The present invention provides an insect or arthropod pesticidal composition. The composition includes active ingredients having a mixture of fatty acids, each of the fatty acids having a straight carbon chain from 6 to 12 carbon atoms long. The fatty acid mixture includes a first fatty acid molecule having a straight carbon chain from 6 to 8 carbon atoms long, and a carboxylic acid group. The fatty acid mixture also includes a second fatty acid molecule having a straight carbon chain from 8 to 12 carbon atoms long, and a carboxylic acid group. The composition also includes a carrier that promotes adherence or absorption or transport across the surface of insects or arthropods to effect their incapacitation or death.