Cooperative Extension
No. 3 (rev. 5/97)
Whiteflies In Arizona: Evaluation of Sampling Plans
The University of Arizona • College of Agriculture
Sampling and action threshold plans for
sweetpotato (a.k.a. silverleaf) whiteflies in
cotton are outlined in the University of Arizona
Cooperative Extension Publication (No.
196006), Sampling Sweetpotato Whiteflies in
Cotton. This trifold publication explains a
binomial sampling scheme using a leaf-turn
technique. These plans were evaluated last
summer within 190 commercial cotton fields
(8,000 acres) in the Laveen-Tolleson area. We
found this sampling protocol to be a practical
and efficient way to track whitefly populations.
How accurate is the sampling
model?
The model on which sampling recommendations
were based was evaluated by resampling the
field data from Laveen-Tolleson numerous times
on a computer. The figure below shows the
correspondence of the observed (dots) and
predicted (line) relationship between % infested
leaves and mean adults per leaf in a field. This
model generally over-predicted the mean number
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% Infested Leaves
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Mean Adults per Leaf
of adult whiteflies observed for densities greater
than 2 per leaf. This means that the recommendations are conservative, resulting in decisions to
spray just before reaching the target threshold.
Are 30 leaves enough?
The sampling model was further evaluated to
determine the amount of variation associated
with different sample sizes. On the average, a
sample size of 30 leaves was more than adequate
to make a treatment decision. However, the
variances were large, so it is not prudent to
sample less than 30 leaves, especially when
whitefly numbers are close to 50% infested.
How long does it take to
sample?
Samplers used stop watches to keep track of the
amount of time spent 1) walking from the truck
to the first sampled cotton plant, 2) sampling
whiteflies within the field and 3) walking back to
the truck. Based on these data, we estimate an
average of 8.5 minutes to sample 30 leaves in a
field. The actual amount of time will vary with
field condition and sampler experience.
Will different samplers get the
same results?
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February 14, 1995
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Whitefly counts were taken by 14 different
samplers within adjacent rows of a field on 12
different dates. These samplers varied in their
amount of experience in sampling whiteflies;
some had two years experience others had two
hours. As shown in the figure below, sampler
variation was not significant. This sampling
method, therefore, provides a common currency
for growers, PCAs and others to compare white-
How accurate is the sampling
method for decision-making?
4.5
P = 0.176
log-transformed
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For a threshold of 5 adults per leaf, the binomial
sampling method (infested-uninfested) provided
the same decision as counting the total number
of adult whiteflies on a leaf 1166 out of 1206
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% Infested Leaves
Adults (Sampler #2)
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Adults (Sampler #1)
fly numbers among fields, even when the samplers are different.
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times (96%). In 35 cases, the binomial method
called for spraying when densities were below
threshold, and 5 times this method called for not
spraying when densities were above threshold. It
is interesting that 4 of these 5 underestimates
occurred on a day following rain. Based on these
results, it is unlikely that a sampler would improperly assess the number of whiteflies by
using this binomial sampling method.
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To find out the effect of time of day on whitefly
numbers, 10 different fields were sampled
multiple times over the course of a day. The
figure below shows the change in whitefly
numbers counted over time in each field. No
consistent effect of time of day was observed. In
the 4 fields indicated with an asterisk the mean
number of whiteflies changed significantly with
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Can the sampling technique
be used any time of day?
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time. Twice the numbers increased and twice the
numbers decreased. Therefore, this method
should be adequate for sampling both in the
morning and in the afternoon.
How much does it cost to
sample for whiteflies?
The cost of sampling the Laveen-Tolleson fields
over an 8 week period was approximately $2.35
an acre. This breaks down to $1.20 / acre for
sampling, $0.23 / acre for travel, $0.72 / acre for
management, and $0.20 / acre for data processing.
Jon Diehl, Assistant in Extension, IPM
Peter Ellsworth, IPM Specialist
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Department of Entomology, Maricopa Agricultural Center, Maricopa, AZ