Introduction
Manduca sexta, commonly known as the tobacco hornworm, is a fascinating insect species that has garnered attention for its pest status, its role as a model organism in scientific research, and its complex feeding behavior. The larval stages of Manduca sexta are known for their voracious appetite, primarily feeding on plants in the family Solanaceae. However, their dietary habits and nutritional requirements are more nuanced than a simple designation as a specialist feeder might suggest. This article delves into the diet and nutrition of Manduca sexta, exploring the factors influencing their food preferences, nutritional needs, and the implications of their feeding habits.
Manduca sexta: An Overview
The tobacco hornworm, Manduca sexta (L.), is a common pest of plants in the family Solanaceae, which includes tobacco, tomato, pepper, eggplant, and various ornamentals and weeds. Caterpillars in the family Sphingidae are known as hornworms, due to their worm-like body shape and the presence of a small, pointed "horn" at their posterior. The adult stage of Manduca sexta is a heavy-bodied moth that resembles a hummingbird, and Manduca adults are commonly referred to as hawkmoths or hummingbird moths. This species may be confused with the tomato hornworm, Manduca quinquemaculata (Haworth), a closely related species that also preferentially feeds on solanaceous plants. These species can be distinguished from one another by comparing the markings on the body of larvae and on the abdomen of the adults.
In addition to its pest status, Manduca sexta is an important model organism in the field of entomology, particularly insect physiology. Manduca sexta has been used for a series of important studies that contributed to the understanding of insect endocrinology and development. Specifically, this species was used to investigate the interactions between endogenous hormones and environmental cues that signal development through multiple instars and the onset of pupation.
Distribution and Life Cycle
The tobacco hornworm is found throughout the United States (north to the southern portion of Canada), Central America, and the Caribbean. The eggs of Manduca sexta are deposited on the leaves of the host plant and hatch one to three days after oviposition. The common name tobacco hornworm refers to the larval stages of Manduca sexta; the caterpillars are robust and bright green, with white, diagonal striped markings and a small protrusion (the "horn" in hornworm) on the last abdominal segment. Manduca sexta larvae undergo four or five instars, gradually increasing in size to about 8 cm (~3 in) in length in the final instar. The tobacco hornworm looks very similar to tomato hornworm, Manduca quinquemaculata, and their range and host plants can overlap. Body markings and horn coloration can be used to distinguish between the two species.
At the end of the final larval instar, the hornworm enters what is considered the prepupal stage. This stage is characterized by wandering behavior and selection of a pupation site, followed by the formation of the pupal cell below the leaf litter or soil substrate. Once the pupal cell is excavated, the prepupal stage transitions into the pupal stage as the insect's cuticle hardens and darkens, forming the pupa. The pupa of Manduca sexta is a dark, reddish-brown color with a maxillary loop at the anterior end and a pointed posterior end. Depending on the time of year and number of generations (two is typical in most areas), overwintering may occur during this stage.
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The adult stage of Manduca sexta is a robust, agile moth known as the Carolina sphinx moth or the tobacco hawkmoth. Adults have a wingspan of 3.75 to 4.75 in (9.5 to 12 cm) for the forewings; hindwings are small in comparison. The wings have a mottled pattern of white, brown, and black with distinct light and dark bands on the hindwings. When at rest, the wings fold back giving the moth a triangular shape and providing camouflage (especially when resting on trees with lichens). The Carolina sphinx moth has six pairs of yellowish-orange spots (five large pairs of spots, with the posterior-most pair comparatively small) arranged vertically on the grayish-brown abdomen.
The adult moths are also referred to as hummingbird moths due to their tendency to fly nimbly between flowers, hovering over each to extract nectar with their long proboscis. Some moths in the Sphingidae family are considered to be beneficial pollinators, an interesting ecological role in contrast to the destructive nature of the larvae. After eclosing (emerging as an adult from the pupal stage), the moths are crepuscular (active at dawn and dusk) and obtain nectar from a variety of flowering plants.
Dietary Specialization and Host Plant Selection
The tobacco hornworm is often described as a specialist feeder, primarily consuming plants from the Solanaceae family. These plants contain steroidal and triterpenoid glycosides, chemical compounds that play an important role in the biology of Manduca sexta. One steroidal glycoside in particular, indioside D, was observed to induce feeding preference in native larvae, causing these individuals to specialize solely on solanaceous foliage. Female moths select plants for oviposition based on chemical cues, such as odor, detected via their antennae. Studies suggest that female moths will not oviposit on plant hosts that have already been infested and exhibit feeding damage, likely detecting a blend of plant volatiles released by the plant in response to the feeding.
