Vapor pressure/volatility pyrethroid insecticides

Since pyrethroids show contact toxicity, the first requirement for insecticides is contact between pyrethroids and insects (flying, crawling). To achieve this, a physical property of pyrethroids, i.e., vapor pressure, is an important indicator. High vapor pressure is usually associated with excellent volatilization however, as the vapor pressures of individual pyrethroids are diverse due to different measurement methods and conditions (for example, temperature), it is difficult to capture the entire picture. 

Pyrethroids generally have low vapor pressures. Although empenthrin (20) volatilizes at room temperature without external energy and is 31 times more volatile than allethrin, its vapor pressure is only 1/88 that of dichlorvos, a volatile organophosphorus compound. As profluthrin (40) is 13 times more volatile than allethrin and volatilizes gradually for 6 months to 1 year at room temperature, it is suitable as an insecticide for the protection of clothes. The vapor pressure of profluthrin is 10 mPa/25°C, almost the same as that of glycerin. 

The majority of pyrethroid insecticides have low volatilities. The heavily used synthetic pyrethroid permethrin is classified as nonvolatile on the basis of its vapor pressure (1.3 x 10 kPa at 20 °C) and is rarely found in indoor air. However, it has recently been reported to be the major pesticide residue found in house dust (USEPA, 2000d). Cyper-methrin [( )-a-cyano-3-phenoxybenzyl-( )-cA,frani -3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate] and cyfluthrin [cyano(4-fluoro-3-phenoxy-phenyl)methyl 3-(2,2-dichloro-ethenyl)-2,2-dimethylcyclopropanecarboxylate] are two other low-volatility pyrethroids commonly used for indoor flea and cockroach control. 

A number of studies have examined the role of various factors such as volatility and solubility on the efficacy of soil insecticides.(14) While the soil itself affects the efficacy of soil insecticides, the major determinate of biological activity is the amount of organic material in the soil.(15) Simmons, Lew, Silverman and Ali studied the effect of pyrethroids and some commercial insecticides on 3 instar southern com rootworm larva (Diabrotica undecimpunctata howardii).(16) They found that a combination of calculated lipophilicity and calculated volatility could predict soil pLCso based on the topical pLDso. The volatility was expressed as the log of the vapor pressure in nun Hg. We re-plotted the difference of the topical pLDso - soil pLCso with the calculated logP and calculated log volatility in a 3D graph shown in Figure 9. It can be seen as the compounds become more volatile (logVp < 5) that is a marked increase in soil toxicity. As the compounds become less lipophilic (more hydrophilic), they also become more toxic in the soil. 

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