Biological control of fruit flies

Researchers turn to nature for help. By Engela Duvenage.

Does nature provide its own little helpers to support the South African agricultural sector in controlling its fruit fly problem? Indeed, said Dr Pia Addison, upon reflecting on four years of biological control research by the Integrated Pest Management section of the Department of Conservation Ecology and Entomology at Stellenbosch University.

Biological control measures fight nature with nature, using insects, microscopic worms, fungi, or other pathogens found naturally in an environment to reduce the effects of specific pests.

Most of the work done by Addison’s team was conducted in the southwestern Cape to search for natural enemies of Cape fruit flies and Mediterranean fruit flies. Studies were conducted on parasitoid wasps, and entomopathogenic nematodes and fungi. It was funded by Hortgro Science, the South African Table Grape Industry, and the National Research Foundation.

“We’re starting to tick the boxes and to get good results,” Addison reported. “So far we have found one option, but it needs more intense research before we can potentially mass produce and release it as a biocontrol agent.”

She cancelled out the possibility of using parasitoid wasps to control fruit flies in the Western Cape, because their natural numbers within the region are simply too low. The use of entomopathogenic nematodes and fungi that are normally found in healthy soils are however showing promise. Entomopathogenic organisms cause disease in insects.

“Entomopathogenic nematodes are valuable tools to have in our pest management toolbox, because they generally attack the soil stages of insects,” Addison explained.

The worm turns on fruit flies

The initial work at Stellenbosch University on entomopathogenic nematodes that attack fruit flies started in 2016. It began with MSc student Monique James screening soil samples collected in 20 orchards for potential candidates. Her work was supervised by Prof. Antoinette Malan.

In due course, two entomopathogenic nematode species were found to be the most effective. The first, Heterorhabditis noenieputensis, was first discovered near the settlement Noenieput between the border of South Africa and Namibia. It targets fruit fly pupae at a high rate. The second, Steinernema yirgalemense, attacks adult flies.

As part of her studies, James also noted the presence of entomopathogenic fungi in local soils. One fungal isolate in particular – MJ06 – was found on Mediterranean fruit flies. Further tests showed that most flies infected with the fungus died.

“MJO6 is part of the Metarhizium robertsii complex which is known to be virulent and quite effective against insects,” noted Addison.

In closing, Addison mentioned that she’d like to investigate the use of tent-like structures called augmentoria. These have an opening and tops that are covered with fine mesh. According to Addison, augmentoria are used in many other African countries to ensure better sanitation within home gardens and smaller orchards, and to control localised fruit fly populations.

She explained that fruit flies emerging from infested fruit are trapped within the augmentorium and die. Adult parasitoid wasps on the other hand can escape through the fine mesh.

In a preliminary study conducted by one of her collaborators, Dr Julien Haran, 800 fruit flies emerged from one kilogram of infested fruits, along with 300 wasps.

What are the benefits of using biocontrol measures on fruit flies?

• These are highly specific and do not harm beneficial organisms like bees and parasitoids.
• Biocontrol is easy to use with other control practices, such as sterile insect techniques and softer pesticides applications such as bait sprays.
• Biocontrol can be used just before harvest when the pest populations are starting to increase because there are no insecticide residues, as would be the case with conventional chemical control programmes.
• No resistance builds up against the biocontrol agents.
• Agents target eggs and pupae. These stages of a fruit fly’s life cycle are very difficult to control using other methods, such as chemical control.
• Prolongs the use of chemicals in an integrated pest management system. It does not discriminate, and removes resistant individuals from the population too.

Image: Entomopathogenic nematodes destroy a pupating fruit fly. Entomopathogenic nematodes ere microscopic worms found naturally in healthy soil. They only attack insects and are very target specific. When a worm finds a larvae or pupae of an insect in the soil, it enters it through natural openings. Once inside, the worm transmits specific bacteria that eventually kills the host insect. When the host insect dies, there are no resources for the worm to feed on. It goes back into the soil to search for a new host. Image supplied by Antoinette Malan | Stellenbosch University.

Below: Entomopathogenic fungi attack a fruit fly. These fungi have a similar life cycle to that of nematodes. They are present in the soil as spores. These attach to insects’ cuticle or covering, and then penetrate it. The fungi’s growth stage occurs within the insect. It then reproduces and eventually kills the host insect. Spores can stay in the soil for extended periods, until a new host is located and the cycle repeats. Image supplied by Monique James | Stellenbosch University.