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Biological pest control measures: researchers now know how to transport tiny nematodes to farms
Author: Engela Duvenage
Published: 08/04/2021

​For the past 18 years, researchers at Stellenbosch University (SU) have studied a selection of nematodes found naturally in South African soils. In the process they have identified quite a few such roundworm species that could be used to biologically control agricultural pests. A recent MSc Entomology graduate, Ms Abongile Nxitywa, has now solved one of the conundrums that have so far stumped the SU research team's efforts to roll out this natural pest control method for use on local farms: how to store and transport them live, and en masse.

Her finding now opens the door for opportunities to commercialise the use of nematodes within the South African agricultural setting, as a substitute for chemical pesticides. Similar commercial endeavours are already underway in America.

“Because of the biosecurity risks involved, it is always better to use local species in pest control," highlights Prof Antoinette Malan, project leader of the Nematode Laboratory in the Department of Conservation Ecology and Entomology.

According to calculations, about 2,5 million nematodes are needed if one is to spray one hectare of orchards against pests such as codling moth, false codling moth, weevils and mealybugs. To do this regularly on a reliable commercial scale will require quite an operation to ensure a steady supply.

“It sounds easy to do, but bear in mind that all the beneficial species we have so far identified are smaller than one millimetre in size," explains Prof Malan, who has been focusing on nematodes since the early 2000s.

In fact, the species are so small that without the benefit of a microscope, one needs seriously good eyesight to see one move.

They are also not as abundant as one might think. Prof Malan says that over the years only 5% of the soil samples that she has collected from all over the country have contained nematodes. These then had to be tested to pinpoint the ones that work against local agricultural pests such as codling moth and false codling moth.

Nematodes are parasitic in nature. They attach to insects, and penetrate their bodies through an opening, such as a pore. Once inside, they release a bacterium which then kills the insect.

Ms Nxitywa, who hails from Mthatha, says she had agricultural science as a school subject, and it was therefore an easy choice to pursue the topic further as a student. She was first introduced to nematodes during her third year of studies, but at that stage the topic did not grab her. In her fourth year as a BScAgric student, she worked with Prof Malan on a related project.

“Prof Malan taught me to love them," she now enthuses.

Ms Nxitywa graduated with a BScAgric in Agronomy and Plant Pathology in 2018, and in 2019 worked for six months as a research assistant in Prof Malan's Nematology Laboratory.

“During this time, she showed an extraordinary ability to master such high-technologically advanced techniques as the molecular identification of nematodes and the successful in vitro liquid mass culture of nematodes," says a very proud Prof Malan. “Her adeptness with such techniques earned her a departmental bursary, and she which led to her promptly switching from being a research assistant to becoming a postgraduate student."

For her MSc project, Ms Nxitywa tackled the issue of how to formulate and optimise the long-term storage and use of parasitic nematodes that prey on insects.

“Their commercialisation as biological control agents are hampered by their short shelf life. I wanted to find a preservation method that can improve their storage, without negatively affecting their infectivity against target pest insects," she explains.

It was easier said than done, because amid the first Covid-19 lockdown period Ms Nxitywa was one of only a few students initially allowed back on campus, and the only one from her lab permitted in the JS Marais Building where the research facilities of the Department of Conservation Ecology and Entomology are situated.

Ms Nxitywa needed large quantities of two South African nematode species, Steinernema jeffreyense and Steinernema yirgalemense, to test her formulations. The Covid restrictions meant that she did not have the benefit of a research assistant or a fellow student to help her in culturing these roundworms in large quantities.

“She did not to let this setback hold her back, and promptly started to culture her own nematodes," Prof Malan tells the story. “Her time as a research assistant came in handy, because she used a technique she had previously learnt from Murray Dunn, a PhD student who is using bioreactors to mass produce nematodes."

In the process, Ms Nxitywa was able to raise about more than enough nematodes for her experiments.

“There was a lot of trial and error," Ms Nxitywa admits.

At long last she could embark on her actual research endeavour: to find a way to keep and transport the tiny organisms for distribution and use on farms.

She tested various mediums, and finally settled on three. One looks like translucent play dough, while another looks like course sea sand is called diatomaceous earth (also known as kieselgur). Nematodes can be stored a high concentration of 700 000 nematodes per grams of diatomaceous earth. Alga-based beads the size of ones that one would typically find on a bracelet were also tested. Ms Nxitywa was able trap up to 5000 nematodes in one such bead.

“Diatomaceous earth formulation would work well for commercial application as it dissolved in water, while in the case of beads, they can act as a slow release of nematodes in undercover production such as blueberries," says Prof Malan.

Ms Mxitywa tested the ability of the various formulations to preserve nematodes under different temperatures and found that 14 degrees Celsius is optimal.

“You can keep them at room temperature, but for only about 4 weeks until a type of fungi starts growing on the medium," she explains.

It was a great moment when after months of trial and error she realised that she was able to keep the nematodes alive for at least 8 weeks under optimal conditions.  

In recognition of her work, Ms Nxitywa received her MScAgric degree in Entomology cum laude during the March graduation ceremony.

One of the four papers she has subsequently prepared as part of her thesis have so far been accepted and will appear in due course in the South African Journal of Enology and Viticulture.

Prof Malan says that ideally it would be great to get a local commercial partner involved to take their research findings further into the commercial space. She however realises that this would come at a great investment cost.

More research in large-scale field trials is still needed to test the effectiveness and persistence of the formulations' use under field conditions. ​