Insects. The challenges of Culicoides colonization have somewhat normalized the use of wild specimens26. We tested wild C. furens, a nuisance species common in U.S. coastal areas, trapped at the University of Florida, Florida Medical Entomology Laboratory (FMEL) in Indian River County. Culicoides were captured using CDC miniature light traps (BioQuip products, Rancho Dominguez, CA, USA) connected to live midge collection chambers27. Traps were baited with an incandescent light bulb, 2 mL of octenol (1-Octen-3-ol) placed in a 15 mL plastic tube with a piece of cotton wool, and ~ 1kg of dry ice contained in an insulated 1.89 L beverage container (Igloo Products Corp., Katy, TX).
Since reference (known susceptible control) C. furens are unavailable (no laboratory colony in existence), we used a laboratory-reared strain of Aedes aegypti (Orlando 1962) which originated from Orlando, FL (Orange County) as a susceptible control. This strain was originally obtained from colonies maintained at the USDA-ARS CMAVE. The Ae. aegypti were reared in an insectary at 26°C and 85% relative humidity. Egg papers were placed in plastic trays containing approximately 2 L of tap water at a density of 200–300 eggs per rearing tray. Larvae were fed a 1:1 mixture of lactalbumin and yeast ad libitum. Pupae were transferred from larval rearing trays to water-filled plastic cups in 30.5 x 30.5 x 30.5 cm Bug Dorm adult rearing cages (BioQuip, Rancho Dominguez, CA, USA). Cotton rounds soaked with 10% sucrose solution were placed inside each cage as carbohydrate source for emerging adults.
CDC Bottle bioassays. The CDC bottle bioassay is commonly used to monitor mosquito insecticide susceptibility19. We adapted this method to calibrate the assay for Culicoides by determining the DD and DT for permethrin. Following the CDC protocol17, we prepared permethrin solutions by diluting technical grade permethrin (100% Chem Service Inc., West Chester, PA, USA) in American Chemical Society (ACS) grade acetone (Fisher Scientific, Hampton, NH). Each bioassay used five 250-ml clear Wheaton bottles with lids (DWK Life Sciences Inc., Millville, NJ), four coated with 1.0 ml of the permethrin solution and one untreated control coated with acetone only. Bottles were coated by swirling the solution inside and then left to dry on a hot dog roller machine with heat turned off to ensure even coating. Control bottles were capped and rolled before preparing bottles containing insecticide. All bottles were then left open to dry in a dark environment to prevent permethrin photodegradation. Preliminary observations showed high mortality of C. furens in control bottles when the bioassay was performed within five hours after the bottles were coated. We tested various drying times (5, 10 and 24 h) and found that 24 hours of drying were necessary to eliminate control mortality.
As recommended by Brogdon and Chan18, when calibrating the CDC bottle bioassay to determine DD and DT for a new insect, we prepared bottles with a range of permethrin concentrations to determine a possible DD. Based on the CDC’s recommended DD of 43 µg permethrin/bottle for susceptible Ae. aegypti17, and the small size of C. furens in comparison to Ae. aegypti, the permethrin concentrations we tested as possible DD were 43, 21.5, and 10.7 µg permethrin/bottle. The permethrin concentration that killed 100% of the wild C. furens between 30 and 60 minutes would be selected as the DD (dose that kills 100% of insects18.
Bottle bioassays were performed at the same time but in separate bottles for susceptible Ae. aegypti and wild C. furens. Mosquitoes and C. furens were aspirated by mouth into each bottle with the goal of 20 individuals per bottle. Mortality was recorded every five minutes for the first 15 minutes and every fifteen minutes until 2-h post-exposure18. After the 2-hrs of exposure, insects were transferred into 8-oz paper food cups with lids and generic mesh to prevent all insects from escaping. Insects were provided with a cotton round with water and mortality was then assessed at 24-h post-exposure28.
Field cage trials. Two field trials were conducted between March and April, 2023 in Indian River County, FL (Table 1), targeting caged susceptible Ae. aegypti and/ or wild C. furens (Table 1). Permanone 30–30 (30% permethrin, 30% piperonyl butoxide) was applied at maximum rate (0.007 lb permethrin/acre) (Table 1) using a truck-mounted ULV aerosol generator that was operated by personnel of the Indian River Mosquito Control District. Spray missions were performed around dusk (1700h and 1800h) on a five-acre plot of low-cut grass at the Indian River County Fairgrounds.
Bioassay cages (Fig. 1) were made of cardboard rings (15.2 cm diameter, 2.5 cm width) covered with mesh26,29. Three mesh types were tested (Fig. 1): stainless steel (SS) McMaster-Carr, Douglasville, GA), no-see-um netting (NN) (Seattle Fabrics, Inc., Seattle, WA), and fine nylon tulle fabric (Hobby Lobby Stores, Inc., Oklahoma, USA), hereafter called mosquito mesh (MM). SS and NN mesh were chosen because preliminary observations indicated they could prevent escape of C. furens (Cooper, unpublished data). Although the MM mesh has openings too large for Culicoides, it was included as it has been used successfully in mosquito cage trials30.
Table 1
Summary of ULV permethrin adulticide field cage trials to evaluate the susceptibility of C. furens in Indian River County, Florida, USA. (SS) stainless steel, McMaster-Carr, Douglasville, GA; (NN) no-see-um netting, Seattle Fabrics, Inc, Seattle, WA.; (MM) mosquito mesh (fine nylon tulle fabric), Hobby Lobby Stores, Inc, Oklahoma City, OK.
Field Trial | Date | Species tested | Mesh type tested | Number of passes | Speed | Wind speed | |
1 | March 1 | Ae. aegypti (susceptible) | SS, NN, MM | 2 | 10 and 5 mph, respectively | 3–5 mph | |
2 | April 6 | Ae. aegypti (susceptible) | SS, NN, MM | 1 | 10 mph | 6–10 mph | |
| | C. furens (wild) | SS, NN | | | |
The mosquitoes-only field cage trial aimed to compare different mesh types for field bioassay cages. Approximately 20 susceptible Ae. aegypti were placed in each cage. After confirming that the mesh types allowed insecticide passage and caused mortality in these mosquitoes, a combined mosquitoes and Culicoides trial was conducted. Each cage contained about 30 wild C. furens and 20 Ae. aegypti, except for the MM cages, which only had Ae. aegypti (Table 1). This design allowed us to evaluate the susceptibility of wild C. furens to permethrin relative to susceptible Ae. aegypti.
Cages were hung approximately 1.3 m above ground on shepherd hooks in five clusters, each with the three mesh types (Fig. 1). Clusters were spaced every five meters along a north-south transect, 20 m downwind of the spray truck path. Two untreated (control) cages of each mesh type were placed 20 m upwind. Mortality was assessed in the field 10 minutes post-exposure, and subsequently recorded at 1, 12 and 24 h post-exposure in the laboratory. Mortality was based on knockdown (inability to stand on legs or have coordinated flight17). Insects were provided a cotton round with tap water and maintained in the cages for 24 hours, before being placed in dry ice to kill any remaining live insects.
Data Analysis. Results from the cage field trials were analyzed by calculating the mean mortality of mosquitoes and Culicoides at various times post-exposure. For each trial, mean mortality and standard deviation were plotted30. For CDC bottle bioassays, time-response survival curves were created for each permethrin dose. Insect survival was assessed using Cox proportional hazards models with the ‘survival’ package in R version 4.2.231. Clustered Cox regression was performed to compare mean mortality across the different permethrin doses for both mosquitoes and Culicoides32.