WHITEFLIES damage crops by sucking plant sap, which weakens plants and causes shoot and leaf distortion. A more serious problem is the large amount of honeydew they secrete onto leaves and fruit. The honeydew is colonized by sooty molds, which reduces the quality of greenhouse vegetables and ornamentals. The main pest in greenhouses is the greenhouse whitefly (Trialeurodes vaporariorum). The sweet potato or silverleaf whitefly (Bemisia tabaci) is usually a problem on ornamentals, such as poinsettia. Sweet potato whitefly can also spread plant viruses. It is important to identify the species of whitefly correctly because the risk of spreading plant viruses differs between the two species as do treatment recommendations. It is difficult to identify whiteflies trapped on sticky traps, therefore examine adults and immature whiteflies on leaves. Both adults and larvae are found on the undersides of leaves, the adults mostly on upper plant leaves, the larvae lower down on the plant. Larvae are flattened, legless, translucent ‘scales’, 1/30 inch (0.8 mm) long. Adult whiteflies are 1/25 inch (1 mm) long and a powdery white.
Greenhouse Whitefly (Trialeurodes vaporariorum) adults are powdery white with wings that form a triangular profile. Pupae have setae arranged in a ring around the top edge of the scale.
Sweet Potato Whitefly (Bemisia tabaci) adults are smaller, less powdery, and slightly yellowish. Their wings form a tent-like canopy over their body, and their abdomen may be seen between the wings. Pupae have setae scattered across the scale.
LIFE CYCLE: A complete greenhouse whitefly life cycle takes 28 days at 68°F (20°C). Females lay pinpoint sized, black eggs in circular patterns on the undersides of leaves. Each female lays up to 200 eggs during her one week lifespan. The eggs hatch in 7 days and the mobile larvae (‘crawlers’) move over the leaves for 5 days, then settles and molts to a sedentary ‘scale’ stage. After 7 days and another molt, the last stage larva remains where it was feeding and pupates; adults emerge in 8 days. The life cycle of the sweet potato whitefly is similar, but the development time is shorter, and more eggs are laid.
CONTROL: To monitor whitefly, hang up yellow sticky cards (3″x 5″) at the rate of 1 trap/500-2000 ft² (50-200 m²) (see Lures & Traps). Traps should be placed just above the plant canopy (if sweet potato whitefly is present, lowering the traps within the canopy may give better results.) Whiteflies tend to remain in one area and gradually move out from the initial site. Check traps weekly for adult whiteflies; traps should be replaced every 3 weeks. Using higher densities of traps helps pinpoints infestation sites more accurately. If monitoring time is limited, however, it is more important to check and record counts on fewer traps (the lower rate) and do it weekly, than to check larger numbers of traps but at longer intervals. With small plants, examining the foliage of 1% of the plants will give a more accurate count. With small plants, examining the foliage of 1% of the plants will give a more accurate count than yellow traps. At 15 traps per 1000 sq.yrd. a count of 1 whitefly per 15 traps is approximately equivalent to 0.1 individual whitefly per plant or 10/100plants. Starting levels of whitefly above 0.1 per plant can lead to hot spots and control problems. Sound Horticulture also offers the parasitic wasp Encarsia formosa and the predatory beetle Delphastus catalinae as whitefly control. Encarsia and Delphastus work well in tandem because Delphastus avoids feeding on the whiteflies that have been parasitized by Encarsia, and Delphastus adults also feed on whitefly eggs (for more information, see Encarsia and Delphastus).