One of my fond memories as a little kid in the 1950’s was joining my dear Italian grandmother on summer afternoons as she dutifully watered her beloved patch of zinnias. One time, Grandma pointed out on the sidewalk a double line of ants busily marching back and forth with great determination, some bearing cargo, never deviating from the neat straight lines. She said, look Raymie, you would have a very hard time getting a group of people to do that so neatly, and look at all those ants with their tiny little brains doing just fine!
That’s just one example of insect behavior that was a mystery at the time. Then in 1959, two German biochemists coined the word “pheromone” to describe natural chemicals involved somehow in signaling. The field took off like a rocket in the next two decades as it was shown that communication by means of specific compounds, usually small organic molecules, is key to the survival of a great variety of creatures. As analytical methods became exquisitely sensitive, minuscule amounts of these compounds could be identified and their structures verified by chemical synthesis. Those ants were following a trail pheromone. Poke a fire ant mound and thousands of ants pour out for battle, mobilized by an alarm pheromone.
In the vernacular, a pheromone is a scent that attracts a mate. Indeed, this is a fascinating area of pheromone science and the one that brings us to the lovely Gregg’s mistflower, Conoclinium (formerly Eupatorium) greggii, a drought-tolerant native that flowers through early autumn and is a favorite source of nectar for monarchs and queen butterflies.
Gregg’s mistflower produces a natural compound called intermedine, which is a pyrrolizidine alkaloid (PA for short). PA’s occur in many plants and are well known to ranchers, being very poisonous to livestock (and humans) as they serve to protect the plants from grazing. However, it turns out that intermedine isn’t poisonous to queen butterflies, but is essential to their reproduction. When you see queens nectaring on Gregg’s mistflower, over 90% of them are males happily imbibing intermedine with the nectar. Then they convert part of the intermedine to a smaller molecule named danaidone which is a sex attractant pheromone that draws in the females. During mating, the male queen passes the remaining unchanged intermedine to the female as a “nuptial gift” that once again manifests itself as a toxin, this time rendering her eggs unpalatable to predators! Thus as the butterfly pollinates the flower, the flower provides a molecule that in two ways enables the butterfly to reproduce!
The science of nuptial gifting in insects was worked out largely by two Cornell University professors, biologist Thomas Eisner and chemist Jerrold Meinwald. The two were good friends who founded the field of “chemical ecology” based on research such as this. I was privileged to meet Jerry Meinwald at a natural products conference some 15 years ago. He gave a great talk, was outgoing and funny, and played the flute beautifully. On receiving the National Medal of Science in 2014, he remarked that his career was “built in large part on collaboration,” much like the natural collaboration between mistflower and butterfly.