Natural and Synthetic Pyrethrins Act as Feeding Deterrents against the Black Blowfly, Phormia regina (Meigen)

SIMPLE SUMMARY: Pyrethrum is a botanical insecticide derived from pyrethrum flowers. Feeding deterrence caused by pyrethrum has been noted for several insect species. However, it is unclear whether the deterrent property results from a single component or from a combination of the six insecticidal active ingredients, known as pyrethrins. Here, we determined the feeding deterrence of natural pyrethrins and their two main components (pyrethrins I and II) on the blowfly, Phormia regina, in a dual-choice feeding assay. We found that natural pyrethrins and synthetic pyrethrins I/II in sucrose solution induced feeding deterrence at a concentration 16 times lower than the lowest concentration at which insecticidal action occurs. These results demonstrate that pyrethrins act as a feeding deterrent at sub-lethal concentrations. At the deterring concentration, feeding bouts were interrupted by intensive grooming of the proboscis, suggesting that pyrethrins acted instantly on the oral gustatory system of flies. The potent feeding deterrence of pyrethrins may provide effective protection for pyrethrum plants by rapidly deterring insects from feeding before insecticidal activities occur. ABSTRACT: Pyrethrum is a botanical insecticide derived from pyrethrum flowers. Feeding deterrence caused by pyrethrum has been reported in several sucking insects; however, there is no account of the cause of deterrence—whether from a single component or the combination of six active ingredients, called pyrethrins. We determined the feeding deterrence of natural pyrethrins, their two main components (pyrethrins I and II), and pyrethroid insecticides on the blowfly, Phormia regina. In a dual-choice feeding assay that minimized tarsal contact with food sources but allowed feeding through proboscises, natural pyrethrins, synthetic pyrethrins I/II, and allethrin were observed to induce deterrence at a concentration 16 times lower than the lowest concentration at which the knockdown rate increased. Feeding bouts were interrupted by intensive grooming of the proboscis at the deterring concentration, but no such grooming was observed to occur while feeding on the unpalatable tastants—NaCl, quinine, and tartaric acid. The underlying mode of action for the feeding deterrence of pyrethrins at sub-lethal concentrations probably occurs on the fly oral gustatory system, while differing from that of unpalatable tastants. The potent feeding deterrence of pyrethrins may provide effective protection for pyrethrum plants by rapidly deterring insects from feeding, before insecticidal activities occur.