Sweetpotato whitefly adults and nymphs. Encarsia inaron parasite laying an egg in an ash whitefly nymph. A parasite has emerged from the round hole in the whitefly nymph at the bottom. Greenhouse whitefly nymphs turn black when they are parasitized. Whiteflies are tiny, sap-sucking insects that may become abundant in vegetable and ornamental plantings, especially during warm weather.