Biology in the News Explained

Cool Bugs #7 – Pipevine swallowtail

The pipevine swallowtail is Battus philenor. In the U.S., it occurs roughly in the southern half of the country, east to west.
The caterpillars of B. philenor (below) feed exclusively on plants in the genus Aristolochia (pipevine). These plants are loaded with toxic compounds called aristolochic acids, which would kill, and thus deter, most herbivores. The pipevine swallowtail, however, is not harmed by aristolochic acids, and instead it sequesters them in its own body to use as a defense against predators. These butterflies are a classic example of aposematism, which means they advertise the fact that they taste bad to predators.
If an insect is black, red, yellow, orange, or any combination of those colors, it is likely to be distasteful. If an insect is green or brown and seems to blend into foliage, it likely tastes good to potential predators, and because of that it is hiding. Aposematic insects do not want to hide, they want to make it clear to the predators out there that they are no good to eat. This is because they are relying on learning by those predators (in the case of butterflies, often birds) learning to associate those colors with bad food. Another better known distasteful butterfly is the monarch, which feeds on milkweed, another plant with nasty chemicals.

Some butterflies which have aposematic coloring do not sequester nasty chemicals. Instead, they use a strategy of mimicry, and rely on the likelihood that predators will mistake them for bad food and avoid them as well. This of course only works if most of the aposematic butterflies do actually taste bad, because if a bird eats a black butterfly and it tastes good, it will not learn to avoid black butterflies, but to eat them. So generally in a population there is a stable balance of truly distasteful butterflies and mimics.
The caterpillars of B. philenor can either be black or red, and this is entirely due to the temperature at which they develop (Nice and Fordyce, 2006). When the temperature is over 30°C, A black caterpillar will overheat, so they become red instead, which keeps them cooler (black absorbs sunlight, and thus heat, much more readily than red). Interestingly, aposematism in B. philenor caterpillars seems to serve a dual function: in addition to deterring predators, the contrasting black or red color also deters a B. philenor adult female from laying more eggs on the same plant already occupied by larvae of the same species (Papaj and Newsom, 2005). This ensures that her offspring will have adequate food left for development.

Adult females lay their eggs preferentially on young foliage. This is probably because younger leaves are more tender and easy to eat by early stage caterpillars. Females determine the suitability of the foliage via chemical receptors (taste buds) on their feet. I established this in an unpublished study in which I stimulated oviposition by females on filter paper using organic extracts of young vs. old Aristolochia foliage (above); they much preferred to oviposit on extracts from young foliage. High pressure liquid chromatographic analysis revealed higher levels of several sugar alcohols in the younger foliage, so the butterflies may use that information to choose oviposition sites. At the time, however, we were unable to measure levels of aristolochic acids in young vs. old foliage, so that may also be a cue instead of or in addition to sugar alcohol levels.

These butterflies can be reared in the laboratory, but not easily. Getting butterflies to mate in a lab is challenging, but B. philenor was often quite accommodating. One could manipulate the genitalia of a male and female into contact, and sometimes get them to hold on and complete the mating (and thus get fertilized eggs in the female). Interestingly, what mattered in lab mating success often was the individual male – certain males could not be induced to mate, while with others we had success with several females.

These insects have been of interest to scientists not only because of their chemical relationship with their host plant, but because of their behavior as well. Many people assume that insects behave only according to instinct, but in fact many species have shown quite good learning ability. Parasitic wasps are often studied for their odor and sometimes visual learning skills, and butterflies as well are good learners. Mainly visual cues, e.g. color and shape, have been shown to be learned by B. philenor. For example, in parts of their range there are multiple species of Aristolochia upon which they feed, and females learn the leaf shape of the dominant species (Papaj 1986). This saves time for females searching for host plants, because ovipositing (egg laying) females can visually scan for potential host plants, and then test leaves of the correct shape for the compounds in Aristolochia, after which they confirm or reject it as a host plant.

A female B. philenor can also simultaneously learn one color associated with egg laying, and another color associated with nectar sources for food (Weiss and Papaj 2003). Similar ability has been found in some parasitic wasps. It actually should not be surprising that insects are good at learning. If your brain is tiny, you have fewer neurons to hardwire different behaviors, so it pays to be flexible anyway.

References

Nice, C.C. & Fordyce, J.A. (2006) How caterpillars avoid overheating: behavioral and phenotypic plasticity of pipevine swallowtail larvae. Oecologia, 146, 541-548.

Papaj, D.R. (1986) Conditioning of leaf-shape discrimination by chemical cues in the butterfly, Battus philenor. Animal Behaviour, 34, 1281-1288.

Papaj, D.R. & Newsom, G.M. (2005) A within-species warning function for an aposematic signal. Proceedings of the Royal Society B-Biological Sciences, 272, 2519-2523.

Weiss, M.R. & Papaj, D.R. (2003) Colour learning in two behavioural contexts: how much can a butterfly keep in mind? Animal Behaviour, 65, 425-434.

Share
tabs-top

Leave a Reply

Your email address will not be published. Required fields are marked *


× three = 18

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>