Tortoise beetles can be quite bizarre, moving around leaves with their colorful and iridescent upper shields that resemble miniature turtle shells.
But as herbivores, leaf-eating tortoise beetles also occupy a unique place in the food web as one of the first line of organisms that converts plant energy into animal protein.
Beetles are the most diverse organisms on the planet –– one out of every four known animal species on Earth is classified as a beetle. About 30% of all known beetle species eat leaves as their primary food source.
Entomologists have spent a long time trying to understand what makes beetles so successful and diverse as an animal group, investigating their eating habits, ability to adapt and other features.
Perhaps one of the most interesting threads of research has focused on the shields that these organisms make and wear on their body for protection.
Caroline Chaboo, a researcher at the University of Nebraska-Lincoln, and her team have been studying how four major species of tortoise beetles build their shields in their native habitats of South Africa and Costa Rica. She and her team found that these shields were made of their old exoskeletons as well as another element: their own poop.
Building the shields
In all the species the team researched, they found that beetles start building their shields as young larvae. Even when they transform from larvae to pupae and then into an adult beetle, they still keep their shields and form a protective chamber during pupation.
“We know these shields protect these animals as a physical barrier,” Chaboo said.
The shields are also mobile, which means the insects can flick them, using them as weapons.
The way they created their shields also differed between species.
“In some species that live as groups, the group flicks [them] together. It multiplies the signal in a way. In a few experiments done with pedestrian predators — the other things living on the plants, like bugs and ants — we can see that [the shield] is a deterrent that reduces the number of enemy attacks. When the shields are removed, [naked larvae] are very quickly picked off and eaten,” she said.
In her previous research, Chaboo found that the behavior of making and wearing shields originated in beetles around the time they emerged on the outside, living on the surface of leaves and trees instead of inside.
“Coming out into the open means you have a completely different arena of predators and parasitoids,” she said.
According to Chaboo, the shield could have been formed as an umbrella, “reducing the impact of sunshine and rain and drying out on tender little bodies,” or a “nasty cheap cover” that would deter predators, who may think the leaf is blotched or damaged.
“With all evolutionary structures, we can’t really know the original ‘why,’ so it is not always easy to separate which reason came first from our array of hypotheses,” Chaboo said.
Living on plants and eating their leaves means these herbivorous beetles have been in an evolutionary arms race with plants. “The plants evolve [to develop] toxins or trichomes or hairs — anything to deter enemies,” Chaboo said. In response, the herbivores counter-evolve to overcome those obstacles.
“We know plants can even signal when they are attacked by an herbivore, [sending] out a chemical signal that calls enemies of that herbivore to the plant,” Chaboo said. “So, that overarching global relationship is millions of years old, and that’s the story of terrestrial life on earth.”
Additionally, the animal’s feces has its own ecosystem of users and decomposers. “There are surely going to be things in this fecal garden that are also a component of the defense,” she said. “If you think of this as a chemical barrier, it’s walking around in its own gaseous halo, so the microorganisms living in there are a component of that hemisphere.”
Thinking beyond the beetle
This is only the story of four subspecies of beetles on Earth, which means Chaboo’s research into beetle shields is only the tip of the iceberg given the sheer amount of beetle species on Earth. Being able to do this research on the species level, Chaboo said, would clarify further how beetle morphology has affected their evolution.
“We have identified that [shield building] appears to be a big driver of speciation in at least two big groups,” says Chaboo. “One group has about 3,000 species and one has about 6,000 species. But then in two other groups, [the behavior’s presence] is very minor.”
For Chaboo, one of the next steps would be to compare the morphology, behaviors and structures of these four subspecies in greater detail to understand what controls the general design and function of these shields, and what creates differences in shield building. But she also wants to understand why certain groups have greater numbers of beetles that build shields while others don’t.
But another thread of research that really interests Chaboo is looking at the ancestors of the tortoise beetle to find why they started living inside leaves in the first place and what made them live on the surface again.
What makes a species valuable?
While Chaboo’s research can help inform ongoing conservation efforts to save leaf-eating beetles, conservation can also often come down to how we value a certain species.
“Every species is unique and tells us something about how life on earth evolved, how it survives, and how it overcomes general problems of living in our environment,” Chaboo said.
Chaboo observed that studying these shield-building behaviors within beetles could teach pest management lessons. Some subspecies of these beetles have become a nuisance in the Philippines, attacking the leaves and fruits of a non-native but highly commodified fruit tree species.
But how can producers combat these beetles when their fecal shields make them impervious to insecticides? “Because I am studying the life cycle,” Chaboo said, “I can say, ‘If you want to tackle a life stage, it will be the larvae and the adults. They are more vulnerable.’”
But these beetles can also be beneficial by acting as biocontrols for certain invasive plant species, because they are picky about what they eat. “You [can] introduce these beetles as a weapon in your arsenal,” she says, “and that is much better than chemicals.”
Often, humans assess the importance of a species based on its impact on mankind’s own survival. But if we only focus on species that directly affect or advance our survival, Chaboo said, we miss the chance to learn key lessons in understanding how life evolves and survives on Earth.
“I am fascinated by the layers of life, and how micro we can get. We have not yet demonstrated the value of everything and we don’t know and we need to have living libraries of taxa,” she said. As we face a future of increasingly extreme environments, learning the unique ways that other species cope becomes even more essential.
“One tragedy in all of this is how we treat Indigenous communities,” Chaboo said. “They have ways of coping with the environment, and if we don’t value their knowledge, it is to our global loss. I want to see [species] conserved and preserved as a way to preserve pages in the book of evolution, how evolution ebbed and flowed, expanded and shrank.”
