Insecticide components of wood protection formulations

Whilst the heavy metal-based wood preservatives have insect protection capability, the organic and organ-metallic systems require the addition of a specific insecticide. 

Most of the older generation of insecticides based on the chlorinated hydrocarbons (lindane, dieldrin, aldrin etc.) have now been removed from these end uses due to both toxicity and environmental concerns. However, the development of insecticides is a rapidly developing area of research with new generations of compounds demonstrating greater specificity, lower mammalian toxicity and reduced environmental burden. 

The selection of the most appropriate insecticide is dependant upon a large number of factors including cost, stability in the preservative formulation, required rate of action and stability in the commodity to be protected. No finite recommendations can be made and ultimate selection will result from research into the performance and permanence of the insecticide in both laboratory and field tests. The most common groups of insecticides used in wood preservation today can be categorized as follows  

1 Pyrethroids. More accurately described as the synthetic pyrethroids, this group of compounds has been used extensively in wood preservation for control of both beetles and termites. Activity, cost and performance characteristics vary depending upon the specific compound but most readily accepted for use in wood preservative formulations are permethrin, cypermethrin, deltamethrin, bifenthrin and cyfluthrin. In terms of activity against wood boring beetles, cyfluthrin is estimated to be 20 times more effective than permethrin, 10 times as effective as cypermethrin and twice as effective as deltamethrin when compared in laboratory evaluative procedures. However, in practice other factors need to be considered, particularly the relative vapour pressures (and therefore evaporative loss) and, in wood in soil contact, the rate of biotransformation by colonising bacteria. The pyrethroids are effective as neurotoxins, and are axonic poisons. 

2 Chloronicotinyl and neonicotinoids. Active as neurotoxins, this group includes the compounds imidacloprid, thiocloprid and thiamethoxam (Xamox). These compounds demonstrate a broad spectrum of activity including beetles and termites. These are very new chemistries for the wood protection industry although some products have already been registered in a few countries for lower hazard end uses. Both imidacloprid and thiocloprid are very specific in their activity towards insect nervous tissue as mimics of the neurotransmitter acetylcholine. 

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