Pyrethroid pesticides exhibit strong chiral characteristics and often consist of multiple chiral enantiomers. While these enantiomers possess identical physicochemical properties, they exhibit distinct insecticidal activity, biotoxicity, and environmental residue levels. Examples include cypermethrin, Beta-Cypermethrin, Alpha-Cypermethrin, and Lambda-Cyhalothrin.
Cypermethrin:
Cypermethrin, the most widely used pyrethroid pesticide, has three chiral centers in its molecular structure and eight enantiomers. The biological activity and toxicity exhibited by the different enantiomers vary significantly.
Cypermethrin consists of eight optical isomers, forming four racemic pairs. The different isomers of cypermethrin exhibit significant differences in their insecticide efficacy and photolysis rates. Their insecticidal activity, from strongest to weakest, is as follows: cis-, trans-, and cis-trans-cypermethrin.
Of the eight isomers of cypermethrin, two of each of the four trans- and four cis-isomers are highly potent.
However, using a single highly potent isomer of cypermethrin as an insecticide not only significantly enhances its insecticidal activity but also reduces toxicity to non-target organisms and adverse environmental impacts. Therefore, beta-cypermethrin and alpha-cypermethrin were developed:
Alpha-cypermethrin
Alpha-cypermethrin is also known as α-cypermethrin. It separates two ineffective or ineffective isomers from a mixture of four cis isomers, resulting in a mixture containing only two highly effective cis isomers in a 1:1 ratio.
Alpha-cypermethrin has twice the insecticidal activity of cypermethrin.
Beta-cypermethrin
Beta-cypermethrin, also known as highly effective cis-trans cypermethrin, is a mixture of the ineffective isomers of the eight-isomer technical of industrial cypermethrin. This is achieved by catalytically isomerizing the ineffective isomers of the technical form of cypermethrin to the highly effective isomers. This results in a racemic mixture of the highly effective cis and trans isomers, i.e., four isomers in a cis:trans ratio of approximately 40:60 or 2:3.
Beta-cypermethrin has the same insecticidal properties as cypermethrin, but its insecticidal potency is approximately twice as high.
Beta-cypermethrin is significantly less toxic to humans and animals, and its toxicity to sanitary pests is equal to or greater than that of alpha-cypermethrin, thus offering certain advantages in sanitary pest control.
Summary:
Since the cis-isomer is generally more bioactive than the trans-isomer, the order of insecticidal activity among the three cypermethrins should be: alpha-cypermethrin ≥ beta-cypermethrin >> cypermethrin.
However, beta-cypermethrin exhibits superior sanitary pest control efficacy compared to the other two.
