Scouting Effects on Applications

December 12, 2003 - 08:22

Research shows scouting reduces the number of insecticide applications.

Western flower thrips cause direct damage by feeding on foliage
and flowers, and indirect damage by transmitting the tospoviruses tomato
spotted wilt and impatiens necrotic spot virus. Both thrips and viruses have a
broad host range. As a result, there is a very low tolerance for the presence
of thrips in most commercial greenhouses. However, the level of thrips
tolerance may vary with the crop production system. In the absence of any
disease, crops grown for their foliage and flowers have a much higher tolerance
for thrips numbers compared with crops grown primarily for the flowers, because
thrips damage to flowers is the main concern.

Similarly, greenhouse-grown vegetables, such as pepper
(Capsicum annuum), have a higher tolerance for thrips because they are grown
for the fruit, which is less susceptible to thrips injury. Tolerance of thrips
populations allows greenhouse producers to structure their pest management
programs around changes in pest numbers rather than reacting to the simple
detection of pests.

Monitoring with colored sticky cards is commonly recommended
to help greenhouse producers track increases and declines in pest populations
throughout the growing season and to assess which management strategies are
providing control. Using sticky cards can also help growers to determine the
spatial distribution of a pest, identify portions of fields or greenhouse
ranges where pests are present, and target localized populations of pests with
insecticides. A pattern of localized applications might slow the onset of
insecticide resistance, a problem that has been particularly troublesome in the
management of western flower thrips.

Although greenhouses are capable of harboring western flower
thrips throughout the year, population increases may be fostered by warmer
temperatures and longer day lengths. Growers monitoring for western flower
thrips should observe seasonal increases in thrips numbers in the spring and
summer months. In temperate regions where day length and temperature
fluctuations are large, some growers may be able to identify times of the year
when the seasonal abundance of western flower thrips tends to stay low, and
pest management inputs, such as insecticides, are minimal, thus possibly saving
costs in labor and insecticide purchases. The objective of this study was to
test this hypothesis by tracking the population dynamics of western flower
thrips in a greenhouse over a two-year period.

Materials and Methods

This study was conducted at the Purdue University
Horticulture and Landscape Architecture Department greenhouses, West Lafayette,
Ind. The greenhouse dimensions were approximately 36 x 391?2 feet,
oriented north-south. The greenhouse contained five upright soil benches 36 x
61?2 feet, and 6 inches in depth. An existing crop of intermixed red and
white cut carnations were used for the study. The plants were grown under
natural daylight conditions. Greenhouse temperatures throughout the year ranged
from 57-78 °F with a relative humidity between 60-70 percent. The growing
medium in the soil benches consisted of 50 percent soil, 25 percent perlite and
25 percent vermiculite. Blue 3- x 5-inch sticky cards (Hummert International)
were used to monitor adult western flower thrips. We chose blue sticky cards
because they have been shown to be highly attractive to adult western flower
thrips. Sticky cards were placed approximately 2-3 inches above the crop
canopy, where thrips are most active. Two sticky cards were used on each of
five benches in the greenhouse. Western flower thrips adults were counted on
fresh sticky cards replaced weekly during winter and twice weekly in late
spring, summer and fall. To determine whether the average weekly catch in the
greenhouse was above a 20-thrips-per-card threshold, we prorated the counts of
individual cards collected by multiplying the average daily card catch by

Populations of adults and immature thrips on flowers were
estimated from a sample of two randomly selected open flowers on each of the
five benches. The greenhouse assistant was the individual responsible for
scouting and would blow into each selected flower to count the number of thrips
present in the open blooms and assess thrips flower injury (petal distortion).
Less than one minute was spent for each flower. Blooms were considered
acceptable or unacceptable based on a subjective evaluation. Flower injury was
subjectively quantified using a numerical rating scale that ranged from 1-5 (1
= no visible injur; 2 = up to 25-percent petal distortion; 3 = up to 50-percent
petal distortion; 4 = up to 75-percent petal distortion and 5 = over 75-percent
petal distortion). All scouting information was recorded in a Microsoft Excel
database system.

The following currently available insecticides and rates
were used during the 24-month study when thrips numbers exceeded the action
threshold: Avid (abamectin; Syngenta Professional Products) at 5.0 fl.oz. per
100 gal.; Duraguard (chlorpyrifos-microencapsulated; Whitmire Micro-Gen
Research Labs) at 30.0 fl.oz. per 100 gal.; Orthene (acephate; Valent USA
Corporation) at 7.5 oz. per 100 gal.; Orthene at 12.5 oz. per 100 gal. plus
Enstar II (kinoprene; Wellmark Int'l) at 7.5 fl.oz. per 100 gal.; Orthene at
7.5 oz. per 100 gal. plus Tame (fenpropathrin; Valent USA Corp.) at 12.5 fl.oz.
per 100 gal.); and Talstar (bifenthrin; FMC Turf & Ornamentals) at 15.0
fl.oz. per 100 gal. (Other chemicals were applied, but are no longer
commercially available.) All insecticide applications were made with a
hydraulic high-volume power sprayer. No insecticide was used continuously for
more than two weeks.

