Nematodes harnessed for biological pest management

TSHIMA RAMAKUWELA and JUSTIN HATTING, ARC-Small Grain Institute

“Going green” and “environmentally friendly” are common terms we encounter increasingly frequent these days. In the agro-chemical industry, the contamination of food by chemicals (i.e. pesticide residues) and their safety to humans and the environment is a world-wide public concern.

The natural enemies of insect pests include predators (e.g., ladybird beetles), parasites (e.g. wasps) and pathogens (disease-causing microbes). The pathogen group is very diverse, comprising fungi, viruses, bacteria, protozoa and nematodes.

Generally, producers are familiar with the root-damaging nematodes known as lesion- and root knot-nematodes. However, another group of nematodes, the so-called entomopathogenic nematodes, does not attack plants, but utilises insects as hosts.

These entomopathogenic nematodes are safe and environmentally friendly, soil-dwelling insect parasites that occur globally in natural and agricultural soils. They live in a symbiotic association with a lethal bacterium, which the entomopathogenic nematodes release into the insect host following penetration thereof.

This bacterium is very potent, capable of killing the insect within 48 hours. The bacterium then serves to digest the internal organs of the host rendering it suitable as food source to aid nematode development within the host (i.e. production of the infective juvenile or IJ stage) (Photo 1).

It is for these reasons that scientists around the globe are conducting research to develop entomopathogenic nematodes based biopesticide products. Such products hold a number of advantages including: There exists no record of acquired or developed resistance against entomopathogenic nematodes, safety towards mammals, non-target organisms and the environment, sustained control due to a durable IJ (non-feeding) stage, host-seeking abilities, persistence in soil, rapid rate of insect mortality (48 hours), compatibility with chemical pesticides and exemption from registration in many countries.

The Insect Pathology unit at the ARC-Small Grain Institute (ARC-SGI) curates a large collection of indigenous entomopathogenic nematodes (>165 isolates) and have recently identified two new species, published as Steinernema tophus n. sp. and Steinernema innovationi n. sp.

Latter species was used as study organism in a PhD thesis by Tshima Ramakuwela (co author of this article). The study covered aspects on the biology, mass production and formulation of the nematode.

As yet, no indigenous entomopathogenic nematodes have been commercialised in South Africa and the research therefore supports efforts to harness our rich biodiversity for the benefit of the local agricultural sector.

With such a product, several soil-borne insect pests could be targeted, including the black maize beetle, cutworms, false wire worms (larval and beetle stages; Photo 2), wire worms and the pupae of the African bollworm residing in the soil.

As these insect pests have different behaviours and occupy specific environmental niches, the use of the correct entomopathogenic nematodes against a particular pest species is critical.

Essentially, three types of IJs are recognised by their foraging strategy, i.e. the cruiser-type, ambusher-type and an intermediate-type (a combination of both cruiser and ambusher).

As the name suggests, the first type actively seeks their host in the soil environment, while the second type lies waiting for the host to pass by, whereupon the IJ will latch onto the insect.

The intermediate strategists are adapted to infecting insects that occur just below the soil surface, such as prepupae of moths, fungus gnats, and/or weevil larvae. The sit-and-wait-strategists or ambushers are characterised by low motility and a tendency to stay near the soil surface.

They tend not to respond to volatile and contact host cues unless presented in an appropriate sequence, and then efficiently infect mobile host species such as cutworms and mole crickets near the soil surface. At the other extreme, the widely foraging strategists or cruisers are characterised by high motility and are distributed throughout the soil profile.

In soil, the infective juveniles can disperse almost 1 m in both horizontal and vertical directions within 30 days. This dispersal, especially for cruiser-type nematodes, allows the entomopathogenic nematodes to actively seek out their hosts.

Factors influencing the motility of infective juveniles are moisture, temperature and soil texture, of which moisture is the most critical, because the nematodes need a water film in the interstitial spaces of soil for effective propulsion.

Different nematode species/strains have different temperature optima and ranges that affect their survival and hence motility. Nematodes lose motility at low temperatures (<15°C) and become inactivated at high temperatures (>30°C).

Soil porosity affects nematode dispersal with less dispersal occurring as soil pores become smaller. Nematodes can also be dispersed great distances, passively by water, as well as by wind, infected hosts, human activity, etc. which may in part account for their wide-spread global distribution.

Entomopathogenic nematodes are commercially available internationally for application in residential gardens, golf courses and agricultural fields. They are infectious to over 200 insect pests including important pests of grains, vegetables, sugarcane and forestry. Some entomopathogenic nematode species have shown good compatibility with chemical pesticides and other biocontrol agents. These entomopathogenic nematodes can therefore be safely incorporated into an integrated pest management strategy.

They furthermore do not require special equipment for application. They can be applied using standard spray equipment for chemicals and through existing irrigation systems. The development of entomopathogenic nematodes based products by the ARC-SGI is being conducted in collaboration with the local company Plant Health Products (Pty) Ltd in KwaZulu-Natal and the Adnan Menderes University in Turkey.

To date, cooperation between the ARC-SGI and PHP (Pty) Ltd has led to the successful registration of the biopesticide Eco-Bb, an environmentally friendly product based on the insect-killing fungus Beauveria bassiana (see www.plant-health.co.za).

For further information, contact Tshima Ramakuwela or Dr Justin Hatting at the ARC-SGI on 058 307 3400.