Those Pesky Whiteflies

by Nathan Jackson

Your plants were thriving and everything was growing great, but now they don’t seem to be producing as well and have an unhealthy appearance. On closer inspection you find tiny white “moths” resting on the underside of the leaves and if you disturb them they flutter and fly around the plant. Whiteflies are a common nuisance in greenhouses, and if left unchecked can cause great damage or even kill your plants.

There are over 1,200 different varieties of whiteflies, although the greenhouse whitefly (Trialeurodes vaporariorum), sweet potato whitefly Bemisia tabaci, and silverleaf whitefly Bemisia argentifolii are the most common varieties found. Whiteflies are not true flies, but are in the same family as mealy bugs, scales, and aphids. They lay their eggs on the underside of leaves, each one on a short stalk. Eggs are pearly white when first laid and gradually become brown or dark grey before hatching.

Young nymphs resemble tiny mealy bugs 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 and this 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.

Whitefly adults and larvae feed by sucking juices from the plant. While feeding, larvae produce honeydew, a sticky sweet substance that promotes growth of a black sooty mold that can eventually kill the plant. Any honeydew or mold should be wiped down with a soapy solution.

Farmers have sprayed whiteflies with pesticides for decades and as a result they are very prone to developing a resistance to pesticides and sprays, and can be very difficult to control. Whitefly Parasites and Whitefly Predators have been used to control whiteflies with great results. Whitefly Parasites (Encarsia formosa is one popular variety) fly around whitefly infestations and lay their egg inside the whitefly pupae. The pupae gradually turns black as the whitefly parasite consumes it from the inside out, eventually with another parasite emerging.
Whitefly parasites are most effective against the common greenhouse whitefly. Whitefly Predators (Delphastus punctillum) can consume from 150 to 600 whitefly eggs per day! They are voracious feeders, and are effective against a number of whitefly varieties, with a preference for sweet potato and silverleaf whitefly.

With any pest, and especially whitefly, early detection is the key to success. Whitefly are attracted to the color yellow making yellow sticky traps a great tool for monitoring and catching adults that may be on your plants. Yellow traps spaced throughout the greenhouse will attract whiteflies and other insects which will get caught in the sticky coating. For heavy infestations a vacuum cleaner can be used to suck the little creatures right out of the air. Follow up with soapy spray and then release some predators or parasites. General feeding predators such as green lacewings or pirate bugs might give some control as well.

Your plants are important. Be vigilant and aware of what is living in, on, and around your garden. At the first sign of trouble take action. The faster you act the better chance you have of wiping out the plague of plant pests that will inevitably find their way to your greenhouse. GG

Nathan Jackson is the owner of Nature’s Control and Ladybug Indoor Gardens. You can send your specific bug questions to This e-mail address is being protected from spam bots, you need JavaScript enabled to view it For more information, to request a catalog, or to find a local distributor, check out www.naturescontrol.com or call (541) 245-6033.