P.C., J.W. Diehl, T.J.
Dennehy, and S.E.
Naranjo. 1995 (Rev. 11/2000). Sampling Sweetpotato Whiteflies
in Cotton . IPM Series No. 2. Publ. No. 194023. University of Arizona,
College of Agriculture and Life Sciences, Cooperative Extension, Tucson,
Arizona. URL: http://cals.arizona.edu/crops/cotton/insects/wf/wfsampl.html
Whitefly Management Starts With Sampling
Because whitefly numbers in cotton can vary from year to year, sampling
helps you avoid costly errors associated with whitefly control:
Sampling should begin once whiteflies are first found in sweep samples and
continue through crop termination. Following the sampling plan outlined
here should add, on average, only eight minutes to any field scouting procedure.
In addition, this method provides a common currency for comparing whitefly
densities among fields, even if counts are taken by different samplers.
- making unnecessary treatments, and
- treating populations too late to achieve adequate control.
Action Thresholds & Percent Infested Leaves
Research shows that insecticides should be applied when a threshold density
of 5 whitefly adults per leaf is present. However, counting exact numbers
of adult whitefly on every leaf is time-consuming and unneccessary. Because
a predictable relationship exists between numbers of adult whiteflies per
leaf and the percentage of leaves infested with just three or more whitefly
adults, no more than three whiteflies need to be counted on a given leaf.
This method increases sampling efficiency and has been confirmed on over
8000 acres of commercial cotton under a wide variety of field conditions.
HOW TO SAMPLE
- DESIGNATE MANAGEMENT UNIT
The size of the area to sample for whiteflies will depend on your
management practices. A management unit is a fairly uniform area of
the same cultivar planted at about the same time. This sampling plan
is for an average field size of 40-80 acres. If your field is larger
than this, take samples from additional sites within the field. Also,
if you plan to implement control in field edges and field centers
separately, then these two areas should be designated as separate
management units and sampled separately. Sample at least weekly.
Samples can be taken any time of day, but samples taken within 24
hours after a rain may not accurately reflect whitefly densities.
- CHOOSE A PLANT TO SAMPLE
Start sampling at least 10 rows into the field. Avoid sampling plants
disturbed by sweep sampling. Choose a plant at random taking
care not to single out tall or heavily infested plants. If you intend
to treat field edges separately, select plants within the first 10
- CHOOSE A LEAF
Once a plant is selected, choose a single main stem node leaf
from the general region of the fifth node down from the terminal-the
first unfolded leaf is main stem node #1 (Figure 1).
Count the nodes on a couple of plants before you begin sampling to
familiarize yourself with the approximate position of the fifth main
stem node. Then make observations from leaves within that general
area of the plant.
- MAKE WHITEFLY COUNTS
Keeping your shadow off the plant, carefully turn the leaf over
by the tip of the leaf blade or the petiole. Tally the leaf as infested
if it contains 3 or more whitefly adults and as uninfested if it contains
less than 3.
- SAMPLE AT LEAST 30 PLANTS
Continue sampling along a diagonal or zigzag line moving over several
rows and taking 5-10 steps before selecting a new plant. The individual
plants sampled should be 10- 15 feet apart. After sampling 15 plants,
move to a new site within the field and sample 15 more (Figure 2).
- CALCULATE THE PERCENTAGE OF INFESTED LEAVES
% infested = infested leaves X 100 total leaves sampled
- COMPARE THIS PERCENTAGE TO THE THRESHOLD OF 57%
* An infested leaf has 3 or more adults.
Track your population relative to the recommended threshold of 5
adults per leaf. This threshold corresponds to 57% of leaves infested.
The adjacent table shows the relationship between average adults per
leaf and percentage of leaves infested. Treat the field if your observed
percentage is greater than 57% and do not spray if it is less. If the
observed percentage is close to the threshold, then you may want to
take additional samples from another site in the field and re-calculate
the percentage. Continue sampling weekly, treating whenever the threshold
Count Sweetpotato Whiteflies Only !
Do not include bandedwinged whiteflies in your whitefly counts because
they rarely warrant pesticide treatment. The bandedwinged whitefly is
often found in cotton, wild sunflower, weeds, or melons in the early spring
and summer. It has gray-colored bands on the wings, while the sweetpotato
whitefly has no bands. The body of the adult bandedwinged whitefly is
ashen-gray in color while the sweetpotato whitefly is a creamy-yellow
Issued in furtherance of Cooperative Extension work, acts
of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture,
James A. Christenson, Director Cooperative Extension, College of Agriculture
and Life Sciences, The University of Arizona.
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