Entomopathogenic Nematodes in the Biological Control of Insect Pests with Reference to Insect Immunity

Abstract

Insects are the successfully living arthropods on earth and challenging humans for food and shelter from several decades. As agricultural pests, insects cause heavy losses to crops and reduce the quality and quantity of human food. Failure of chemical pesticides and development of resistance of insects have become the major problem for researchers and are challenging mankind to search out for better solutions. Biological control management has been found as an emerging key of success, and in this context, entomopathogenic nematodes are in concern as potential biological control agents. These nematodes parasitize and enter the body of insect host and kill them quickly without the development of any resistance and failure. Several species of the entomopathogenic nematode genera Steinernema and Heterorhabditis have been identified around the world, and few of them have been commercialized for the biomanagement of several insect pests. The killing capabilities of these nematodes were enhanced severalfold by their symbiont bacteria of the genera Xenorhabdus and Photorhabdus which reside in the alimentary canal of these rhabditid nematodes. The symbiont bacteria produce severe toxins which ultimately results in the death of insect hosts. For the successful establishment of a biological species as biocontrol agent of insects, it is necessary to study the immune responses of host and defensive mechanisms of the parasites. Insects comprise innate immune system where cellular and humoral components combat with the invading pathogens. When entomopathogenic nematodes enter in the haemocoel of insect’s body, both the immune components are triggered and try to evade pathogenicity of invading nematode–bacterium complexes. The mechanisms of nematode immunomodulation display the variation of modulatory approaches developed by different types of parasitic nematodes to cripple the host immune responses. Both the nematode and bacteria produce chemical factors which protect them from the insect’s immune system attack. The cellular and humoral responses are avoided and broken by toxins produced by nematode–bacterium complexes included different proteins producing immunodeficiency in insects. These nematodes were found to be safe to the environments and soil flora and fauna and also have exemption from one area to another by several countries.