Toxicity, retention and distribution of [14c]hexaflumuron in the last larval instar of Leptinotarsa decemlineata, Spodoptera littoralis and Spodoptera exigua

The toxicity of the benzoylphenylurea hexaflumuron to newly ecdysed last‐instar larvae of Leptinotarsa decemlineata, Spodoptera littoralis and Spodoptera exigua was determined, together with the rate of retention and distribution of hexaflumuron during a 72‐h period. The results of the latter were considered in connection with the toxicity of this compound. Hexaflumuron was 54 and 19 times more toxic to L. decemlineata and S. littoralis respectively than to S. exigua. The amount of radioactivity, expressed as [14C]hexaflumuron retained in the larval body of L. decemlineata was high compared to that in the two other insect species. After 72h, >90% of ingested [14C]hexaflumuron was excreted by last‐instar larval S. littoralis and S. exigua, compared with 76% excreted by L. decemlineata larvae. The amount of radiolabelled residues recovered from the different tissues of L. decemlineata was more than five times greater than that recovered from S. littoralis and S. exigua after 24 h. This difference substantially increased after 48 and 72 h, reflecting a faster excretion, especially in S. exigua. These results suggest that the greater toxicity of hexaflumuron towards L. decemlineata is probably due to a greater retention of hexaflumuron in the larvae. Similarly, the greater toxicity of hexaflumuron towards S. littoralis compared with S. exigua larvae may result from a greater retention of pesticide in the former species.