GROWER 101: Whiting Out Whiteflies
Whiteflies can be very damaging to greenhouse crops andfield and greenhouse-grown cut flowers because of their broad host range,resistance to insecticides and potential to vector a variety of plant virusdiseases. High populations of whiteflies can weaken plants, causing chloroticfoliage and reduced vigor.
There are several whitefly species, but the two that causethe majority of problems to greenhouse growers are the silverleaf whitefly,Bemisia argentifolii, and the greenhouse whitefly, Trialeurodes vaporariorum.One reason these whiteflies are serious pests is that they attack a wide rangeof floral crops. In addition, their sheer presence deters customers, causingeconomic loss to growers and sellers of plants.
Whitefly is a general pest of many greenhouse plants, butnot all plants are prime hosts for whitefly. Whitefly magnets include verbena,fuchsia, regal geranium, lantana, nicotiana, ageratum and of course,poinsettia. If you are growing herbs, keep a close eye on rosemary, Balm ofGilead, African blue basil, sage, lavender and salvia for whitefly feeding.
Whitefly eggs appear very tiny and spindle-shaped, usuallystand vertically on the leaf surface and are attached to the leaf by a tinypedicel or “stalk” at the base of the egg. For many species, eggsare white when first laid and turn dark gray (greenhouse whitefly) oramber-brown (silverleaf whitefly) with time. The four nymphal stages areidentified by their relative sizes; length and width increase with eachsuccessive molt. The best life stage to use for identification is the”pupal” stage, which is the last nymphal stage before adultemergence. A 10x hand lens or dissecting microscope will be needed to examinepupae closely enough to see these characters and differentiate accuratelybetween the species.
Pupa of the greenhouse whitefly are oval and have elevatedsides that are very straight and perpendicular to the leaf surface. Seen fromabove, the greenhouse whitefly, unlike the siverleaf whitefly, has a tiny”fringe” of wax filaments around the top “rim” of thepupa. There are several pairs of long wax filaments that arise from the topsurface of the pupa, especially on hairy leaves. Adult greenhouse whitefliesare somewhat larger than silverleaf whiteflies. The wings lie fairly flat overthe abdomen, almost parallel with the leaf surface. When viewed from above, thebody of the silverleaf whitefly can be seen through the wings when at rest.From the same viewpoint, the body of the greenhouse whitefly cannot be seenthrough the wings.
The pupa of the silverleaf whitefly appears from side viewto be more rounded, dome-shaped or even pointed. Several pairs of longer waxfilaments may arise from the top surface of the pupa, but these are usuallyshorter on silverleaf whitefly compared to greenhouse whitefly. The color ofsilverleaf whitefly nymphs tends to be more yellow than the whitish greenhousewhitefly nymphs. The wings of the adult silverleaf whitefly are held close andtent-like against the abdomen at approximately a 45-degree angle to the leafsurface.
Sampling for whiteflies is critical to establish whether atreatment threshold has been reached and determine whether a treatment iseffective. Whitefly infestation can be monitored using a combination of yellowsticky traps and foliage inspection. Yellow sticky cards work in greenhousesbut are impossible to use outdoors. They attract so many other species ofinsects that the cards become “unreadable.” The location andrelative numbers of adults can be monitored with yellow sticky traps, whilenymphs must be monitored by frequent foliage inspection. Monitoring for nymphalstages is crucial for predicting infestations.
Sticky cards are best used at 1-2 cards per 1,000 sq. ft. ofgrowing area and should be checked on a minimum of a weekly basis. Thethreshold of adults found on a card per day and the numbers of nymphs per leafoften changes according to the maturity of the crop. Early in a crop cycle, agrower may tolerate 0.5 whiteflies per day on cards. Near sale, growers mayhave an increased tolerance of adults, allowing two whiteflies per card perday.
If you have a small number of plants, you can do a visualinspection of the foliage for the presence of whitefly eggs, sessile stages andadults. Separate infested plants and make plans to treat them. For a largenumber of plants, immediately put out sticky cards (at least one card per 1,000sq ft), and determine if you have whitefly “hotspots” that need tobe treated. A good choice in summer is to treat with an insect growth regulator(IGRs).
A decision pops up in late August or early September afteryou pinch back poinsettia plants. The standard practice for most greenhousemanagers growing poinsettia crops has been to treat the substrate with a drenchor granule application of imidacloprid (Marathon) about 7-10 days afterpinching the plants. Syngenta Professional Products is still working on EPAapproval of its systemic insecticide Thiamexthoxam (Flagship) for greenhouseuse. In our trials, Thiamexthoxam has worked well on whitefly, but it lookslike growers will have to wait for EPA approval for this product.
If whiteflies are a problem on your poinsettia crop, laterin the season you can usually clean up a population with applications ofSanmite (pyridaben), Avid (abamectin) or Dithio (sulfotepp — if you canfind it on the market). More information about pesticide efficacy can be foundon page 58.