Insect Birth Control

The identification and commercial synthesis of insect pheromones opened the door to using pheromones to control insect populations. The pioneering work on pink bollworm in cotton by Harry Shorey in the 1970s was the first demonstration that pheromones could control insect populations. The first commercial pheromone product for insect control was the pheromone for the pink bollworm.

The successful control of pink bollworm led to large scale pheromone trials against a variety of pests, including Heliothis and Helicoverpa moths in cotton and gypsy moth. However, pheromones were not working as well for control of these other pests. Harry Shorey had been lucky and chose the right pest. Pink bollworm has a number of characteristics that led to successful control by pheromones. Some of the other pests have characteristics that make control by pheromones far more difficult.

Pheromone control of pink bollworm works best at low population densities and eventually fails if the population rises above a threshold level. Mating disruption to prevent population explosions of pink bollworm was successful because pink bollworm populations could be driven to very low levels at the start of the season by eliminating pink bollworm over wintering sites. If the pink bollworm started the season at high populations the mating disruption strategy would not have worked. However, this aspect of pink bollworm control using pheromones was not well understood by people working with different pests.

Originally, pheromone control was thought to work by causing males to become habituated to the pheromone odor. It was believed that constant stimulation with pheromone odor would lead the male moths to become unresponsive altogether and fail seek females. If pheromones were working for pink bollworm control by habituating the male moths, then pheromones should work on other moth species by habituation. However, the idea that pheromone control of pink bollworm depended on habituation was a misperception. This misperception was reinforced by a marketing quirk.

The brochure for the commercial pink bollworm product featured a picture of a male pink bollworm resting on a cotton leaf next to a pheromone dispenser. The photo was widely distributed and shown at scientific meetings, Presenters often gave the interpretation that the male moth was indeed habituated and unresponsive to the pheromone odor. However, this was not the case. The professional photographer who took the photo used a dead moth, strategically staged on the leaf. This is what professional photographers do- artfully stage the subjects and photograph them. Of course the moth was unresponsive to the pheromone dispenser, it was dead. A live moth would have been frantically beating its wings and trying to mate with the dispenser. A picture of a frantically moving male moth would have been difficult to capture, so the photographer used a dead moth instead.

About this time, a debate was underway whether mating disruption worked by habituation or by a mechanism of “false trail following”. Habituation would have the males becoming unresponsive. False trail following would have the males following odor trails emitted from the pheromone dispensers (false trails) which were in competition with pheromone trails released by female moths. The difference in the two mechanisms is important to mating disruption strategy. If the moths were habituated, then pheromone control should work more or less the same way at high moth densities as at low densities. However, if the pheromone was working by false trail following, the probability of a male finding a female rather than a pheromone dispenser increases at high densities. At high enough densities, pheromones would become ineffective because moths would find each other and mate in spite of the pheromone treatment.

In reality, both mechanisms may operate depending on the species and the treatment. However, at the time all our data on our commercial product pointed to false trail following as the mechanism for pink bollworm control. The pink bollworm was an ideal target for control by pheromones because its populations were low at the beginning of the season and could be kept at a low level by pheromone treatments that kept the population from rapidly increasing.

Attempts to control other insect species with pheromones were not as successful because mating disruption was not effective at the higher moth densities of those pests. At the time, the USDA was treating outbreak populations of gypsy moth with pheromones and not having great success. Through the early 1980s, the prevailing belief was that gypsy moth pheromone could work by habituation of the male moth. When it was recognized that the mechanism was false trail following and not habituation, the idea of using pheromones at high moth densities was reevaluated. This eventually led to development of the current strategy of using pheromones to eradicate low density populations of gypsy moth.