Whiteflies in Greenhouse Crops - Biology, Damage and Management
Table of Contents
Whiteflies are a major pest of greenhouse crops, including tomatoes and cucumbers, as well as many ornamental species, such as poinsettia, gerbera and a number of spring crops. Many weed species are also hosts of whiteflies and often serve as sources of infestations.
Adult whiteflies are small, winged, white insects about 1.5-2 mm long (Figures 1 and 2). Eggs are laid on the underside of the youngest leaves and are too small to be seen clearly without a microscope (Figures 3a and 3b). A female whitefly may lay up to 300 eggs during her lifetime and live as long as 42 days at 18°C and 8 days at 27°C.
Figure 1. Adult greenhouse whitefly.
Figure 2. Adult Bemisia.
Figure 3a. Recently laid (lighter coloured) and more mature (darker) eggs.
Figure 3b. Circular egg-laying pattern of greenhouse whitefly on some crops.
After hatching, the eggs undergo four stages, or instars, before becoming adults. The first instar, or nymphal stage, hatches in 5-10 days (Figure 4). Sometimes called crawlers, whiteflies at this stage are flat and scale-like and crawl around for a short while before becoming immobile. The second and third nymphal stages are followed by the fourth, or pupal, stage. The adult emerges from the pupal stage (Figures 5a and 5b). Old pupal skins and adults may be found on the underside of lower leaves, which may have symptoms of wilt.
Figure 4. Early larval stage recently hatched from egg.
Figure 5a. Pupa of greenhouse whitefly.
Figure 5b. Pupa of Bemisia.
On average, the whitefly completes its life cycle in 35 days at 18°C and 18 days at 30°C. Whiteflies have no special overwintering stage and can usually survive as long as there is some kind of vegetation around.
There are two whiteflies of concern to growers in Ontario: the greenhouse whitefly (GWF) (Trialeurodes vaporariorum) (Figure 1) and Bemisia (Figure 2). There are actually two Bemisia species: Bemisia tabaci, also known as sweetpotato whitefly, and Bemisia argentifolii, also known as silverleaf whitefly. Because the two species cannot be easily differentiated, they are simply referred to as Bemisia in this factsheet.
Although the GWF and Bemisia adults look very similar, there are some differences. Bemisia is slightly smaller than GWF, and its body is more yellow in colour. At rest, Bemisia holds its wings in a tent-like position above its body, whereas GWF holds them flatter and more parallel to the surface on which it is resting.
The major diagnostic differences between GWF and Bemisia occur in the pupal stages. The GWF pupa (Figure 5a) is raised off the leaf surface and is surrounded by a fringe of hairs. In contrast, the Bemisia pupa (Figure 5b) sits flat on the leaf and does not have a fringe. These features are best seen with a microscope. In addition:
Figure 6a. Honeydew residue on poinsettia.
Figure 6b. Sooty mould on tomato.
Figure 6c. 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.
There are currently a number of commercially available biological control agents for whiteflies:
Additionally, there are two microbial products, Beauveria bassiana and Isaria fumosorosea, that are fungal pathogens of whiteflies.
Encarsia is more effective against GWF but will still provide some control of Bemisia. Adult Encarsia are about 0.6 mm long and have a black thorax and yellow abdomen (Figure 9a). They kill whiteflies mainly by laying eggs into the immature stages (mostly the third and fourth instars). Adult Encarsia can live for a few days to a month depending on temperature. Females lay 50-350 eggs during their lifetime.
GWF pupae turn black 10-14 days after being parasitized (Figure 9b). On average, another 2 weeks are needed before the adult wasp emerges. When the adult is fully developed, it cuts an opening in the top portion of the black scale before emerging.
Parasitism by Encarsia of Bemisia is not as effective as in GWF. Parasitized Bemisia turn a tan to brown colour and are not as easily noticeable (Figure 9c). To control Bemisia with Encarsia, consider higher introduction rates.
Figure 9a. Adult Encarsia formosa.
Figure 9b. Greenhouse whitefly pupa parasitized by Encarsia formosa.
Figure 9c. Bemisia pupa parasitized by Encarsia formosa.
Success with Encarsia can be improved in a number of ways:
Eretmocerus eremicus and Eretmocerus mundus
E. eremicus parasitizes both GWF and Bemisia, while E. mundus is specific to Bemisia. Adult Eretmocerus resemble Encarsia in shape and size but are entirely yellow (Figure 10a). Although this wasp species will host feed on all four whitefly instars, it prefers to parasitize second and third instars. Adults live for 1-2 weeks and lay approximately 100 eggs during their lifetime. The majority of these eggs are laid within the first 6 days after emerging from their pupal cases. Because this wasp thrives at higher temperatures than Encarsia, they are best released later in the year, around March.
Parasitized whiteflies are a slightly different colour from those that are not parasitized, but this difference is not as noticeable as it is with Encarsia (Figure 10b and 10c). Tips for increasing success with this parasitoid are similar to those listed for Encarsia. Eretmocerus is very attracted to yellow sticky cards and tape, so it may be necessary to reduce the use of these tools, particularly when trying to establish an Eretmocerus population.
Figure 10a. Adult Eretmocerus eremicus.
Figure 10b. Greenhouse whitefly pupa parasitized by Eretmocerus.
Figure 10c. Bemisia parasitized by Eretmocerus.
Both adult (Figure 11a) and larval (Figure 11b) Delphastus feed on whiteflies, particularly on the eggs and nymphs (however, they avoid parasitized scales). This behaviour makes them compatible with the use of parasitic wasps. Use these beetles to supplement the activity of parasitic wasps for reducing whitefly populations. Adult Delphastus live for 6-9 weeks and reportedly need at least 10 whitefly eggs per day to reproduce. Their need for large numbers of whitefly to sustain their development makes them more suited to crops with higher tolerances for whiteflies, such as vegetable crops or cut gerbera.
Figure 11a. Adult Delphastus catalinae feeding on whitefly larva.
Figure 11b. Larva of Delphastus catalinae.
Dicyphus (Figures 12a and 12b) is a predatory bug with piercing/sucking mouthparts. These bugs are general feeders. However, they appear to prefer whiteflies, feeding particularly on the eggs and larval stages. To a lesser extent, they feed on aphids, spider mites, thrips, leafminer larvae and moth eggs. In the absence of prey, a high population of Dicyphus may cause damage to tomato fruits and flowering crops such as gerbera. To manage whiteflies, it is best to use Dicyphus together with the parasitic wasps Encarsia formosa and Eretmocerus eremicus. The use of banker plants such as mullein (Verbascum thapsus) (Figure 12c) may help establish Dicyphus when whitefly population densities are low.
Figure 12a. Adult Dicyphus hesperus.
Figure 12b. Light-green, recently emerged adult Dicyphus.
Figure 12c. Mullein banker plant in tomato crop.
There are three forms of physical control for whiteflies.
Figure 13. Use of yellow tapes for mass trapping.
Figure 14a. Hand-vacuuming adult whiteflies in tomato crop.
Figure 14b. Hand-vacuuming adult whiteflies in gerbera.
Whiteflies can develop resistance to many pesticides. Judicious use will extend the useful life of pesticides and delay build-up of resistance. Always use pesticides in conjunction with a regular monitoring program, using action thresholds and rotating chemical classes. At the same time, make use of all available control strategies.
For more information:
Toll Free: 1-877-424-1300