Caterpillars balance picky eating, violent thrashing to avoid birds, ants

A gang of caterpillars descended on my potted dillweed and turned it into dill stallks. If any birds got hold of one of these caterpillars, I bet they were tasty. (Tinkerbrad/The Daily Campus)

Biologists tend to love the caterpillar. This common Connecticut resident is helping researchers, including those at University of Connecticut, understand food webs in our forests and our worldwide ecosystems.

Among hundreds of caterpillar species examined worldwide, scientists have found that nearly 70 percent of them are dietary specialists – eating only one type of plant – with the remaining 30 percent being dietary generalists – eating many kinds of plants.

Researchers at the UConn collect and rear caterpillars in the wild to better understand the feeding behaviors and relationships of different species in nature. Other researchers are using this information to understand broader patterns that hold ecosystems together.   

For biologists studying caterpillars, there isn’t yet a good explanation as to why so many caterpillars specialize, but evidence suggests that specialization could be a consequence of their place in food webs.

Research from Mike Singer’s laboratory at Wesleyan University and his colleagues shows that caterpillars eating only one species of plant tend to be eaten by birds less often than caterpillars that eat a wide variety of plants.

For a caterpillar, specialization opens doors for the evolution of various traits that can help the insect avoid being eaten by birds. They can store toxic compounds they acquire from their host plant so that birds eating them think twice before going for a second helping. They can also camouflage with their host plant so well that birds wouldn’t be able to see them.

However, both toxicity and camouflage are difficult to acquire if a caterpillar is feeding on many different kinds of plants. Specialization also enables a caterpillar to overcome the many chemical defenses plants use to prevent being eaten by caterpillars – it’s hard to evolve tolerance to many different toxic chemicals, but much easier to evolve tolerance to just one.

Generalists aren’t so lucky in being able to dodge these predatory birds or toxic plants, but why do they exist if specialization seems to be the favored life style? The answer: Ants.

Singer and colleagues are finding that ants may have a bigger role to play in the caterpillar food web than previously thought. While birds tend to eat generalist caterpillars, ants go for the specialists.

Generalist caterpillars are fussy. They will thrash violently and move quickly if an ant gets too close, fending off any ant that dares to approach. While it may be futile to thrash and run to avoid a bird, generalists have a leg up on specialists because they can avoid ants. The behavior of specialist caterpillars is quite different because they tend to sit still when predatory ants approach.  

This is just one piece of the puzzle of why so many caterpillars specialize, but ants are bringing biologists closer to answering the larger question of what maintains such differences in animals worldwide.

Within the greater food web, caterpillars aren’t just attacked by birds and ants. An extremely diverse and abundant group of wasps called the parasitoids are also constantly attacking caterpillars by injecting their eggs into or on the caterpillar body. These eggs eventually eat and kill the helpless caterpillar, hatching into a new parasitoid ready to find another victim.

Research on how different parts of an ecosystem – mammals, plants, parasites and herbivores – interact allow biologists to reconstruct food webs and make predictions about how humans will affect them in the future.

Habitat destruction and climate change are just two examples of human influence on the environment. In the face of these threats, scientists will need information about these complex food webs in order to save imperiled ecosystems.  


Diler Haji is a campus correspondent for The Daily Campus. He can be reached via email at diler.haji@uconn.edu.