Susceptibility of white grubs from forestry and sugarcane plantations in South Africa to entomopathogenic nematodes

Abstract

The control of white grub (Coleoptera: Scarabaeidae) pests of sugarcane and forest plantations is difficult due to their cryptic nature and resistance to chemicals. This study evaluated the potential use of entomopathogenic nematodes (EPNs) as an alternative control method. Laboratory bioassays were performed with 12 locally isolated EPN species to determine the susceptibility of third instar larvae of the white grubs Schizonycha affinis Boheman, Pegylis sommeri Burmeister, Monochelus sp. and Maladera sp. 4. Concentration trials to determine lethal dosages for three of the white grub species were performed using Heterorhabditis zealandica Poinar MJ2C. Bioassays were performed to determine whether nematodes could develop inside the cadavers of S. affinis and P. sommeri as these had shown the lowest susceptibility to EPNs. The mortality percentage of the white grubs, although varying significantly, was found to be low for most of the EPN species, except H. zealandica. The highest percentage mortality of white grubs was observed at four weeks post inoculation with the rate of mortality being highest in the first week. Schizonycha affinis had the lowest LD50 of 38 Infective juveniles (IJs) per larva after 28 days, compared to Maladera sp. 4, with 284 IJs per larva, and P. sommeri, with 1035 IJs per larva. The dissection of insect cadavers revealed possible limiting factors for low susceptibility, due to the nematodes and their associated symbiotic bacteria’s inability to infect the insect haemocoel, with no EPNs being found inside some of the cadavers.