Although carnation flowers were harvested periodically
during the study, there was always a continuous supply of flowers all year
long, and the plant population was maintained at a relatively consistent level.

Relationships among average estimates of thrips populations
in flowers, card catches and flower damage ranking on each sampling date were
assessed by Pearson's correlation coefficient.

Results and Discussion

In our study, the numbers of western flower thrips were
highest from May through September for both years 1 and 2 (See Figure 1,
right). This is similar to the seasonal pattern of thrips observed in other
cropping systems and is most likely due to the higher reproduction rate and
faster development time at high summer temperatures. This may also be the
reason why the insecticide applications were not able to prevent flower injury
from occurring despite two to three insecticide applications per week during
the summer months. However, scouting for thrips using the blue sticky cards
allowed us to identify December through March as a time of the year when weekly
populations were below 20 thrips per card, and as a result, no insecticides
were applied (See Figure 2, page 32). This suggests that greenhouse producers
don't need to use insecticides continuously during the year, as thrips numbers
may be low enough during certain times that sprays are not warranted. This then
lowers worker exposure to insecticide residues and possible allergic reactions,
reduces environmental contamination, decreases labor costs associated with
making applications, prevents possible flower injury (phytotoxicity) and may
even lessen the potential for thrips populations to develop a resistance to

Over the course of the study, the number of unacceptable
blooms (with greater than 25-percent petal distortion) was greatest during the
months when western flower thrips were most abundant (See Figure 3, page 32).
Although we did not determine whether consumers will tolerate up to 25-percent
distortion in carnations, other studies have reported public tolerances of
between 20- to 25-percent petal distortion on chrysanthemum blooms.

The seasonal periodicity of estimates of thrips abundance
and flower damage suggests that these measures should be correlated. This
hypothesis is generally supported by our correlation analyses. In years 1 and
2, the daily estimate of thrips caught on cards was significantly correlated
with estimates of thrips in flowers (year 1: r = 0.40, n = 59, P = 0.0016; year
2: r = 0.73, df = 74, P<0.0001) and with the rank of flower damage (year 1:
r = 0.40, n = 59, P = 0.0016; year 2: r = 0.47, n = 74, P<0.001). Thrips
counts per flower, however, were not correlated with rank of flower damage in
year1 (r = 0.11, n = 59, P = 0.41), but they were correlated in year 2 (r =
0.36, n = 74, P = 0.0016). This inconsistency across years suggests a weak
relationship between our estimates of thrips numbers per flower and flower

Sticky cards provide a coarse measure of thrips populations
that can be used to identify times of year when thrips must be managed
intensively. The threshold value of 20 thrips per card per week appeared to be
adequate for these purposes. Furthermore, our findings support the arbitrary
action threshold of 10-20 thrips per sticky card per week to time insecticide
applications. However, the abundance of damaged flowers in the summer months
indicates that Á some refinements are needed during the times of year
when thrips populations are most active and likely to damage flowers in the
greenhouse. Other studies have indicated that sampling open blooms for adult
western flower thrips provides better information and may be more cost
effective in making management decisions when thrips are abundant.

Our findings show that maintaining detailed records and
assessing population trends are important in making fact-based pest management
decisions on when to apply insecticides. In addition, this may allow greenhouse
producers to incorporate the use of biological control agents such as
parasitoids and/or predators into their pest management programs. There is
minimal information available to greenhouse producers to associate scouting
with a reduction in insecticide use and greenhouse producers need to identify
reasons to justify the costs of scouting whether it is hiring a professional scout
or designating an employee. This case study over a two-year period is the first
to demonstrate that routinely scouting for thrips throughout the year can lead
to fewer insecticide applications and thus possible cost savings in labor and
insecticide purchases.

In conclusion, despite their limitations, sticky cards are
likely to increase greenhouse producer adoption of integrated pest management
practices by engaging them in a process that helps them identify times of the
year when insecticide use is not required for thrips management.

About The Author

Raymond Cloyd is assistant professor, extension specialist in Ornamental Entomology/Integrated Pest Management at the University of Illinois Department of Natural Resources and Environmental Sciences, Urbana, Ill. Clifford Sadof is professor in the Department of Entomology at Purdue University, West Lafayette, Ind. They may be reached by E-mail at

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