Chemical Warfare in the Garden
Posted by Richard Conniff on May 18, 2014
I love cabbage and wish I could grow it in my garden. But every time I try, the leaves end up peppered with holes. Flea beetles—little leaping insects, less than a 10th of an inch in length—are the culprits. Meanwhile, their larvae are doing terrible things to the roots. I am, however, both too organically inclined to attack them with pesticides and too lazy or distracted to do it with organic methods.
It turns out that flea beetles are even trickier than I knew, according to a new study in Proceedings of the National Academy of Sciences. Cabbages have devised what ought to be a perfectly adequate defense. Their tissues contain compounds called glucosinolates, which leak out when the plant is damaged—for instance, by an insect bite. The leaking fluid comes into contact with a plant enzyme called myrosinase, normally stored separately. The combination of the two components triggers the “mustard oil bomb.” For us, that’s just part of the taste of cabbage. For beetles, it’s
a toxic and foul-smelling chemical defense.
Yet when the beetles should by all rights be fleeing in horror or falling down dead, they say, in effect, “Really? Is that all you got?” Then they resume eating. In fact, the mustard oil bomb causes male flea beetles to release a pheromone that attracts other flea beetles to the scene. The cabbage’s primary means of defense instead becomes a dinner bell for insect pests: “Yes, we’ll take mustard with that!”
It gets worse, according to a research team led by Franziska Bera at the Max Planck Institute for Chemical Ecology. Just in case the plant doesn’t release glucosinolates, the beetles have sequestered their own supply within their bodies. They also carry a stash of myrosinase, a bit like suicide bombers belted up for self-immolation. One possibility is that the beetles have adapted the mustard oil bomb as a defense against their own enemies, as some aphids do. But the researchers suggest that it may also amplify the male beetles’ “come and get it” signal. Either way, the beetles avoid what the new study calls “auto-intoxication.” Nothing dies except my cabbages.
This sort of chemical warfare arms race is common in the natural world, and apparently also in my garden. Tomatoes, for instance, have their own form of chemical defense. But when hungry larvae of Colorado potato beetles light into the leaves of a tomato plant, they inoculate the plant with bacterial pathogens, according to a study published last year. That tricks the tomato into thinking it’s been attacked by a pathogen rather than an herbivore. The poor confused tomato suppresses its chemical warfare against leaf eaters and ramps up its antibacterial defenses instead. Thus the beetle grows fat, the study reports, by “hijacking the defensive machinery of the host plant for its own benefit.”
I shouldn’t make it sound as though the plants are always on the losing side of this relationship. They’ve evolved a host of mechanical defenses, including thicker leaves, hairs, thorns, spines, and even sticky extrusions that can trap and hold insects, along with an arsenal of chemical weapons, including terpenoids, alkaloids, anthocyanins, phenols, and quinones, to kill or slow the growth of their leaf-eating tormentors. In fact, plants and insects have been living together, quarrelling, and one-upping one another’s weaponry for 350 million years.
Even if I were the oldest gardener on Earth, I would still be a newcomer in the garden. That should probably fill me with a sense of awe before the wonder and complexity of nature.
Mainly, though, I’m wondering if there will be anything left for me to eat.