Specialized receptor neurons for pheromones and host plant odors in the boll weevil, Anthonomus grandis Boh. (Coleoptera: Curculionidae)

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Abstract
Antennal olfactory receptor neurons in the boll weevil, Anthonomus grandis , were investigated through single neuron recordings. Receptor neurons for both pheromone components and host plant odors were associated with type I sensilla within the sensory band regions. Nine types of receptor neurons were identified, based on their responsiveness to the four aggregation pheromone components and selected host plant odors. Three receptor neuron types responded to either compound I, II or IV of the aggregation pheromone. Dose—response curves were similar for each of these receptor neuron types, which differed only in their key compound. In each instance, I neurons responded primarily to (+)-I, the optical isomer produced by the boll weevil which was found to be active in field tests. Receptor neurons for II also responded to a lesser degree to III, its aldehydic analog, at the same stimulus load. Six additional receptor neuron types responded to selected host plant odors: β-caryophyllene, trans -2-hexen-l-o1 and other six carbon alcohols and aldehydes, trans -β-ocimene, benzaldehyde, linalool, and B-bisabolol. These neurons were as responsive as, or in some cases more responsive at the same stimulus load as receptor neurons for pheromone components. Receptor neurons responsive to six-carbon alcohols and aldehydes were generally most responsive to trans-2-hexen-l-ol. Receptor neurons for other plant odors responded principally to only one compound among the odorants tested. However, responses of these neurons were not uniform, suggesting possible specialization for other unidentified key odorants. Comparisons were also made between single neuron and electroantennogram responses. The results indicate that the boll weevil, a narrowly oligaphagous insect, detects its host plant at some distance, and utilizes information about a wide range of chemical structures in its olfactory-mediated behavior.

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