Do I Have Greenhouse Whiteflies or Sweet-Potato Whiteflies?

Both these whitefly species show up in greenhouses, often at the same time. The difference is important, because whitefly parasites (Encarsia formosa), our least-expensive whitefly control, work better when used against greenhouse whiteflies than when used against sweet-potato whiteflies. With sweet-potato whitefly, additional parasite releases are required (perhaps continuously as often as every 2 weeks), and at higher dosages, and other controls such as Whitefly Predators and Yellow Traps may become needed as well. Monitoring Whitefly Parasite progress against sweet-potato whitefly is also more difficult, because the pupal stages of sweet potato whitefly don't turn "jet" black in appearance like greenhouse whitefly pupa do when parasitized, instead changing only slightly to a milky, somewhat darker color. To really tell these whitefly species apart, it helps to know a little about whitefly biology.

Whiteflies lay eggs on the bottoms of leaves, each one on a short stalk. Eggs are pearly white when first laid, and greenhouse whitefly eggs gradually become brown before hatching, while sweet-potato whitefly eggs age to a dark grey color instead. Young nymphs of both species resemble tiny mealybugs or aphids when first hatched, and don't move very far before settling down to a sedentary existence. After the first molt, they lose their legs and resemble small, white scales. These nymphs are oval, flattened, and appear translucent, with a white, green, or yellowish cast.

After nymphs molt through a total of four 'instar' or growth stages to a pupa-like stage, the adult whitefly emerges through a T-shaped slit cut in the outer skin. All four stages look similar in appearance, growing a little larger with each molt. The last part of the fourth nymphal stage is commonly referred to as the pupal stage. This pupal stage is the most reliable difference between greenhouse whiteflies and sweet-potato whiteflies. The pupa of greenhouse whitefly is oval with straight, flat sides that are perpendicular to the leaf surface, forming a distinct "rim" with a fringe of short hairs around the top edge. The sweet-potato whitefly pupa is oval too, but appears more rounded, or dome-shaped from the side view, and lacks the fringe of hairs. Both species have several pairs of longer hairs rising from the top of the pupa, which are usually longer with greenhouse whitefly than sweet-potato whitefly, but these aren't considered reliable for identification.

When resting, adult whiteflies hold their wings differently, another clue in identification. Greenhouse whitefly adults are slightly larger than sweet-potato whitefly adults, and hold their wings fairly flat over their bodies, nearly parallel to the leaf surface. Sweet-potato whiteflies are slightly more yellow, and hold their wings in a peak over their bodies, at a 45° angle to the leaf surface.

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