Laboratory evaluation of boric acid plus porridge oats and iodofenphos gel as toxic baits against the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae)

Adults of a field strain of Blattella germanica (L.) resistant to dieldrin and bendiocarb were used to evaluate two insecticide bait formulations, 50% boric acid plus 50% porridge oats and 0·25% iodofenphos gel in comparison with gel alone and culture food. All cockroaches from the boric acid plus oats treatment died in less than 20 days, whereas some that fed on iodofenphos gel, culture food or gel blank survived for 30 days. Both insecticides caused females to drop their oothecae prematurely, and fewer eggs from the boric acid plus oats treatments hatched. More cockroaches exposed to boric acid plus oats bait died outside the harbourages than those from the iodofenphos gel treatments. Boric acid plus oats or iodofenphos gel offered together with culture food were eaten throughout the treatments. With iodofenphos gel alone, no bait was eaten after four days, but mortality continued for 21 days. The minimum amount of insecticide bait required to kill all 100 cockroaches was about 1340 mg iodofenphos gel, and 100·0 mg boric acid plus oats. After exposure for 24 h, all cockroaches died within nine days after exposure to boric acid plus oats, whereas with iodofenphos gel some survived after 21 days. Two further applications of gel bait in the iodofenphos experiments on days 13 and 17 failed to increase mortality although some bait was eaten. B. germanica was killed more quickly by both insecticide baits when deprived of drinking water.