Cats and Rats
For the Mint
But what's in it for the catnip plant, a member of the mint family? Surely Nepeta cataria did not evolve its signature compound to intoxicate cats. It does not need them for pollination or propagation. Might another creature be the intended target for nepetalactone? Tom Eisner asked this question almost 40 years ago. In a paper so clear and free of jargon (typical of Eisner's work; see Macroscope) that I can read every word of it with my first-year chemistry class, he describes his findings. Here is a piece of it:
This possibility [that catnip may be a defensive substance, protecting the plant against phytophagous insects] was investigated by a series of simple experiments.
One of these consisted in observing the response of a variety of insects to the vapors emanating from the tip of a fine capillary tube filled with pure liquid nepetalactone, and pointed to their bodies from a few millimeters away. The insects tested (Table 1) were a mixed assortment that had come to rest at night on an illuminated surface. The majority (part A) showed a distinct avoidance response, which varied somewhat with the particular species. The caddis-flies flew away. The alleculid beetles fell to the ground (as do many beetles when disturbed). The remainder simply turned away from the capillary and walked off.
So nepetalactone is an insect repellant. This might be of evolutionary advantage, part of a web of defense that the catnip plant uses to protect itself from herbivorous predators. As my friend Haruko Kazama at the International Christian University in Tokyo says, "Plants are sessile not because they are primitive and cannot move, but because they have such exquisite mechanisms to sense and respond to their environment. They do not need to run away...."
Remarkably, nepetalactone next turned up in an insect, the walking stick. Both life forms are trying to avoid being eaten, and of course they are not alone in this desire. Several plants and animals use other, similar molecules as insect repellants. Nepetalactone is reported to be stronger (by weight) than DEET as a mosquito repellant. And, in a bizarre twist of its usual role, the compound is made (along with related molecules) by female aphids to attract males. Unfortunately, novel use can be risky: Parasitic wasps that prey on the aphids can also sense the molecule. This has led to commercial production of nepetalactone to control aphid infestations via wasp recruitment.