Understanding Female Sex Pheromones

Although his results were not conclusive he did find that females were less active in the presence of female sex pheromones than under control conditions, supporting ideas that the effect of the secretion on the female is to inhibit movement.

Electrophysiological studies have been made (Grant 1970, l971b; Birch l97l; Grant et al. 1972) in an attempt to circumvent this behavioral block. The results of these investigations indicate that male scent brushes or even compounds identified from the scent brushes do not elicit antennal responses that are specific either with respect to sex tested or to chemical compounds employed; nor in general is there any difference between male and female responsiveness.

It seems that no consistent distinction can be drawn between electroantennograms (EAGs) produced by scent- brush compounds and related chemicals (Birch 1971; Grant et al. 1972). In contrast, the female sex pheromones usually have lower threshold values than any other chemicals related or not, and produce larger EAGs than other compounds at concentrations within the normal physiological range (Grant et al. I972).

The conclusion must be that the ability to perceive an odor stimulus is not necessarily correlated with naturally intraspecific biologically active compounds. Further behavioral evidence is needed to determine which component(s) are actually the pheromones.

Evolution of male aphrodisiacs in Lepidoptera

The immediate function of the male pheromones from both the hair-pencils of the Daniinae and the brush-organs of the Noctuidae is to cause inhibition of locomotion in the female, promoting successful mating by blocking her normal escape-flight response to a strange object. In addition to this immediate effect there is also the possibility that the pheromone may also function in a number of other ways: (1) as a species-specific signal to prevent cross breeding; (2) as a primer pheromone to initiate oogenesis; (3) to deter other males; (4) to repel predators.

One immediately attractive theory is that the male pheromones provide a species-specific signal preventing cross-mating, particularly since now we know that considerable cross-attraction occurs to synthetic pheromones and to some extent naturally (Jacobson 1972). However, several species of Noctuidae appear to have similar or identical male pheromones (fig. 7.3). In particular, three species of Leuca-nia all produce a mixture of benzaldehyde and iso-butyric acid in the same ratio (80/20). All these species can occur in the same locality in England.

All have larvae which apparently feed on the same variety of common grasses and all emerge at the same time of year. It is possible that the female sex pheromones are different, but the male antennae of each species respond equally (EAG) to the female pheromones of the other two. The only obvious difference is that each species flies at a different though overlapping time of the night (Birch unpublished). EAG recordings from several noctuid species indicate that the male scent brushes and compounds isolated from them are very unspecific.

So far no threshold differences have been found for substances from one species as opposed to another. For example, Grant (l97lb) found that eight noctuid species all responded similarly to the hair-pencils of Trichoplusiani.

Working with Pseudaletia unipuncta Grant et al. (1972) also found that both male and female antennae responded equally well to each chemical extracted from the brushes (fig. 7.3) and that scent brush compounds were no more stimulating than other compounds. Any specificity in the Noctuidae remains to be found.