Quantification of canavanine, 2-aminoethanol, and cyanamide in Aphis craccivora and its host plants, Robinia pseudoacacia and Vicia angustifolia: effects of these compounds on larval survivorship of Harmonia axyridis.
The cowpea aphid Aphis craccivora that infests the black locust Robinia pseudoacacia shows toxicity to its predator, the multicolored Asian ladybird beetle, Harmonia axyridis. In contrast, the same aphid species that infests the common vetch, Vicia angustifolia, is suitable prey for H. axyridis larvae. Previously, it was reported that the toxicity of A. craccivora infesting R. pseudoacacia was due to canavanine and 2-aminoethanol, but there was some doubt about the toxicity of these compounds and their concentrations in the aphids. In the present study, we determined the concentrations of cyanamide, canavanine, and 2-aminoethanol in A. craccivora infesting the two host plants. In the extracts of A. craccivora that infested either of the host plants, canavanine was undetectable, and 2-aminoethanol was detected at the concentration of 3.0-4.0 μg/g fresh weight. Cyanamide was detected in the extract of A. craccivora that infested R. pseudoacacia (7.7 μg/g fresh weight) but not in that infesting V. angustifolia. The toxicity of canavanine, 2-aminoethanol, and cyanamide was evaluated against H. axyridis larvae in a bioassay by using an artificial diet containing these compounds at various concentrations. Cyanamide exhibited 10-100 times stronger toxicity than canavanine and 2-aminoethanol. These results indicate that the toxicity is at least partly due to cyanamide, which is present in the toxic A. craccivora that infests R. pseudoacacia but absent from the non-toxic A. craccivora that infests V. angustifolia.
