Entomopathogenic Nematodes- bioagents for management of insect pests

Introduction

Use of chemical pesticides in agriculture posed a concern about hazardous effects of pesticides on environment. There is an urgent need for methods that are eco-friendly. Development of resistance and emergence of new biotypes is another concern about using chemical pesticides which drive us for a change in pest management methods. Biological control is an attractive alternative which is environmentally safe and feasible in crop protection. Understanding the biology and adaptability of  bio-control agents has an important role in successful application of bio-agent based products in different crop ecosystems. Entomopathogenic Nematodes (EPNs) are effective bio-control agents for the management of insect pests, especially soil dwelling insects. Two genera, Steinernema and Heterorhabditis are highly virulent EPNs, killing the insect host within 1-2 days. Steinernema and Heterorhabditis has symbiotic bacteria Xenorhabdus and Photorhabdus respectively in their guts which they will  release upon entering the insect.

Biology of EPNs

Third stage juvenile of the EPN species are infective. They are free-living, non-feeding and possess highly sensitive chemoreceptors which can detect host cues.  When a insect has been located, the nematodes enter into the insect body cavity via natural body openings (mouth, anus, spiracles) or  cuticle. After entering the insect haemolymph, bacteria released produceds toxins with insecticidal properties and the insect gets killed with 24-48 h due to septicemia (blood poisoning). In  the dead insect , nematodes and bacteria feed on insect body contents and reproduce inside the  cadaver. After completing several generations,  thousands of  infective juveniles emerge out of cadaver and search  for a fresh host.

Advantages over chemical pesticides

1. Highly lethal to  important insect pests with least off-target effects.
2. Highly safe to humans and environment. No safety equipment required, no residual effects, no groundwater contamination and are safe to pollinators.
3. Kills insects within 24-48 hours
4. Easy to mass produce and are compatible with standard agrochemical equipment and irrigation systems for application.

Mass production and Formulations

There are two methods of mass production, In vivo and In vitro mass production methods s. In vivo mass production is done using greater wax moth larvae or rice moth larvae. It is oriented for small scale production.This method is more laborious. In vitro mass multiplication is done using solid or liquid media. Bacteria from EPNs is isolated and inoculated in media.After 2 days incubation at 27 degrees, fresh IJ are inoculated.After 2-3 weeks,  newly emerged IJs can be harvested through white's trap. This method is used for large scale production.

Formulation plays a significant role in commercialization of bioagents. EPNs can be formulated either with active nematodes in various substrates or with reduced or arrested mobility(partial of complete anhydrobiosis). A number of formulations are available with good nematode storage and application including activated charcoal, alginate and polyacrylamide gels, baits, clay, paste, peat, polyurethane sponge, vermiculite, and water-dispersible granules. The shelf-life is variable among the formulations and it mainly depends on the activity of infective juveniles.EPNs are highly sensitive to temperature fluctuations and soil moistures in environment. So researchers  has to keep this problem in mind and develop suitable EPN formulations which improves the shelflife and protection from environmental fluctuations under field conditions.

Source: B.Gayatri and P Duraimurugan, ICAR-Indian Institute of Oilseeds Reserach, Telangana, Hyderabad