the direct role that digestion and nutritive value have on the growth and development rates of the species, as well as the direction of food preference that the species exhibits. In addition to the purely scientific interest that exists in the species growth and development, however, this research will also outline possibilities for the control of Manduca sexta populations via manipulations of the food supply, which has already been shown to highly affect adult development at the organism and molecular levels (Nijhout et al. 2007; Pauchet et al. 2010). It is expected that noticeable differences in the growth rates will be observed due to the variations of food source in this research, with definite implications for future research and the practical control of the Manduca sexta.

Discussion

The results of the research are generally in keeping with the research expectations and hypothesis. There is a clear growth benefit to the laboratory-created food source, which was designed as an optimum nutritional source for the Manduca sexta, while the larvae of this species showed a definite preference for their natural food source, the leaves of the tobacco and tomato plants. These results have several interesting implications both in purely scientific terms and in the area of practical agricultural concerns.

First, it is interesting to note that nutritional and growth rate benefits do not translate to food source preferences in the Manduca sexta, any more than they do for most human beings and other anecdotally observed species (domesticated dogs, for instance). Despite markedly increased growth rates when fed solely with the laboratory-created food source of Mulberry leaf powder, one-percent soy flour, corn meal, agar, and beet powder, the Mandcua sexta larvae ate far more of the provided tobacco and tomato plant leaves when given the opportunity than they did the laboratory-created food source or the provided radish leaves. This suggests that even the relatively simple digestive and sensory mechanisms of these insects can lead to clear non-nutrient-based preferences in food sources, raising questions for future research concerning the degree to which "taste" plays a role in insect food selection, and possibly food selection in all members of the animal kingdom.

The fact that radish leaves also appeared to be a negative preference to the Manduca sexta larvae is also of interest to the research at hand. A comparison simply between the natural preferred food sources of the species (i.e. The leaves of the tobacco and tomato plants) and the laboratory-created food source could suggest a variety of reasons of differences in preference, many of them involving the artificiality of the laboratory-created food. The inclusion of radish plant leaves in the study as a fourth alternative food source makes it clear that the artificiality of the laboratory created food was not the primary cause of the negative preference, if indeed it was a cause at all. Instead, the Manduca sexta showed a clear preference for tobacco and tomato leaves even amongst non-artificial alternatives.

This does note bode well for the more practical aims of this research experiment, which had hoped to determine methods for exerting control of the consumption of agricultural products (i.e. tobacco and tomato plants) by the Manduca sexta larvae. Though a laboratory-created food source provides better nutritive value to the caterpillars, they generally will not eat it when their preferred food source is available, meaning that this research has not shown a viable method for deterring the destruction of tobacco and tomato plants via the eating of their leaves by the Manduca sexta larva. Had the increased nutritive value of the laboratory-created food also proven to be a source of positive preference for the larva, further research might have provided… [END OF PREVIEW]

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