Put the brakes on thrips resistance

Western flower thrips, Frankliniella occidentalis, is still one of the most destructive insect pests in greenhouse production systems. Besides causing direct feeding injury to plants, western flower thrips vectors the tospoviruses impatiens necrotic spot virus and tomato spotted wilt virus.

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Spinosad is effective

Since its introduction in the 1990s, the principal insecticide relied on by greenhouse growers to control western flower thrips populations is Conserve, which contains the active ingredient spinosad.

Spinosad is derived from the Actinomycete bacteria Saccharopolyspora spinosa that contains metabolites called spinosyns. Conserve, the trade name for the formulation registered for use in greenhouses, contains spinosyns A (85 percent) and D (15 percent).

Spinosad is one of the most effective insecticides for controlling both western flower thrips nymphs and adults. This insecticide has both contact and ingestion activity. Control typically takes one to three days with residual activity up to two weeks.

The mode of action of spinosad entails excitation of the insect nervous system, which leads to paralysis and death. Spinosad’s mode of action is similar to the neonicotinoid-based insecticides and macrocyclic lactone insecticides/miticides. However, the neonicotionoid-based insecticides -- acetamiprid (TriStar), clothianidin (Celero), dinotefuran (Safari), imidacloprid ( Marathon) and thiamethoxam (Flagship) -- are active at a different target site than spinosad. The macrocyclic lactone insecticide/miticide abamectin (Avid) also binds to a different target site in the insect/mite nervous system.

Decreased sensitivity to spinosad

Just like any insecticide or miticide, continued reliance on spinosad will lead to potential resistant western flower thrips populations. For example, certain populations of western flower thrips, particularly in California, have expressed diminished sensitivity to spinosad. Furthermore, greenhouse growers throughout the United States have indicated that spinosad no longer provides the level of thrips control (based on mortality) that they had observed previously.

Research at the University of Illinois demonstrated that greenhouse populations of western flower thrips either developed resistance to spinosad or were already predisposed to be resistant to spinosad.

Western flower thrips was first reported in Australia in 1993. Populations of western flower thrips in Australia have been reported to be resistant to spinosad under laboratory conditions.

In general, western flower thrips resistance to most insecticides is recessive and primarily polyfactorial, which means that different resistance mechanisms confer resistance in different populations with variable mechanisms coexisting in the same population. Furthermore, a single gene may be responsible for resistance indicating that the level of resistance may be extremely high in a population. Certain insect pests including the diamondback moth (Plutella xylostella), tobacco budworm (Helicoverpa virescens), beet armyworm (Spodoptera exigua), oriental fruit fly (Bactrocera dorsalis) and common housefly (Musca domestica) have also demonstrated to be resistant to spinosad.

Preserving effective controls

To preserve the longevity of spinosad, greenhouse growers must rotate spinosad with other insecticides possessing different modes of action. These insecticides are registered for use on thrips and have different modes of action:

* Spinosad (Conserve): nicotinic acetylcholine receptor agonist.

* Methiocarb (Mesurol): acetylcholine esterase inhibitor.

*Chlorfenapyr (Pylon): oxidative phosphorylation uncoupler.

* Abamectin (Avid): gamma-amino butyric acid chloride channel activator.

*Acephate (Orthene): acetylcholine esterase inhibitor.

* Novaluron (Pedestal): chitin synthesis inhibitor.

* Kinoprene (Enstar II): juvenile hormone mimic.

Growers should also obtain the spray records from plant suppliers to determine which insecticides and rotations have been used. This will avoid exposing any existing western flower thrips populations to chemicals with similar modes of action that have been used previously thus decreasing the selection pressure on these thrips populations. Any western flower thrips populations that migrate into non-screened greenhouses from weeds or other crops (corn, soybean, vegetables and/or fruits) may already possess genes for resistance at very low frequencies due to previous exposure to insecticides that are similar to those labeled for use in greenhouses.

Spinosad is registered for use on agricultural crops and is available under different trade names including Entrust (organic formulation), Success (vegetables, fruits, herbs, ornamentals and forestry), SpinTor (leafy vegetables, root and tuber vegetables, fruits and citrus) and Tracer (cotton, chickpeas, soybean and sorghum).

Greenhouse growers must establish monitoring or scouting programs that will detect when western flower thrips populations are present or on the rise so as to appropriately time spinosad applications at the susceptible nymph and adult life stages. It is also extremely important that greenhouse growers employ proper sanitation and cultural practices, which may significantly alleviate western flower thrips problems.

Critical rotation programs

The days of new active ingredients regularly entering the marketplace for use in greenhouses are long gone. The ornamentals industry is still awaiting the registration of Overture (active ingredient is pyridalyl), which has activity on western flower thrips. However, once Overture is commercially available for greenhouse use, the prospect for any new active ingredients for thrips control is slim.

Delaying the rate of resistance to spinosad is possible. As such, it is mandatory that greenhouse growers develop and implement rotation programs that use different modes of action as opposed to solely relying on spinosad.

The benefit of a rotation program is to sustain the longevity and usefulness of spinosad in the future. Insecticide resistance is an international issue since the expanding international trade of plant material may not only spread western flower thrips populations among countries but may also spread populations of western flower thrips with resistance genes. As a result, international collaboration among greenhouse growers is required in order to avoid spreading resistant populations of western flower thrips throughout the world. Finally, monitoring crops for thrips, and implementing proper sanitation and cultural practices will avoid relying strictly on insecticides to manage western flower thrips. Realistically, spinosad resistance is just a matter of time.

Contact me if you are interested in additional information about insecticide or miticide resistance or if you require assistance in developing a rotation program for western flower thrips or any other greenhouse arthropod pests, including aphids, whiteflies, spider mites, mealybugs and fungus gnats.

Mention of any insecticides, both common and trade name, in this article does not constitute an endorsement.

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- Raymond Cloyd

July 2008