Improved sentinel method for surveillance and collection of filth fly parasitoids

Authors: Geden, Christopher - Chris; Johnson, Dana; Taylor, David

Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2020
Publication Date: 11/2/2020
Citation: Geden, C.J., Johnson, D.M., Taylor, D.B. 2020. Improved sentinel method for surveillance and collection of filth fly parasitoids. Journal of Insect Science. 20(6):1–7. https://doi.org/10.1093/jisesa/ieaa026.
DOI: https://doi.org/10.1093/jisesa/ieaa026

Interpretive Summary: House flies have many important natural enemies, including parasitic wasps ("parasitoids") that attack the fly in the pupal stage. Current methods for monitoring and collecting parasitoids from the field have advantages as well as major liabilities. Wild fly pupae can be collected from farms and held in the lab for parasitoid emergence. This method gives a good picture of the species present, but it depends on the ability of the investigator to find pupae in the field and resutls can be distorted by the tendency to collect pupae from a small number of hot spots. The second method, placement of "sentinel" pupae in screen bags, is easy to use but is strongly biased towards collecting certain species of parasitoids. In this paper, researchers with USDA-ARS-CMAVE in Gainesville (Florida) describe a novel approach that combines the advantages of both current methods. Fly larvae, along the rearing media in which they grow, are placed in the field and allowed to pupae in place. The containers are protected against animals by placing them in wire Havahart traps. The new method was tested on cattle farms in Florida and Nebraska. The method returned far more parasitoids than the other two methods and provided a species composition profile that closely matched collections of wild pupae. This will benefit researchers and biocontrol businesses that must make periodic collections of these important biological control agents.

Technical Abstract: Parasitoids are important natural enemies of house flies and other muscoid flies. The two most commonly used methods for collecting fly parasitoids from the field have distinct advantages and disadvantages. Collections of wild pupae depend on the ability to find pupae in sufficient numbers and are prone to localized distortions in relative species abundance because of overrepresentation of samples from fly-breeding hot spots. Placement and retrieval of sentinel pupae is convenient and allows consistent sampling over time but is strongly biased in favor of Muscidifurax spp over Spalangia spp. An improved sentinel method is described that combines some of the advantages of these two methods. Fly medium containing larvae is placed in containers, topped with a screen mesh bag of pupae, and placed in vertebrate-proof wire cages. Cages are placed at sites of actual or potential fly breeding and retrieved 3-7 days later. The modified method collected species profiles that resembled proportions from collections of wild pupae much more closely than those from sentinel pupal bags. Pupae from the collections that were housed individually had a higher proportion of emerged Spalangia species than pupae that were held in groups. A method is described for isolating pupae in 96-well tissue culture plates for parasitoid emergence. Use of the plate method provided a substantial saving of time and labor over the use of individual gelatin capsules for pupal isolation.