Research Matters - Prolonging activity of pyrethroids

Pyrethrum is an effective insecticide with low toxicity to humans, but it is rapidly destroyed by ultraviolet light.

Modern, synthetic pyrethroids are generally more stable. Recently, specialised plastic films have been developed for protected cropping, and while some transmit more ultraviolet than normal, others block ultraviolet transmission.

In the experiments reported here, open-ended tunnels were clad with three film plastics - a standard film, one that transmitted more ultraviolet than the standard and an ultraviolet-opaque film. Control areas were outdoors.

Strips of cotton - soaked in a 0.04 per cent solution of the synthetic pyrethroid cypermethrin - were air-dried and attached to hardboard. Control strips contained no pyrethroid. Each week, plastic cages containing two adult "confused flour beetles" were placed in contact with the cotton strips and the condition of the beetles was assessed 48 hours later.

The effective life of cypermethrin increased as the ultraviolet-blocking ability of the film-plastic cladding increased. Indeed, the pyrethroid still killed beetles even after it had stood under the ultraviolet-opaque film for six months.

Evidently, if cypermethrin is used under such films, more time must elapse before beneficial insects can be introduced. However, the ability to prolong the life of pyrethroid insecticides could be an advantage for some growers.

- Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick

Plastic Films for Polytunnels Can Prolong the Effective Residual Life of Cypermethrin to Over Six Months by van Emden and Hadley (2011). Journal of Horticultural Science & Biotechnology 86 (2): 196-200. The authors' abstract of the manuscript can be seen in full at

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