Larval host plants include Datura wrightii (jimsonweed), Nicotiana attenuata (wild tobacco), Proboscidea parviflora (devil's claw), Lycopersicon esculentum (tomato), Capsicum annuum (bell pepper), and Solanum tuberosum (potato). They have been recorded on other vegetables such as eggplant, pepper, and potato, but this is rare. Several Solanum spp. weeds are reported to serve as hosts.
Feeding Behavior and Nutritional Requirements
The larval stages of Manduca sexta are voracious feeders. Larvae feed by consuming the leaves of solanaceous plants, often stripping entire leaves to the midrib and causing complete defoliating of the plants. Though considered a common garden pest, tobacco hornworm can cause significant economic damage to tobacco crops and occasionally tomato and potato crops.
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Manduca sexta L. larvae exhibit broad food acceptance with regard to nutrient content during the first 3 days of the last stadium. Larvae fed diets with a constant combined level of casein and sucrose, but variable ratios, display a linear relationship between protein and carbohydrate intake. Larvae grow best on a diet with equal nutrients, but will consume an excess of one nutrient in order to obtain an adequate amount of the other, as nutrient ratio shifts. Parasitized larvae feed similarly, but the nutrient ratio does not affect growth.
Unparasitized larvae regulate intake of protein and carbohydrate when offered choices of protein-biased and carbohydrate-biased diets having combined nutrient levels of 120 g/l, but with variable ratios. Larvae normally consume equal amounts of nutrients, regardless of ratio, and grow similarly. As combined nutrient level is reduced in one diet, larvae abandon regulation and feed randomly. Parasitized larvae offered choice diets with 120 g/l combined nutrients do not regulate nutrient intake. Consumption of nutrients varies widely, but growth is unaffected.
Innate vs. Learned Food Preferences
Food preference can drive an organism to seek a specific food source even if another food, which can provide needed nutrition, is easier to obtain. Food preference can develop in different ways, including innate preference, i.e., organisms display food preference at birth, or learned preference, i.e., organisms develop food preference after previous experience with that food. The tobacco hornworm, Manduca sexta, shows learned preference; however, we lack information about innate food preference in Manduca. Manduca sexta larvae eat many different foods until they feed on a solanaceous plant, when they become more specific in their diet.
One study focused on naive Manduca sexta larvae to determine if they show innate food preferences. Because early juveniles seek out solanaceous plants if they hatch on a different family of plants, it was predicted that Manduca sexta prefer solanaceous plants over artificial food designed for captive animals, and that Manduca prefer plants that have not been fed upon previously. To test these predictions, Manduca sexta were allowed to choose from live plants with damaged leaves (to simulate previous feeding by other insects), live plants with undamaged leaves, or lab food. It was found that there is no significant difference between latency time, or between the plant and lab food trials.
The Influence of Rearing Diet on Food Acceptance
Larval Manduca sexta has a natural affinity for plants in the family Solanaceae. It has been previously demonstrated that this preference for specific host plants must be induced by prior feeding experience on those plants. However, it is not yet clear whether effects of these behavioral modifications are isolated to the rearing plant or extend to other closely related plants.
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Cohorts of larvae were reared on a single solanaceous plant (tobacco) or one of several non-solanaceous plants to the 5th instar, and later tested in an isolated, no-choice experiment. Survivability of the larvae (from 1st to 5th instar) in each cohort was recorded. To determine the palatability of a specific plant, data was gathered on how much leaf area the caterpillars consumed at hourly intervals. This information was then graphed as a percentage of the leaf remaining over time which was used to determine the rate of feeding.
Experimental results showed that there was an overall higher larval acceptance for tobacco regardless of their rearing history. The results obtained for larvae feeding on non-solanaceous plants were considerably more varied. Cabbage, for example, appeared surprisingly palatable across all rearing backgrounds (despite extremely low survivability on cabbage). Consumption of rapeseed and turnip varied more depending on diet history. Survivability was highest on tobacco, moderate on rapeseed and turnip and low in cabbage and soybean, where only a single larva for each survived to the fifth instar. The survivorship data demonstrates that the nutritional value and general suitability of non-solanaceous plants varied greatly. The consumption data suggests that while the acceptance of tobacco was universally high and inflexible, the acceptance of non-solanaceous plants (even within the same family) were much more heterogeneous, with varying degrees of flexibility. Increases in the palatability of tobacco were therefore not observed. In contrast, some evidence of both inductions for the rearing plant and for other related plants were observed within the non-solanaceous plants tested.
Oligophagy and the Role of Chemical Constituents
Manduca sexta is an insect that in nature primarily feeds on plants within the family Solanaceae. The family of plants preferred by the Tobacco Hornworm includes well-known plants like tomato, potato, tobacco, and other nightshades. The tobacco hornworm is known to eat certain plants outside of the solanaceous family. Some previous studies have shown that tobacco hornworms are willing to feed on, and can even be raised on, non-solanaceous plants. This raises the question as to whether these larvae are truly oligophagous. Nearly every behavioral study shows that the oligophagous nature of the tobacco hornworms can be influenced by the plant they were reared on.
