The very hungry, angry caterpillars

In the absence of milkweed--their favorite food--monarch butterfly caterpillars (Danaus plexippus) go from peaceful feeders to aggressive fighters. Researchers reporting in the journal iScience on November 19 observed that caterpillars with less access to food were more likely to lunge at others to knock them aside, and caterpillars were most aggressive during the final stages before metamorphosis.

"I saw that there were basically no published papers on aggression in this or any other caterpillar species, but there's a lot of exciting work that's been done on fruit flies where they've found single-pheromone receptors or single genes that trigger aggression," says lead author Alex Keene, a professor of biological sciences at Florida Atlantic University. "Now we might be able to take that powerful neurobiology and genetics and study it in a more ecologically relevant organism."

While Keene usually studies neurobiology using fruit flies, he was inspired to investigate monarchs after observing their combative behavior in his own garden. Because they are large and recognizable compared to many other insects, monarchs are a useful indicator of the status of pollinator populations in an ecosystem. If monarchs are declining, other pollinators are likely to be declining as well.

Monarchs also impact the milkweed plants they consume--at their largest and hungriest phase, a single caterpillar may eat an entire milkweed leaf in under 5 minutes. "If you compare that to a fruit fly where there are lot of larvae on one piece of rotting fruit, there is less competition there," says Keene. "But each of these caterpillars will at some point in their developmental cycle encounter resource limitation."

To model this resource limitation, the researchers faced the challenge of maintaining a population of monarchs. Inspired by his own butterfly garden at home, Keene and his team built an open milkweed garden behind their Boca Raton-based lab and let nature do the work of collecting caterpillars. Back in the lab, the researchers placed caterpillars into groups with different amounts of milkweed. The results were clear--the less food, the more likely caterpillars were to try to headbutt each other out of the way to get their fill. But the process of getting to that result was anything but simple.

"We definitely had a lot of challenges. We had a hard time breeding the monarchs in the lab, and we found that almost every nursery sells their milkweed with pesticides. So, we ended up having to grow our own," Keen says. "But I like to say that resilience is one of the main characteristics scientists have to have because most of what we do doesn't work."

While this research showed that the caterpillars respond aggressively to limited food, the researchers still hope to learn more about what drives this response in their brains, which is important for learning more about how these responses work outside the lab.

"One of the fundamental problems with work like this is that we're testing animals in a very derived setting. And that's not what brains evolved to do," Keene says. "So now that we have this invertebrate model in a relatively controlled setting, but doing an ecologically relevant behavior, that becomes important in terms of looking at the mechanism and function of this behavior in more complex organisms."

###

The authors were supported by the National Science Foundation.

iScience, Keene et al.: "Aggression is induced by resource limitation in the monarch caterpillar" https://www.cell.com/iscience/fulltext/S2589-0042(20)30988-3

iScience (@iScience_CP) is an open-access journal from Cell Press that provides a platform for original research and interdisciplinary thinking in the life, physical, and earth sciences. The primary criterion for publication in iScience is a significant contribution to a relevant field combined with robust results and underlying methodology. Visit: http://www.cell.com/iscience. To receive Cell Press media alerts, contact press@cell.com.

---

---