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South African entomopathogenic nematodes effectively kill fungus gnat pests
Author: Agil Katumanyane
Published: 10/11/2017

Fungus gnats are serious pests in South Africa's undercover farming, often harming production of various crops. The larvae of fungus gnats are soil-dwelling and, if uncontrolled, can chew through a plant's underground root system and severely harm the plant's ability to grow. Currently, the most common way that growers control fungus gnat populations is by spraying chemical insecticides, which can be harmful to the environment and human health. Entomopathogenic nematodes (EPNs) are tiny soil-dwelling worm-like organisms which attack and kill insects. EPNs are found naturally in the soil and are not known to harm humans. In a recently completed MSc study, it was found that local EPNs which are found naturally in the soil of the Western Cape, may be very effective at killing fungus gnats. Therefore, EPNs might prove to be a powerful alternative to chemical insecticides by killing the pest insects without harming humans or the environment.

Though fungus gnats naturally feed on fungi and organic matter, there exist a few species that are specialised pests of living plants. The most harmful fungus gnats in South Africa are believed to have been introduced from the Northern hemisphere. Some of the crops affected so far include mushrooms, greenhouse-grown cucumbers, tomatoes, chrysanthemums, blueberries, vegetables, herbs and tree nursery beds. The effects of the fungus gnats on these crops include direct physical damage on the crops' root system through their larval feeding. Consequently, the symptoms presented by the affected crop will resemble those of a plant deprived of a healthy root system. Since fungus gnats naturally feed on fungi, they are also responsible for dispersing fungal pathogens, and other soil borne pathogens, when they move from one plant to the other. The adults, though physically harmless, are quite a nuisance to farm workers, and also disperse fungal spores.

The control of fungus gnats has been dominated by the use of chemical insecticides which unfortunately pose risks to farm workers' health and of fungus gnats-resistance development. Furthermore, the cryptic habitats of the fungus gnats make control through the use of chemicals difficult. Considering such hindrances, as well as the increased consumer demand for chemical free products, alternative management practices such as physical, cultural and biocontrol methods, are being sought after. Such management practices are ideal, since fungus gnats attack crops where the application of chemicals is relatively inappropriate. This is the case for crops such as mushrooms and vegetables that have short life cycles, and for which the application of chemicals is restricted only up to a certain growth stage. Fungus gnats are also pests of houseplants, which are found in very close proximity with human beings, and where application of chemicals is undesirable.

Biocontrol for fungus gnats has been achieved through the use of biocontrol agents such as EPNs, predatory mites, rove beetles and soil bacterium. Worldwide, control using EPNs has been dominated by use of the EPN, Steinernema feltiae, which has also been adopted by some South African producers. It is important to note that S. feltiae is an alien EPN species that has, to-date, not been isolated from South Africa. Thus the effects of this EPN on South Africa's biodiversity are still unknown and the importation of this species is illegal. More still, S. feltiae has been determined to be less adapted to the environmental conditions of high temperatures that prevail in South Africa's undercover production.

Ms. Agil Katumanyane, who recently completed an MSc, focussed on determining the potential for using local EPNs to control fungus gnats in undercover farms in South Africa. The project was carried out in the Department of Conservation Ecology and Entomology and was supervised by the nematologists Prof. Antoinette P. Malan and Dr Tiarin Ferreira. Funding was provided by NemaBio (Pty) Ltd. and the Technology and Human Resources for Industry Programme (THRIP: TP14062571871).

Laboratory bioassays and field trials were performed on the larvae of the fungus gnats. Field trials were performed in a commercial cucumber farm based in Paarl, Western Cape Province. The local EPNs that were tested, were found to positively control the fungus gnats both during the laboratory and field trials. Another ongoing study by the PhD student, Nicholas Kagimu, is focussed on formulating the EPNs into a marketable product. We hope that once these local EPNs are formulated, they will be availed to producers for use, at an affordable price. This will provide a more sustainable approach to controlling the fungus gnats, as well as help prevent the need to import foreign nematodes, whose effect on South Africa's biodiversity, is still unknown.