Biology Of Whiteflies In Greenhouse Crops
Table of Contents
Whiteflies are a major pest of greenhouse crops including tomatoes, cucumbers and many ornamental species, particularly poinsettia, gerbera, and a number of spring crops. Many weed species are also hosts of whiteflies and often serve as sources of infestations. Whiteflies damage crops by reducing their vigour, facilitating growth of sooty mould on leaves and fruits, transmitting viral diseases, and by reducing crop quality by their visible presence (see Factsheet Management of Whiteflies in Greenhouse Crops, Order No. 03-067).
Adult whiteflies (Figures 1 and 2) are small, winged, white insects about 1.5-2 mm long. Eggs (Figures 3a and 3b) are laid on the underside of the youngest leaves, and are too small to be seen clearly without the aid of a microscope. A female whitefly may lay up to 300 eggs during her lifetime, and live as long as 42 days at 18oC and 8 days at 27oC. After hatching, the eggs undergo 4 stages or instars before becoming adults. The first instar or larval stage (sometimes called crawlers) (Figure 4) hatches in 5-10 days. They are flat and scale-like, and crawl around for a short while before becoming immobile. The second and third instars or larval stages are followed by the fourth instar or pupal (Figures 5a and 5b) stage, from which the adult emerges. Old pupal skins and adults may be found on the underside of lower leaves, which may have symptoms of wilt. On average, the whitefly completes its life cycle in 35 days at 18oC and 18 days at 30oC. Whiteflies have no special overwintering stage and can usually survive as long as there is some kind of vegetation around.
There are 2 whiteflies of concern to growers in Ontario, the greenhouse whitefly (GWF) (Trialeurodes vaporariorum) (Figure 1), and the silverleaf whitefly (SLWF) (Bemisia argentifolii) (Figure 2). The GWF and SLWF adults look very similar, although there are some differences. The SLWF is slightly smaller than GWF, and its body is more yellow in colour. At rest, the SLWF holds its wings tent-like above its body, whereas the GWF holds them flatter and more parallel to the surface on which it is resting. The major diagnostic differences between GWF and SLWF occur in the pupal stages. The GWF pupa (Figure 5a) is raised off the leaf surface and is surrounded by a fringe of hairs whereas the SLWF pupa (Figure 5b) sits flat on the leaf and does not have a fringe. These features are best seen with a microscope. Additional differences are: (a) the SLWF flies in a more direct manner whereas the GWF assumes a more haphazard flight pattern, (b) the life cycle of SLWF takes longer to complete, and (c) SLWF is less adapted to cold conditions.
Figure 1. Adult greenhouse whitefly.
Figure 2. Adult silverleaf whitefly.
Figure 3a. Recently laid (lighter coloured) and more mature (darker) eggs.
Figure 3b. Circular egg-laying pattern of greenhouse whiteflyon some crops.
Figure 4. Early larval stage recently hatched from egg.
Figure 5a. Pupa of greenhouse whitefly.
Figure 5b. Pupa of silverleaf whitefly.
Figure 6a. Sooty mould on tomato.
Figure 6b. Sooty mould on gerbera.
Figure 7. Pseudo yellows disease on cucumber.
Figure 8. Large numbers of whiteflies reduce the quality and marketability of ornamental crops.
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