Transport and kinetics of diflubenzuron and pyriproxyfen in the beet armyworm Spodoptera exigua and its predator Podisus maculiventris

Transport and retention of the insect growth regulators (IGRs) diflubenzuron and pyriproxyfen in larvae of the beet armyworm Spodoptera exigua (Hübner) and in nymphs of the predatory bug Podisus maculiventris (Say) were investigated. In a first experiment, the retention of orally administered [¹⁴C]radiolabeled isotopes of both compounds in fifth-instar larvae of the beet armyworm was studied. Rate of excretion of both IGRs in S. exigua caterpillars was high, with a 50% excretion time of approximately 6 h after intake. In a second experiment, the transport of the compounds from prey to predator and their retention inside the predator were studied. Fifth-instar nymphs of P. maculiventris were allowed to feed on caterpillars that had been given contaminated food. For both diflubenzuron and pyriproxyfen, more than 80% of the amount of radiolabel applied was recovered in consumed prey. Low levels of radioactivity (c. 3% of the applied amount of radiolabel) were also found in the fluid regurgitated by the prey larvae when attacked by the predatory bugs. Relatively small amounts of radiolabel (c. 8 and 15% of the amount orally applied to the prey for diflubenzuron and pyriproxyfen, respectively) were ingested by P. maculiventris nymphs when feeding on beet armyworm caterpillars. The data suggest that the predators did not use gut content as food. The pattern of excretion in nymphs of P. maculiventris differed between compounds. For diflubenzuron, there was a drastic decrease of radioactivity inside the predator body of around 40% within the first 6 h and then the level of retained radiolabel remained stable at 3-4% up to 72 h. For pyriproxyfen, a slow decrease of radioactivity inside the body was observed and at 72 h only 2% of the applied quantity was detected. Results of this study are discussed in relation to the findings from previous studies on the toxicity of both IGRs to P. maculiventris.