Comparison of mosquito cuticle penetration and feeding assays to evaluate novel water soluble insecticides

Authors: STOOPS, CRAIG - Us Army Medical Department; Estep Iii, Alden; Choi, Man-Yeon

Submitted to: Army Medical Department Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: The spread of insecticide resistance is a direct threat to military readiness and health and has prompted medical entomologists to consider new insecticides to control mosquito vectors. Water soluble insecticides such as molecular insecticides, including RNA interference technology, represent a possible revolutionary solution but are difficult to get through the cuticle of the insect which is designed to limit water penetration. In this study, we use a water soluble, flourescent dye as a surrogate for an insecticidal molecule to assess both uptake and excretion from mosquitoes when administered by feeding or by application in various vehicles to the cuticle. We observed that feeding was the most effective method of delivery, as assessed by the level of flourescence, followed by acetone administration to the cuticle. Notably, cuticular administration was significantly less effective than feeding but there was evidence that a portion of the dye crossed the cuticle. This manuscript provides important basic information to guide future studies with water soluble pesticides.

Technical Abstract: The spread of insecticide resistance is a direct threat to military Force Health Protection and has prompted Department of Defense medical entomologists to consider new insecticides to control mosquito vectors. Water soluble insecticides such as molecular insecticides, including RNA interference technology, represent a possible revolutionary solution. Feeding currently is the best method of delivering molecular insecticides, but making their use operationally significant for the military will involve designing contact insecticides. Because molecular insecticides are highly polar molecules formulating a contact insecticide needed by the military will require an understanding of how these types of molecules penetrate the cuticle to reach susceptible tissues. To understand the difference between feeding and topical application, laboratory-reared adult Aedes aegypti (L.) were treated with combinations of Rhodamine B dye in water-soluble oil (MPEG), acetone or were fed Rhodamine B dye and sugar water. Untreated controls were fed sugar water only. To measure the dye uptake, relative fluorescence intensity of fecal spots were measured and whole specimens were processed and read in a NOVOstar microplate fluorescence reader. The order of intensity of fecal spot fluorescence for topical applications was Control < Acetone < MPEG < Fed. The relative order of fluorescence intensity of specimens read in a NOVO star microplate reader was Control < MPEG Oil < Acetone < Fed. Both oil and acetone were effective in disrupting the physical barrier and setting up the necessary diffusion gradient but feeding appeared to be the most effective way to deliver water soluble molecules.