GROWER 101: Whiting Out Whiteflies

August 9, 2002 - 11:12

With whitefly control, it is best to get on the starting block as soon as young plants arrive at your greenhouse. Here?s how.

Whiteflies can be very damaging to greenhouse crops and
field and greenhouse-grown cut flowers because of their broad host range,
resistance to insecticides and potential to vector a variety of plant virus
diseases. High populations of whiteflies can weaken plants, causing chlorotic
foliage and reduced vigor.

There are several whitefly species, but the two that cause
the 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 range
of floral crops. In addition, their sheer presence deters customers, causing
economic loss to growers and sellers of plants.

Whitefly is a general pest of many greenhouse plants, but
not 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 of
Gilead, African blue basil, sage, lavender and salvia for whitefly feeding.

Identifying

Whitefly eggs appear very tiny and spindle-shaped, usually
stand vertically on the leaf surface and are attached to the leaf by a tiny
pedicel or “stalk” at the base of the egg. For many species, eggs
are white when first laid and turn dark gray (greenhouse whitefly) or
amber-brown (silverleaf whitefly) with time. The four nymphal stages are
identified by their relative sizes; length and width increase with each
successive molt. The best life stage to use for identification is the
“pupal” stage, which is the last nymphal stage before adult
emergence. A 10x hand lens or dissecting microscope will be needed to examine
pupae closely enough to see these characters and differentiate accurately
between the species.

Pupa of the greenhouse whitefly are oval and have elevated
sides that are very straight and perpendicular to the leaf surface. Seen from
above, the greenhouse whitefly, unlike the siverleaf whitefly, has a tiny
“fringe” of wax filaments around the top “rim” of the
pupa. There are several pairs of long wax filaments that arise from the top
surface of the pupa, especially on hairy leaves. Adult greenhouse whiteflies
are somewhat larger than silverleaf whiteflies. The wings lie fairly flat over
the abdomen, almost parallel with the leaf surface. When viewed from above, the
body 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 seen
through the wings.

The pupa of the silverleaf whitefly appears from side view
to be more rounded, dome-shaped or even pointed. Several pairs of longer wax
filaments may arise from the top surface of the pupa, but these are usually
shorter on silverleaf whitefly compared to greenhouse whitefly. The color of
silverleaf whitefly nymphs tends to be more yellow than the whitish greenhouse
whitefly nymphs. The wings of the adult silverleaf whitefly are held close and
tent-like against the abdomen at approximately a 45-degree angle to the leaf
surface.

Monitoring

Sampling for whiteflies is critical to establish whether a
treatment threshold has been reached and determine whether a treatment is
effective. Whitefly infestation can be monitored using a combination of yellow
sticky traps and foliage inspection. Yellow sticky cards work in greenhouses
but are impossible to use outdoors. They attract so many other species of
insects that the cards become “unreadable.” The location and
relative numbers of adults can be monitored with yellow sticky traps, while
nymphs must be monitored by frequent foliage inspection. Monitoring for nymphal
stages is crucial for predicting infestations.

Sticky cards are best used at 1-2 cards per 1,000 sq. ft. of
growing area and should be checked on a minimum of a weekly basis. The
threshold of adults found on a card per day and the numbers of nymphs per leaf
often changes according to the maturity of the crop. Early in a crop cycle, a
grower may tolerate 0.5 whiteflies per day on cards. Near sale, growers may
have an increased tolerance of adults, allowing two whiteflies per card per
day.

Eliminating

If you have a small number of plants, you can do a visual
inspection of the foliage for the presence of whitefly eggs, sessile stages and
adults. Separate infested plants and make plans to treat them. For a large
number of plants, immediately put out sticky cards (at least one card per 1,000
sq ft), and determine if you have whitefly “hotspots” that need to
be 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 after
you pinch back poinsettia plants. The standard practice for most greenhouse
managers growing poinsettia crops has been to treat the substrate with a drench
or granule application of imidacloprid (Marathon) about 7-10 days after
pinching the plants. Syngenta Professional Products is still working on EPA
approval of its systemic insecticide Thiamexthoxam (Flagship) for greenhouse
use. In our trials, Thiamexthoxam has worked well on whitefly, but it looks
like growers will have to wait for EPA approval for this product.

If whiteflies are a problem on your poinsettia crop, later
in the season you can usually clean up a population with applications of
Sanmite (pyridaben), Avid (abamectin) or Dithio (sulfotepp — if you can
find it on the market). More information about pesticide efficacy can be found
on page 58.

About The Author

Stanton Gill is a regional specialist at the CMREC, University of Maryland Cooperative Extension and an adjunct professor at Montgomery College, Montgomery County, Md. He may be reached by phone at (301) 596-9413 or E-mail at sg10@umail.umd.edu.

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