A new study by Earthwatch has revealed a fatal flaw in the diet of choice for hungry caterpillars. Scientists have long thought that caterpillars eat plant toxins to protect themselves from predation. But surprising new findings show that although these toxins make them unpalatable to birds and spiders, they weaken the caterpillar's immune system, leaving them more vulnerable to their greatest enemy of all - wasps and flies (parasitoids).
Parasitoids lay their eggs inside the caterpillar and the growing larvae feast on its tissues, destroying it from within.
"This is not at all what I expected," said Earthwatch scientist Lee Dyer, co-author on the new study published in June in Ecology Letters and associate professor at the University of Nevada Reno. "The caterpillars may be protecting themselves from birds and spiders who shun this toxic diet, but by weakening their immune system they are defenceless against their greatest threat - parasitoids."
Angela Smilanich, lead author of the study and now a postdoctoral scholar at Wesleyan University, worked with volunteers from Earthwatch to implant glass beads, which mimick the parasitoids, into the bodies of caterpillars that had eaten various amounts of plant toxins, and then watched to see what happened. Those who ate the most toxins had the weakest immune response and couldn't protect themselves.
"A lot of caterpillar control efforts have employed parasitoids, with mixed results," said Smilanich, who performed the research while at Tulane University. "What this study shows is that we can do a better job of predicting if the parasitoid method of bio-control would work, if we look at the immune system of the caterpillar first."
This conclusion was further substantiated in another paper Smilanich and Dyer published in Ecology this month, based on a 15-year data set collected by Earthwatch volunteers in Costa Rica, that showed the caterpillar's immune system is its best defense against parasitoids.
"These results turn some commonly held assumptions about the caterpillar's relationship with its predators on their head," said Earthwatch Senior Research Director Nat Spring. "It's very exciting to see how Earthwatch volunteers have contributed to such a big step forward in this research by Angela, Lee and others."
By better understanding the basic ways in which the organisms within the ecosystem interact, scientists can speculate about the implications of climate change, which is the major focus of the Earthwatch project Climate Change and Caterpillars that Dyer and Smilanich lead in Costa Rica, Ecuador, New Orleans and Arizona.
Dyer led earlier studies that showed that extreme weather events associated with climate change (like strong hurricanes) would cause the parasitoids to decline, leading to more caterpillar outbreaks - much to the dismay of farmers and forestry workers worldwide for whom caterpillars cause great damage.
"But this new information means it is virtually impossible to make a generalisation about caterpillars and climate change," said Dyer, because models also suggest that with a changing climate plants would increase their toxicity. In light of the new study, that would mean that caterpillar populations would fall whereas the parasitoids would rise.
The Earthwatch study reveals surprising new findings.
Many caterpillars resemble the plants on which they feed.
Identifying caterpillars in Costa Rica.