Behavioral antagonism in the moth Helicoverpa zea in response to pheromone blends of three sympatric heliothine moth species is explained by one type of antennal neuron.

We have discovered a type of sensillum on the antennae of male corn earworm moths, Helicoverpa zea, that houses two types of receptor neuron (RN) that explains both the upwind flight attraction of males to their own species' pheromone blend and arrestment to the quite similar blends emitted by females of three other sympatric North American heliothine species. The first RN type is a large-spiking neuron that is most sensitive to (Z)-9-hexadecenal (Z9-16:Ald), the secondary H. zea pheromone component that along with the major component, (Z)-11-hexadecenal, causes attraction to the female. This RN is also responsive to (Z)-9-tetradecenal (Z9-14:Ald; not a H. zea pheromone component) at higher dosages. The sensitivity of this RN thus explains the attraction that has been observed in other studies when small proportions of Z9-14:Ald are added to Z11-16:Ald to mimic the conspecific blend. The second type of RN in this sensillum is a small-spiking neuron that is again responsive to Z9-14:Ald (which in larger proportions acts as a strong antagonist to upwind flight), but this RN is actually more sensitive to two other strong behavioral antagonists, (Z)-11-hexadecenyl acetate and (Z)-11-hexadecenol. Thus, activation of this single broadly tuned 'antagonist' RN could explain why H. zea males will orient only to their conspecific females. These three compounds are emitted by females of three other North American species, H. subflexa, H. phloxiphaga, and H. virescens, as agonists in their blends, which also contain the H. zea components Z11-16:Ald and Z9-16:Ald. This antagonist RN may also explain why a blend of Z11-16:Ald and a small amount of Z9-14:Ald is never as attractive to H. zea males as the conspecific blend. Enhanced specificity for the conspecific blend arises because the antagonist RN is never stimulated to fire, even when large proportions of the pheromone component, Z9-16:Ald, are added to Z11-16:Ald. When Z9-14:Ald is used instead of Z9-16:Ald, however, and the proportion of Z9-14:Ald becomes too great, the threshold of the antagonist neuron as well as that of the agonist neuron is exceeded, and the upwind flight response begins to be suppressed.