Scouting Effects on Applications
Western flower thrips cause direct damage by feeding on foliageand flowers, and indirect damage by transmitting the tospoviruses tomatospotted wilt and impatiens necrotic spot virus. Both thrips and viruses have abroad host range. As a result, there is a very low tolerance for the presenceof thrips in most commercial greenhouses. However, the level of thripstolerance may vary with the crop production system. In the absence of anydisease, crops grown for their foliage and flowers have a much higher tolerancefor thrips numbers compared with crops grown primarily for the flowers, becausethrips 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 grownfor the fruit, which is less susceptible to thrips injury. Tolerance of thripspopulations allows greenhouse producers to structure their pest managementprograms around changes in pest numbers rather than reacting to the simpledetection of pests.
Monitoring with colored sticky cards is commonly recommendedto help greenhouse producers track increases and declines in pest populationsthroughout the growing season and to assess which management strategies areproviding control. Using sticky cards can also help growers to determine thespatial distribution of a pest, identify portions of fields or greenhouseranges where pests are present, and target localized populations of pests withinsecticides. A pattern of localized applications might slow the onset ofinsecticide resistance, a problem that has been particularly troublesome in themanagement of western flower thrips.
Although greenhouses are capable of harboring western flowerthrips throughout the year, population increases may be fostered by warmertemperatures and longer day lengths. Growers monitoring for western flowerthrips should observe seasonal increases in thrips numbers in the spring andsummer months. In temperate regions where day length and temperaturefluctuations are large, some growers may be able to identify times of the yearwhen the seasonal abundance of western flower thrips tends to stay low, andpest management inputs, such as insecticides, are minimal, thus possibly savingcosts in labor and insecticide purchases. The objective of this study was totest this hypothesis by tracking the population dynamics of western flowerthrips in a greenhouse over a two-year period.
Materials and Methods
This study was conducted at the Purdue UniversityHorticulture 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 x61?2 feet, and 6 inches in depth. An existing crop of intermixed red andwhite cut carnations were used for the study. The plants were grown undernatural daylight conditions. Greenhouse temperatures throughout the year rangedfrom 57-78 ¡F with a relative humidity between 60-70 percent. The growingmedium in the soil benches consisted of 50 percent soil, 25 percent perlite and25 percent vermiculite. Blue 3- x 5-inch sticky cards (Hummert International)were used to monitor adult western flower thrips. We chose blue sticky cardsbecause they have been shown to be highly attractive to adult western flowerthrips. Sticky cards were placed approximately 2-3 inches above the cropcanopy, where thrips are most active. Two sticky cards were used on each offive benches in the greenhouse. Western flower thrips adults were counted onfresh sticky cards replaced weekly during winter and twice weekly in latespring, summer and fall. To determine whether the average weekly catch in thegreenhouse was above a 20-thrips-per-card threshold, we prorated the counts ofindividual cards collected by multiplying the average daily card catch byseven.
Populations of adults and immature thrips on flowers wereestimated from a sample of two randomly selected open flowers on each of thefive benches. The greenhouse assistant was the individual responsible forscouting and would blow into each selected flower to count the number of thripspresent in the open blooms and assess thrips flower injury (petal distortion).Less than one minute was spent for each flower. Blooms were consideredacceptable or unacceptable based on a subjective evaluation. Flower injury wassubjectively 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-percentpetal distortion; 4 = up to 75-percent petal distortion and 5 = over 75-percentpetal distortion). All scouting information was recorded in a Microsoft Exceldatabase system.
The following currently available insecticides and rateswere used during the 24-month study when thrips numbers exceeded the actionthreshold: Avid (abamectin; Syngenta Professional Products) at 5.0 fl.oz. per100 gal.; Duraguard (chlorpyrifos-microencapsulated; Whitmire Micro-GenResearch Labs) at 30.0 fl.oz. per 100 gal.; Orthene (acephate; Valent USACorporation) at 7.5 oz. per 100 gal.; Orthene at 12.5 oz. per 100 gal. plusEnstar II (kinoprene; Wellmark Int’l) at 7.5 fl.oz. per 100 gal.; Orthene at7.5 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 15.0fl.oz. per 100 gal. (Other chemicals were applied, but are no longercommercially available.) All insecticide applications were made with ahydraulic high-volume power sprayer. No insecticide was used continuously formore than two weeks.
