Tomatoes infuse their leaves with toxins to turn insects against each other
Tomato plants do not want to be eaten. The 31 pounds of tomatoes each American gobbles per year is fine, because that helps with seed dispersal and gives us a reason to plant more tomatoes. The problem for the plants is that too many bugs bypass the red fruit and eat the leaves of the plant, leaving it with no way to produce its own food through photosynthesis. To defend themselves, the plants have found a way to control the bugs’ appetites and populations— they get the bugs to eat each other.
This pest-control concept is actually based on normal behavior in various pest herbivorous insects, like mottled willow moth caterpillars (Spodoptera exigua). When these bugs can’t get enough nutritious food, they don’t really have the means to travel to find something better, as they’re trying their best to hoard calories in preparation for metamorphosis. So when the leaves are scarce, or just low enough quality, the insects will start eating each other instead as the last local source of nutrients and calories.
Turning up the toxins
Tomato plants (Solanum lycopersicum) have evolved to exploit this quirk of pest ecology. Tomatoes in danger can start producing extra toxins in their leaves that make them less nutritious to eat. Manipulating leaf-quality like this then convinces caterpillars it’s time to switch to cannibalism. In experiments, caterpillars offered more toxic leaves started munching caterpillar corpses much sooner than their peers. From the tomato plant’s perspective, adjusting the chemistry of leaves may be energetically costly, so they don’t make their leaves less attractive all the time. When circumstances demand it, this strategy does work well enough to make a measurable difference in just how much each plant gets eaten.
The last layer of this defensive strategy is that a tomato plant doesn’t need to get bitten to start raise its defenses. Like a variety of other plants, tomato plants can warn each other about the arrival of herbivores. They emit a compound called methyl jasmonate (MeJA) that can be detected by nearby plants, giving them a chance to start toxifying their leaves before the insects begin their buffet. There is some interest in manipulating this warning system, since presumably farmers could release MeJA to warn crops whenever they wanted. However, it might be best to follow the tomato plants’ lead on this, since constant warnings and toxic leaves could stress the plants while selecting for only the hardiest, toughest insects around.
Source: Plants turn caterpillars into cannibals by Laura Castells, Nature