One study wanted to determine if Tobacco Hornworms were inherently oligophagous insects, as seen in nature, or if they were induced to oligophagy based on the plants they were reared on. To test this, an experiment was conducted under no-choice conditions: the insects were feed only one type of plant until they were first instar larvae. This included an array of solanaceous and non-solanaceous plants. The test concluded that first instar larvae ate a variety of non-solanaceous plants within 24 hours showing polyphagous characteristics. Thus, behavioral patterns, even when increasing the variety of plants, is still consistent with results in which acceptable non-solanaceous reared larvae exhibit a loss of oligophagy that solanaceous reared larvae have and even induce a preference towards certain acceptable non-solanaceous plants.
It has been suggested that the host (solanaceous) preferability could be from the larvae becoming dependent on the chemical constituents of the solanaceous plants. Therefore, to induce a host specificity, the Tobacco Hornworms must become dependent on solanaceous chemical extracts. To directly investigate the role of chemical constituents of the solanaceous plants on the induction of influence preferences, larvae were given a choice of a non-solanaceous disk (cowpea) treated with extract from a solanaceous plant (potato) or cowpea disk alone. The no-choice test showed that larvae reared on a solanaceous diet accepted any solanaceous foliage offered but rejected non-solanaceous foliage (cowpea). In contrast, larvae reared on a non-solanaceous diet equally accepted solanaceous and non-solanaceous foliage . This further reinforced the idea that rearing influenced the strength of preferability because rearing on non-solanaceous foliage decreased the preferability for solanaceous foliage. The choice test showed that a much higher percentage of larvae reared on the solanaceous diet took their first bite on the cowpea disc treated with potato extracts than the control (cowpea). However, larvae reared on the non-solanaceous diet did not discriminate between the control disk and the cowpea treated with potato extract disk. Also, the percentage of larvae taking their first bite of a potato disk with cowpea extract was relatively the same regardless of their reared diet being solanaceous or non-solanaceous.
Laboratory Rearing and Artificial Diets
In the lab they can be successfully reared on artificial diet. Manduca sexta larvae reared under laboratory settings will not feed exclusively on solanaceous plants, accepting plant tissue from a variety of other plants in different plant families. However, caterpillars reared initially on a solanaceous diet were significantly less likely to feed on non-solanaceous leaves, even when no other food option was presented. Additional studies suggest that olfactory cues are supplemented by gustatory (taste-based) cues in Manduca sexta larvae, and that feeding preference is largely a function of early feeding experiences.
A professional agar diet is available for rearing Manduca sexta, which is quick and easy to prepare. The diet contains high density nutrients and provides high feed conversion. The diet was specially formulated for Manduca sexta, and produces teal colored worms.
Management of Manduca sexta as a Pest
The larval stages of Manduca sexta are voracious feeders. Larvae feed by consuming the leaves of solanaceous plants, often stripping entire leaves to the midrib and causing complete defoliating of the plants. Though considered a common garden pest, tobacco hornworm can cause significant economic damage to tobacco crops and occasionally tomato and potato crops.
Tobacco hornworms can be controlled in various ways and immediate management is recommended if this pest is detected in a garden or field setting. In smaller operations, like a home garden, hand picking and destruction of the caterpillars is an effective way to reduce the population. This method of cultural control requires regular scouting of solanaceous plant species, looking for signs of feeding damage.
Biological control is another method of management, using Bacillus thuringiensis (Bt), a naturally occurring soil bacterium that produces a protein that acts as a fatal endotoxin when consumed by Lepidopteran larvae (and the larvae of some other insect taxa). This product is widely available and safe for use around pollinating insects because it has to be consumed in order to be effective. Additionally, Bt is suitable for use in organic growing operations.
Tobacco hornworms have several natural enemies, including vertebrate species that feed on caterpillars, such as birds and small mammals, and insects like lacewing and lady beetle larvae that consume the eggs and early instar larvae. Wasps are a common predator of hornworms. Paper wasps and other insects that provision prey for their young will take hornworms from the host plant, paralyze them, and place them into the nest cells containing the wasp's eggs as a future food source. Parasitoid wasps, like Cotesia congregata, use hornworms as a food source for their developing young. These wasps deposit their eggs inside the hornworm's body and the larval wasps develop within the caterpillar, feeding on it as they progress through their life cycle. When pupation takes place, the immature wasps spin small, white, silken cocoons that protrude from the body of the still-living caterpillar.