This photo shows one of the main tools being used to stop New World screwworm. The green pupae are sterile flies before they emerge. Each ground release unit holds about 80,000 flies and is refilled every other day. As the flies emerge, they disperse into the surrounding area in search of females. Sterile flies are also being released by air. So far this month, USDA has released more than 19.5 million sterile flies (via air and ground) as part of the effort to contain and eradicate New World screwworm.