Although carnation flowers were harvested periodicallyduring the study, there was always a continuous supply of flowers all yearlong, and the plant population was maintained at a relatively consistent level.
Relationships among average estimates of thrips populationsin flowers, card catches and flower damage ranking on each sampling date wereassessed by Pearson’s correlation coefficient.
Results and Discussion
In our study, the numbers of western flower thrips werehighest 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 othercropping systems and is most likely due to the higher reproduction rate andfaster development time at high summer temperatures. This may also be thereason why the insecticide applications were not able to prevent flower injuryfrom occurring despite two to three insecticide applications per week duringthe summer months. However, scouting for thrips using the blue sticky cardsallowed us to identify December through March as a time of the year when weeklypopulations were below 20 thrips per card, and as a result, no insecticideswere applied (See Figure 2, page 32). This suggests that greenhouse producersdon’t need to use insecticides continuously during the year, as thrips numbersmay be low enough during certain times that sprays are not warranted. This thenlowers worker exposure to insecticide residues and possible allergic reactions,reduces environmental contamination, decreases labor costs associated withmaking applications, prevents possible flower injury (phytotoxicity) and mayeven lessen the potential for thrips populations to develop a resistance toinsecticides.
Over the course of the study, the number of unacceptableblooms (with greater than 25-percent petal distortion) was greatest during themonths when western flower thrips were most abundant (See Figure 3, page 32).Although we did not determine whether consumers will tolerate up to 25-percentdistortion in carnations, other studies have reported public tolerances ofbetween 20- to 25-percent petal distortion on chrysanthemum blooms.
The seasonal periodicity of estimates of thrips abundanceand flower damage suggests that these measures should be correlated. Thishypothesis is generally supported by our correlation analyses. In years 1 and2, the daily estimate of thrips caught on cards was significantly correlatedwith estimates of thrips in flowers (year 1: r = 0.40, n = 59, P = 0.0016; year2: 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). Thripscounts per flower, however, were not correlated with rank of flower damage inyear1 (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 weakrelationship between our estimates of thrips numbers per flower and flowerdamage.
Sticky cards provide a coarse measure of thrips populationsthat can be used to identify times of year when thrips must be managedintensively. The threshold value of 20 thrips per card per week appeared to beadequate for these purposes. Furthermore, our findings support the arbitraryaction threshold of 10-20 thrips per sticky card per week to time insecticideapplications. However, the abundance of damaged flowers in the summer monthsindicates that ç some refinements are needed during the times of yearwhen thrips populations are most active and likely to damage flowers in thegreenhouse. Other studies have indicated that sampling open blooms for adultwestern flower thrips provides better information and may be more costeffective in making management decisions when thrips are abundant.
Our findings show that maintaining detailed records andassessing population trends are important in making fact-based pest managementdecisions on when to apply insecticides. In addition, this may allow greenhouseproducers to incorporate the use of biological control agents such asparasitoids and/or predators into their pest management programs. There isminimal information available to greenhouse producers to associate scoutingwith a reduction in insecticide use and greenhouse producers need to identifyreasons to justify the costs of scouting whether it is hiring a professional scoutor designating an employee. This case study over a two-year period is the firstto demonstrate that routinely scouting for thrips throughout the year can leadto fewer insecticide applications and thus possible cost savings in labor andinsecticide purchases.
In conclusion, despite their limitations, sticky cards arelikely to increase greenhouse producer adoption of integrated pest managementpractices by engaging them in a process that helps them identify times of theyear when insecticide use is not required for thrips management.
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