Method of insecticide delivery affects horizontal transfer of fipronil in the German cockroach (Dictyoptera : Blattellidae)

Horizontal transmission of insecticide occurs when foragers contact or inge st an insecticide, return to the aggregation or nest, and translocate the i nsecticide to the shelter and its vicinity. Relatively more sedentary membe rs of the population then contact or eat tire translocated insecticide and die. We evaluated three different methods of delivering fipronil to adult m ale German cockroaches, Blattella germanica (L.), for their potential to ca use such secondary mortality ill various developmental stages of the cockro ach. Adult males topically treated with 5 ng of fipronil (approximate to LD 99) caused low mortality in untreated nymphs and no mortality in untreated adults within tile same aggregation. Males exposed to residual fipronil on a glass surface translocated more insecticide, resulting in higher mortalit y of cockroaches they contacted, but only early instars were affected and n o adult mortality was observed. Ingested fipronil bait, however, was most e ffectively translocated, and caused high mortality of untreated adults and nymphs. Ingestion of fipronil also caused greater secondary kill compared w ith a topical application of 25 ng, approximately the same amount recovered from the exterior of males that ingested 1 mg of 0.05% fipronil bait. Seco ndly mortality in the untreated population was significantly affected by th e duration of contact between the treated and untreated cockroaches, the qu antity and freshness of excretions from the treated insects, and the access ibility of the secretions to untreated cockroaches. The mechanisms that cau se secondary kill may include ingestion of excreted fipronil residues, cann ibalism of bait-fed cockroaches, as well as contact with fipronil-contamina ted substrates.