Caterpillar chemistry

Vismia baccifera

The tropical plant Vismia baccifera protects itself by producing a number of repellent chemicals, including three compounds that are toxic to living cells. Few plant-eating insects can stomach such a cocktail, but for those that can, the advantages are clear - less competition for a meal, and a chemical toolkit they can use in their own defense.

One of them is the skipper butterfly Pyrrhopyge thericles. Their caterpillars only eat plants of the genus Vismia. In contrast the caterpillars of the large saturniid, Periphoba arcaei, have a much broader diet, including Vismia plants and many others. The caterpillars of both species are brightly coloured, one with conspicuous stripes, and the other blue-green with bristles. For potential predators such flashy looks are associated with toxicity. Now it would seem that the skipper butterfly that exclusively consumed plants containing toxic chemicals would more easily incorporate toxins into its body than the one with a broad diet. 

A new study by the Smithsonian Tropical Research Institute (STRI) in Panama found the opposite. They compared the diets of the caterpillars of the two species at several life stages and tested for the presence and concentration of plant toxins called vismiones. While two vismione compounds are found at a ratio of 1:6 in the plants, in the specialist butterfly caterpillars the compounds were barely detectable, and at roughly equal ratios. Meanwhile, the generalist moth caterpillars contained significant quantities of the rarer of the two compounds, suggesting that they were able to actively store this plant chemical in their own bodies. Both caterpillars' fecal matter revealed a 1:2 ratio of the plant compounds, indicating that their bodies might uptake compounds selectively or convert molecules of one type over the other.

We know very little about just how each plant-eating insect handles these chemicals--how they store them or eliminate them. Some insects might isolate the compounds so they do not cause them harm, while others might convert the molecules into forms that are harder to detect. Insects that process harmful toxins without damaging their own cells have a survival advantage. For a generalist species, the ability to sequester toxic compounds might be an early evolutionary breakthrough, the first step along the pathway to becoming a toxic plant specialist.