Effects of insecticide sublethal concentrations on the ringlegged earwig, Euborellia annulipes (Lucas)

Integrated pest control of the sugarcane stem borer (SSB), Chila infuscatellus, recommends the use of a voracious predator as the ring legged earwig (RE), Euborellia annulipes (Lucas) and insecticide application. This study was conducted to determine sublethal effects of two recommended pyrethroid insecticides, cypermethrin and deltamethrin, on predation, the number of eggs/batch, % egg hatch and the developmental period of the offspring of RE adults. Sublethal concentrations of cypermethrin (10% EC) and deltamethrin (3% EC) at LC1 and LC10 were topically applied with a micro-applicator on tested individuals with direct and indirect methods. They were applied directly once on adults of RE or on ten 3rd instar larvae of SSB which were then fed to RE adults. Surviving adults were further investigated. Insecticide-treated RE adults with LC1 and LC10 of cypermethrin showed significantly slower onset of predation averaged 14.20 and 18.35 h, lower number of preys killed, averaged 4.8 and 4.2 larvae/d, respectively, as compared to those of untreated ones. The corresponding figures of treated adults with deltamethrin were 30.5 and 42.2 h, 4.6 and 3.8 larvae/d respectively. Similar results were obtained on RE adults that fed on insecticide-treated 3rd instar SSB larvae with the same concentrations of insecticides. The only different result was recorded on the number of preys killed by indirectly treated adults with LC1 of cypermethrin and deltamethrin averaged 9.6 and 8.8 larvae/d, respectively, which was not significantly different as compared to those of untreated ones. Insecticide-treated RE adults with LC10 of cypermethrin and deltamethrin showed significantly lower number of eggs/batch averaged 31.4 and 27.4 eggs, lower % egg hatch averaged 29.8 and 31.7%, prolonged developmental period of the offspring averaged 91.8 and 113.0 d, respectively, as compared to those of untreated ones. The corresponding figures of those from RE adults that fed on insecticide-treated 3rd instar SSB larvae were 30.7 and 37.5 eggs, 36.1 and 32.8%, 95.6 and 113.7 d, respectively, as compared to those of untreated ones. This study concluded that sublethal concentrations of cypermethrin and deltamethrin at LC1 and LC10 which did not kill RE adults reduced their predation, lowered the number of eggs/batch and % egg hatch, and prolonged the developmental period of their offspring. It is suggested that RE may not be totally safe for combination use with cypermethrin and deltamethrin in controlling SSB.