The plum curculio (PC),Conotrachelus nenupharHerbst (Coleoptera: Curculionidae), is a key pest of stone and pome fruit in North America. Though grandisoic acid (GA) was identified as a male-produced aggregation pheromone for this species, other components likely exist, as have been identified for various curculionids. To better determine these components, an understanding of the conditions necessary for optimum pheromone production and attraction is needed, this is essential for the improvement of monitoring techniques and to achieve better biological control. The goal of this study was to determine the biotic and abiotic factors influencing both the response to pheromones and pheromone production. Tests were conducted in a dual-choice still-air vertical olfactometer using live male PCs as odour sources and live females as responders, to determine which physiological factors (age, number of males, mating status) influenced female response to males. Head-space collections of GA production under various conditions (airflow rate and frequency, collection zone strata, variation of humidity, temperature, and presence of a harbourage) were also done, as were electroantennograms (EAG) using synthetic pheromone mixtures. Results revealed that for both strains, the odour of two virgin mature males elicited significantly greater and more consistent attraction from mature virgin females than other ages and numbers of males when compared with the control. Head-space collections indicate that male PC have increased production of GA under high humidity in the presence of fruit, indicating that these conditions are necessary for optimal pheromone production and collection. EAG studies revealed significant responses to GrandLures I, II, III/IV and to the positive enantiomer of GA, and the amplitude of the signal varied with concentration. Our data identify the optimal physiological state and conditions at which pheromone collections should be performed, and what physiological life stages respond to these stimuli. These results have implications for optimising monitoring tools for this serious crop pest. This species has a northern univoltine strain and a southern multivoltine strain, both of which were examined in this study.
Plum curculio, Conotrachelus nenuphar (Herbst), one of the most important pests of apple in eastern and central North America, is usually managed in New England apple orchards by multiple full-block insecticide applications. Efforts to reduce insecticide inputs against plum curculio include using an "attract and kill" approach: odor-baited trap trees deployed in the perimeter row of apple orchards. The standard approach is based on baiting apple trees with two olfactory stimuli, the fruit volatile benzaldehyde and the aggregation pheromone of plum curculio, grandisoic acid. We attempted to improve attraction, aggregation, and retention of adult plum curculios within specific baited trap tree canopies within apple orchards using an additional host plant volatile found to be highly stimulating in electroantennogram studies, trans-2-hexenal. We also attempted to increase aggregation using increased release rates of grandisoic acid. We found that trans-2-hexenal did not provide increased aggregation when deployed as an additional attractant within trap trees or when conversely deployed as a "push" component or repellent in perimeter trees lateral to the baited trap tree. Although increasing the release rate of grandisoic acid 5× actually appeared to increase overall aggregation within trap trees, it was not significantly different than that obtained using the standard dose. Therefore, we believe that the standard olfactory stimuli are sufficient to provide aggregation within trap trees, but that other means should be used to manage them after their arrival.
An infestation of Histiostoma Kramer sp. mites (Acari: Histiostomatidae) occurred in rearing colonies of the univoltine strain of plum curculio (PC), Conotrachelus nenuphar (Herbst) (Coleoptera: Curculionidae), established in 2010 in southern Québec, Canada. Tests conducted in a two-choice still-air vertical olfactometer with mite-infested and noninfested PC revealed that the number of beetles responding by walking towards normally attractive synthetic and natural odours was significantly lower for those infested with mites. Those mite-infested curculios that did discriminate between test odours and odour-free air all responded positively to the test volatiles, similar to the behaviour of noninfested insects. This indicates that mites affect PC ability to physically move towards attractive volatiles but not odour preference.
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