Western flower thrip Insecticide Resistance

Western Flower Thrips Cycle


Farmers face multiple challenges in the crop production stages. Among these challenges are pests and notably the western flower thrip. Customarily, farmers use pesticides to address Thrips. However, pesticides are not always efficient in controlling the pest. This is mainly because of resistance.

Farmers have increasingly reported cases of resistance towards pesticide both locally and internationally. The most notable signs have been; Reduced efficacy at the recommended rate, shorter insecticide residuality and increased use rates required to achieve efficacy.

According to the IRAC (Insecticide Resistance Action Committee) definition: “Resistance is a heritable change in sensitivity of a pest that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest.”

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Western flower thrip Overview

Western flower thrips (Frankliniella occidentalis) is an economically important pest in ornamentals, vegetables and fruits around the world, owing to its role as a virus vector, Its capacity to transmit the tomato spotted wilt virus (TSWV), together with its high polyphagous behaviour, its adaptation capacity and its high reproductive potential, makes repeated insecticide  treatment necessary for its control.

Western flower thrips are difficult to detect and control because of their small size, and tendency to hide in protected plant parts. Early signs of infestation are distorted terminals, deformed young leaves, stippled or scarred petals, buds, or leaves, and tiny greenish-black fecal specks on the plant.

Use of insecticides has been the primary strategy for controlling western flower thrips, which has led to the development of resistance to the major insecticide groups.

Insecticide resistance is more alarming because fewer compounds are available owing to rising standards of environmental and toxicological safety. Maintaining sufficient pesticide diversity allows compounds that do not show cross-resistance to be to be applied in alternating sequence. However, It is important to preserve existing insecticides by developing and implementing resistance management strategies.


  • The overlapping generations of western flower thrips increases the exposure of different life stages  to insecticides thereby promoting resistance.
  • The haplodiploid sex determination characteristic of thrips. In this type of sex determination, females are diploid, but males are haploid. Because males are haploid, their alleles are exposed directly to selection, which enable alleles for resistance to become fixed rapidly in a population
  • The inherent metabolic detoxification pathways predispose the western flower thrips to overcome insecticides.
  • Western flower thrips has a short life cycle and high female fecundity, which enhances the potential to develop resistance. At temperatures between 20 and 25°C, western flower thrips requires only 2–3weeks are  to develop from egg to adult, but at higher temperatures it takes less than 10days.
  • Repeated use of insecticide with one mode of action (MoA) removes susceptible western flower thrips from the population, leaving resistant western flower thrips to reproduce and pass on resistance traits to their offspring. Ultimately, resistant western flower thrips will dominate the population, and insecticide effectiveness will decrease.
  • Dosage: Application of Insecticide rates lower than the recommended application rate can accelerate resistance.
  • The continuous availability of suitable host plants enables Western flower thrips populations to be active and reproduce throughout the year

Warning Signs of Resistance Development.

  • Important to note: Reduced length of time with insecticide residual control and Increased use rates required to achieve effective control, tolerance is increasing, action must be taken to reduce selection pressure.

Resistance to Delay Strategies.

Resistance management tactics will not prevent resistance, they can only delay resistance.

The delay may be as long or longer than the product’s “life cycle.” However, we can influence some factors that contribute to the rate of resistance development. Selection pressure, which is a factor we can influence, is the primary contributor to resistance.

Selection pressure is influenced by many factors, but it is directly related to the “intensity” of insecticide exposure that a pest population experiences. The more an insecticide is used, the greater the selection pressure.

The most direct way to reduce selection pressure is to reduce the frequency of spraying insecticide with one MoA.

Reducing Selection Pressure

The key to managing resistance is to reduce selection pressure by rotating between insecticides with different modes of action and reducing the number of insecticide applications.

It may be necessary to use non-chemical control methods and to rotate insecticides that may not provide the highest levels of control.  The choices you make about which products to use and how often to use them directly impacts selection pressure. These decisions determine whether resistance develops and how quickly it develops. 

If you encounter poor Western flower thrips control after an insecticide application, DO NOT spray the same product again at a higher rate or shorter spray interval and hope for better control. Determine if poor control resulted from application error, equipment failure, or unfavourable environmental conditions during or after application. If none of these occurred then your Western flower thrips population may be becoming resistant.

At times farmers are  faced with difficult  choices when they experience pest complexes within a crop especially where only one chemistry is the primary tool for the management of Western flower thrips.

In such scenarios the rule of thumb is to maximize the use of other effective products for more secondary pests.

Case Study in Roses:

Thrips are a primary pest in roses and persist in the crop for multiple generations.

A rose producer follows the label for thrips by using Delegate 250WDG in two consecutive pest generations, and then rotates to a different mode of action insecticide.

The next month, caterpillars and leafminers required treatment and Delegate 250WDG was sprayed, but thrips have reinfested the field.

Multiple consecutive generations of thrips within the field were exposed to Delegate 250WDG

Correct Recommendation: Use a different IRAC MoA Group for caterpillars and leafminer, which are secondary pests.

Resistance Management in Western flower thrips.

The availability of multiple tactics and multiple insecticides with diverse MoA’s is essential for a successful integrated resistance management program. Utilize all effective  chemical and non-chemical measures to manage Western flower thrip. These include;

A good monitoring program that allows resistance to be caught early, is enough to enable resistance to be reversed or slowed.

Carefully follow the specific label guidelines within use directions especially in regard to resistance management,i.e. (Re-treatment interval and number of applications)

Avoid use of the same active ingredient or MoA on consecutive generations of insects within a single planting or across sequential plantings. However, two applications to reduce a single thrips generation is acceptable. Treat the next generation with a different active ingredient that has a different mode of action.

 Applications should be targeted against early insect developmental stages whenever possible.

Treat Western flower thrips only at threshold, 2-3 Thrips per bud is a good number to begin sprays.

Adjust application equipment and technique to ensure good plant coverage.

Monitor treated insect populations in the field for loss of effectiveness performance of insecticide.

Avoid using less than labelled rates of any insecticide when applied alone or in tank mixtures.

Base insecticide use on comprehensive IPM programs including crop rotations.

Use “soft” chemistries to maintain natural enemies.

Maximize the number of alternative host plants at field edges, such as Sunflower that harbor natural enemies of western flower thrips. Do not treat alternative hosts with herbicides or insecticides.

Use UV-reflective/Metallized plastic mulch to reduce the number of Western flower thrips adults that move into planted fields.

Maintain good sanitation by destroying crop material immediately after final harvest. This can be done by removing plastic and disking under the crop.

Migration patterns of the Western flower thrips between or among hosts at different times of the year, ensure that you clear the alternative hosts around the greenhouse eg; Black Nightshade, Lambs quarter, Chinese lantern, Leeks and Amaranthus.

Use only registered Insecticides


Resistance can still occur even if all users follow all label guidelines and restrictions. Availability and use of “effective” rotation insecticides overshadow all other factors in resistant management. The choices a crop producer makes about which products to use and how often to use them directly impacts selection pressure and determines whether resistance develops and how quickly it develops.

Ultimately Resistance Management is In the Hands of the Crop Producer”

Everlyne Pamba is the Integrated Field Scientist Lead, East South &Central Africa ESCA


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