While there’s a famous horror-movie spoof about killer tomatoes, no one seems to have made one about caterpillars — the insect pests that eat the juicy red fruits of summer.
Perhaps the time is ripe, with inspiration from a new study at Pennsylvania State University.
Scientists found that a caterpillar called the tomato fruit worm not only chomps on tomatoes and their leaves, but also deposits enzyme-laden saliva on the plant, interfering with its ability to cry for help.
If it all sounds a bit improbable, starting with the concept of plants crying for help, scientists also scoffed at that idea when it was first proposed a few decades ago. But it has been shown time and time again that when under attack, plants can emit chemical distress signals, causing their peers to mount some sort of defense. A classic example is the smell of a freshly mown lawn, which prompts the release of protective compounds in nearby blades of grass that have yet to be cut.
In some cases, plant distress signals can even summon help from other species. That’s what happens with the tomato. When caterpillars nibble on the plant’s leaves, the leaf pores release volatile chemicals that are detected by a type of parasite: a wasp that lays eggs inside caterpillars. (Not to overwork the horror-movie analogy, but as with the hapless astronauts in the "Aliens" franchise, it doesn’t end well for the caterpillar.)
So the Penn State researchers wondered: Could the caterpillars strike back? In a series of experiments, they found that the answer was yes, illustrating a previously unknown strategy in nature’s age-old contest between predator and prey.
The key is the enzyme in the caterpillar’s saliva, which inhibits the opening of pores in tomato-plant leaves, said Po-An Lin, the study’s lead author. That means the leaves are less able to release the distress signals, and presumably less able to attract wasps that could come to the rescue, he said.
The researchers did not use actual wasps in the study, as the insects’ ability to detect these signals is well established. But the team measured the diameter of the leaf pores and the levels of distress signals that were emitted after being exposed to caterpillar saliva.
The scientists did so both for caterpillars whose saliva contained the enzyme and for those that did not, as a result of having their genes “edited” using the popular laboratory technique called CRISPR. Sure enough, the enzyme-deficient saliva failed to silence the plant’s distress call.
That made for a convincing case, said Chris Martine, a Bucknell University plant geneticist who was not involved in the study.