Ecology & Evolutionary Biology
Whiteflies

Greenhouse Whitefly - Trialeurodes vaporarium

Damage: Greenhouse whiteflies are worldwide pests of greenhouse-grown ornamentals and vegetables. First discovered in England in 1856, they were found in the United States in 1870. Tropical Central or South America are suggested origins of the greenhouse whitefly. Greenhouse whiteflies infest a wide variety of ornamental and vegetable crops, and they can survive outdoors during the growing season, particularly in sheltered locations. Even trees may be infested (redbud, Kentucky coffee berry, and avocado). Infested plants become chlorotic and unthrifty. Secondary infections of honeydew and sooty mold further detract from the appearance of the crop.

Life Cycle: Unless controlled, greenhouse whiteflies may completely destroy the commercial value of a floricultural crop. Greenhouse whiteflies reproduce slowly (a generation every 30 to 45 days), but each female may lay up to 400 eggs and live as long as 2 months. Adults are usually found on the lower surface of new leaves. Here they insert their eggs that hatch 5 to 7 days later. The new crawlers move about the plant for a day or two, often from leaf to leaf before inserting their mouthparts to feed. Once this occurs, they probably do not move again until mature. The crawlers molt into nymphs and then pupae. Finally, a new generation of whitish-yellow adults emerges. They are soon covered by a white, waxy bloom.

Scouting: Adults congregate and oviposition is heaviest on the undersides of younger leaves. Crawler movement is limited and subsequent immature stages are immobile so that development occurs on the undersides of leaves close to the site of oviposition. Continued growth of the plant after oviposition causes successively older stages of the whitefly to be found on successively older leaves. Look for adults on new growth as well as eggs which are often laid in a semi-circular arrangement on the underside of the leaf. Look for larva and pupa farther down the stem. If adults/eggs are absent, check larval/pupal scales for signs of hatching out - you may be looking at scales that are no longer occupied.

Silverleaf Whitefly - Bemesia argentifolii

Description: Except for slightly smaller size, slightly more yellowish color, and slightly more slender appearance, silverleaf whiteflies resemble other whiteflies. The eggs are tiny, cylindrical, white to brownish, and inserted into the leaf tissue on the underside of the leaf. Nymphs are translucent, pale yellow, flat scale-like insects that feed on the lower leaf surface (this is the "egg" stage that growers refer to). Silverleaf whitefly pupae are pale yellow, scale- like insects that sometimes have noticeable waxy hairs (sometimes they appear to be bare). The sides are not perpendicular and there is no tiny fringe of waxy filaments around the margin.

Damage: Infested plants become sticky with honeydew and dark with sooty molds. Silverleaf whitefly "crawlers" hatch and crawl about until they insert thread-like mouthparts into the lower surface of the leaf to feed. They tuck the legs and antennae underneath and settle down closely to the leaf surface. Crawlers molt into scale-like nymphs that also suck out sap. Nymphs grow and molt a second and third time into a nonfeeding state called the pupa. The adult whitefly develops within the pupa. Adults emerge from the pupal skin through a T-shaped slit about a month after the time the egg was laid.

Life Cycle: Females live about 4 weeks and lay 28 to 300 eggs each on the lower leaf surface. In hot weather, development may take only two weeks; in cool weather, development takes much longer.

Scouting: Silverleaf whitefly can be differentiated from the more commonly found Greenhouse Whitefly by careful observation of adults and pupae. Adult silverleaf whiteflies have white wings and a yellow body; they are slightly waxy with no dark markings or bands. Their wings are held somewhat vertically tilted like the peaked roof of a house, instead of flat over their bodies like the greenhouse whitefly. The yellow body is visible between the wings. Pupae of the silverleaf whitefly have no waxy filaments around their edges as do most other species of whiteflies. Most whiteflies produce a lot of white wax in their colonies; silverleaf whitefly has almost none. Also, the Greenhouse Whitefly pupa, when viewed from the side, has vertical sides almost as if the pupa had been stamped out with a cookie cutter. The Silverleaf and most other whitefly species have pupae that slope gradually towards the margins.

Whitefly Biocontrols

Whitefly Parasitoid - Encarsia formosa

To successfully reproduce in greenhouses, E. formosa must locate potential hosts, assess host quality, and use nymphs appropriately for host feeding or parasitism. Following release into greenhouses, E. formosa employs visual and olfactory cues to find infested host plants. When searching new leaves, the parasitoid does not distinguish between upper and lower surfaces and shows no preference for middles or edges of leaves. The rate at which hosts are encountered is dependent on the parasitoids' walking speed, whitefly size, and number of hosts on a leaf. Walking speed is reduced by leaf venation, high trichome densities, excessive honeydew, encounters with nymphs suitable for host feeding and parasitism, decreasing temperature, low barometric pressure, and smaller egg loads.

Encarsia formosa is a solitary endoparasitoid that matures 8-10 eggs per day. Daily egg maturation and oviposition rates decline as wasps age. Adults obtain energy and nutrients by consuming honeydew and hemolymph of hosts that are pierced with the ovipositor, but in which no egg is deposited.

Killing hosts for nutritional purposes is termed host feeding. Encarsia formosa will host feed on all pre-imaginal stages of T. vaporariorum except the egg, but prefers second instar nymphs and pupae. However, the pupae and all nymphal stages of B. tabaci are used equally for host feeding. To host feed, E. formosa wounds nymphs or pupae by probing with the ovipositor for up to six minutes and feeds from wounds which wasps may enlarge with their mandibles. This probing followed by feeding kills hosts. Nymphs that have been used for feeding are not used for oviposition, and previously parasitized whiteflies are not used for host feeding (Hoddle et al. 1998).

Encarsia formosa will oviposit in all immature stages of T. vaporariorum, except the egg and the mobile first instar, and in all immature stages of B. tabaci older than the settled first instar nymph. Encarsia formosa prefers to oviposit in third, fourth, and prepupal nymphs of both T. vaporariorum and B. tabaci. The rate of successful emergence of the parasitoid is highest from these preferred stages. Encarsia formosa does not oviposit in up to 50% of suitable hosts in the preferred stages even when these are not parasitized or mutilated from host feeding. Such hosts may be parasitized at a later encounter.

Encarsia formosa reared on T. vaporariorum can lay five eggs per day (will oviposit a total of 59 eggs before death), host feed on three nymphs per day, and kill on average a total of 95 nymphs over a 12 day life expectancy. Adult females chew a round exit hole on the dorsal surface of fourth instar nymphs before emerging. At 21°C, and with third instar T. vaporariorum as hosts, the time from oviposition to adult emergence is 25 days.

Scouting: The most obvious sign of successful parasitization by Encarsia are the blackened scales found on the leaves. Note that the scale only turns black during the last half of the 20 day development cycle within the host. An obvious hole will be noted in the scale if the parasite has already emerged. Only the presence of unhatched scales should be used in quantifying the presence of Encarsia for scouting purposes. Adult wasps will also frequently be seen walking on the upper and lower surfaces of leaves looking for prey but due to their small size, the scout must pay close attention to see them. Adult wasps are also attracted to yellow and blue sticky cards thus when Encarsia levels are relatively high, sticky cards use must be discontinued or limited to one day per week.

Mixed species Whitefly Parasitoid - Eretmocerus sp.

Whitefly Predator - Delphastus pusillus

Life Cycle:

Scouting:

Insidious Pirate Bug - Orius insidiosus