Entomopathogenic nematode interactions with white grubs from forest and sugarcane plantations

Abstract

In South Africa, white grubs are important pests of sugarcane and wattle. There is need for a sustainable management alternative since their control has mainly been through the use of chemical insecticides. Such alternatives can include the use of entomopathogenic nematodes, EPNs. A review of the available literature revealed that EPNs can potentially provide a higher percentage control of white grubs, when compared to chemical insecticides. However, this control efficacy is characterized by several inconsistencies that can be attributed to differences in the EPN species, environmental conditions, and the resistance of white grubs to EPN infection. To increase efficacy of EPNs against white grubs, these inconsistencies need to be minimized, for example by using locally isolated EPN species that are better adapted to the local environment. In this project, I investigated the potential use of locally isolated EPNs as an alternative biological control method for white grubs in South Africa. I examined EPN associations with white grubs collected from wattle and sugarcane plantations in the Mpumalanga and KwaZulu-Natal provinces of South Africa. Using bioassays, EPN pathogenicity towards white grubs as well as the white grub resistance mechanisms to EPN infection were evaluated using eleven locally isolated EPNs. The results indicated that the mortality percentage of the white grubs varied significantly but was low for most of the EPN species tested, except for the EPN Heterorhabditis zealandica MJ2C. When H. zealandica MJ2C was used in a probit test, its varying lethal dosages were revealed for the different white grubs. The experiments to investigate the physiological defence mechanisms of the grubs showed that the ability of the nematodes to penetrate the white grubs, the encapsulation rates and growth in the haemolymph varied between the EPNs and the white grub species. However, the different species of the EPN symbiotic bacteria were able to grow in the haemolymph of all the white grub species tested, implying that white grubs avoid initial attack of EPNs as a resistance mechanism. During a survey for white grubs in the KwaZulu-Natal province in South Africa, sporadic incidence of larvae with symptoms of nematode infections were detected. Incubation of the infected cadavers resulted in isolation of three EPN species. This is the first report of locally occurring EPNs found naturally associated with white grub species in South Africa. This project also resulted in the description of a new EPN species from South Africa, described as Steinernema bertusi Katumanyane, Malan, Tiedt & Hurley (Rhabditida: Steinernematidae). The newly described EPN species was the fifth species to be included in the Cameroonense-clade, the 12th Steinernema species described from South Africa, of which only a 100 species are currently described in the world.