Review of research advances in the screwworm eradication program over the past 25 years

Abstract

New World screwworms, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae: Chrysomyinae), are devastating pests of warm-blooded animals that cause significant economic damage to livestock. The successful campaign to eradicate screwworms from continental North America, led by the US Department of Agriculture and using the sterile insect technique, continues to receive research support that has resulted in improved technologies for all aspects of the program. The process and ingredients for mass-rearing screwworms is more efficient and sustainable, and there is now a standardized protocol for developing new strains used in mass rearing. Cryopreservation of screwworm embryos allows strains to be preserved and recovered if necessary and also reduces rearing requirements for backup and research strains. Sterile fly release procedures and equipment have been updated leading to optimized sterile fly release rates. Surveillance for screwworm infestations and outbreaks have incorporated new trap designs, habitat analysis, and molecular genetic techniques that enhance monitoring the progress of the program as well as early detection and response to outbreaks. Genetic analyses of screwworm populations across their current range have increased the understanding of genetic differentiation, which may aide in developing new strains and determining the geographic origin of screwworms causing outbreaks when they occur. The ability to release only sterile males, which has been a goal of the program for over 60 years, has recently been accomplished through the development of transgenic sexing strains. The strains carry a conditional female lethal gene and are comparable to the wild-type strain for several biological parameters that are important for mass production and performance in the field. The strains should improve efficiency of population suppression of the current and future eradication and prevention programs against screwworms. These research advances as well as future considerations are presented.

Agricultural production: assessment of the potential use of Cas9-mediated gene drive systems for agricultural pest control

“…Subsequently, the program was extended to eradicate screwworm from all of Central America (VargasTeran, Hofmann, and Tweddle 2005). Currently, to prevent re-infestation from South America, sterilized flies are being released in a 'buffer zone' in Eastern Panama and along the border with Colombia (Scott et al 2017). The screwworm mass rearing facility is in Pacora, Panama, and is run by the U.S.-Panamanian Commission for the Eradication and Prevention of Screwworms (Comisión Panamá-Estados Unidos para la Erradicación y Prevención del Gusano Barrenador del Ganado or COPEG).…”

“…In addition, economical mass rearing methods were developed (Melvin and Bushland 1936;Chaudhury and Skoda 2007), sterilization by radiation had only a small negative effect on male fitness (Crystal 1979), and there were no significant mating barriers across the eradication zone (Lachance et al 1982). More recently, efficient methods were developed for distributing chilled sterile flies by air using modern GPS-guided navigation (Tween and Rendon 2007;Scott et al 2017). Other factors that contributed to the success of the program that we think are also applicable to future gene drive programs include:…”

“…Screwworm remains a problem throughout most of tropical South America and on some Caribbean islands, notably Cuba and Jamaica (Scott et al 2017). It may not be necessary to use a gene drive system for eradication and/or suppression of populations on islands and in regions in South America west of the Andes.…”

Size matters in insects – an introduction

“…Many insects are mass reared for insect pest control programs. For instance, the success of the sterile insect technique depends critically on the fitness of the released males, as these need to favorably compete with wild males (Orozco‐Dávila et al., ; Scott et al., ). The articles in this current special issue on insect fitness clearly demonstrate that larger males have a competitive advantage and that adult size depends on nutritional settings, stressing the importance of optimal conditions in insect mass rearing.…”

The Sterile Insect Technique (SIT) – an introduction

“…Some SIT programs are decades old, and Scott et al (2017) critically reviewed findings from the last 25 years on the original target species, C. hominivorax. Important advances in this program during this period have been the development of methods to cryogenically preserve embryos, the application of molecular genetic techniques to quantify population differences, and the recent breakthrough of being able to release only males via the development of a transgenic sexing strain carrying alleles lethal to females.…”