Effectiveness of Entomopathogenic Nematodes against the African White Rice Stem Borer Maliarpha separatella Rag

Abstract

The efficacy of three entomopathogenic nematodes (EPN) species Steinernema carpocapsae Weiser, Steinernema karii Waturu, Hunt & Reid, Heterorhabdtids indica Poinar, Karunaka & David and two EPN isolates collected from Coastal Kenya (EX MOMBASA) and Rift valley (EX NAKURU) was evaluated against larvae of African white rice stem borer, Maliarpha separatella Rag. The experiment was carried out using no- choice modified filter paper bioassay at KARI-Mwea. Whatman filter paper was substituted with white cotton cloth discs. The activity of the biological agents under study was determined at 25℃ and 65% relative humidity, with concentrations of 50, 100 and 200 infective juveniles (IJs) per one M. separatella third instar larva. The larvae were confined in 60mm plastic petri dishes in darkness as the larvae spend their entire life inside the rice stems. The mortality rate was determined at 24, 48, 72 and 96 hours after application of the nematode suspension. Significant virulence was obtained with all the nematode species at 200 infective juveniles (IJs). There was low mortality at 50 and 100 concentration rates. All the EPNs significantly reduced M. separatella larvae after 48 hours in the following order H. indica> EX NAKURU> S. carpocapsae> EX MOMBASA >S. karii. The number of nematodes from the infected cadavers after 48 hours post infection was H. indica (553), EX –MSA (294), EX-NKU (242), S. karii (168) and S. carpocapsae (157). S. karii took the longest time (96 hours) to kill all the test insects. In conclusion, the study shows that EPNs are effective against M. separatella and could be used within an integrated pest management strategy for the pest. There is need to carry out further studies to determine effective dosages under field conditions.