Soil’s Trojan horse

Is an affordable and effective biocontrol method at hand for the deciduous-fruit industry? After more than a decade of researching entomopathogenic nematodes, two Stellenbosch University researchers seem to think so. By Esté Beerwinkel.

It’s dark. A population of false codling moth larvae overwinters under the soil’s surface. They have survived the chemical spray applications and will emerge after winter. But, before this false codling moth population can see the light of day, entomopathogenic nematodes sneak into the soil like Trojan horses. These nematodes scope out the false codling moth, infiltrate their bodies and release bacteria that destroy the pest from the inside out.

Entomopathogenic nematodes (EPNs) are threadlike worms that provide ecosystem services as a biological control agent. They are indigenous, safe to use around humans, and environmentally friendly —as opposed to destructive chemicals. And soon, they’ll be a commercialised biocontrol method.

The first EPN found in South Africa was in 1953 in the Eastern Cape on the maize beetle. Although it is 65 years later, research on EPNs is still in its infancy —mostly championed by Antoinette Malan, associate professor in nematology at Stellenbosch University (SU), and her team.

Nematology at SU was started by Bertus Meyer in 1973. Before his retirement in 2002, Meyer started projects on nematodes and EPNs, which was extended by Malan and her students.

“In 2005, Matthew [Addison, Hortgro Science Programme Manager and SU researcher] came and said ‘Antoinette, I have R30 000. Would you like to use it for EPN research?’ Back then I was still working on plant-parasitic nematodes, and EPNs were something foreign to me.”

More than a decade since that first Hortgro funded project, a lot has been achieved — including the discovery of ten new EPN species. But Addison recalls its initial lukewarm reception. “When we first started working on EPNs, the industry was sceptical about it being just another biocontrol method, but we believe any potential and local biocontrol agent is important.”

According to Malan, other countries such as the United States, depend on EPNs for pest control to a great degree. “Aside from being environmentally friendly, nematodes are not discriminative against chemical-resistant insects. They will wipe out all pests. And ironically, add to the chemical’s lifespan. If you implement an integrated pest management system, including nematodes makes the chemicals you apply last longer. EPNs are also persistent and survive beyond any chemical’s lifespan.

“In Florida, they get much better results from using nematodes than they do from applying chemicals against the citrus weevil. Larry Duncan [a professor of nematology at the University of Florida] claims he has never been to an orchard and not found a nematode. When I heard this, I yelped, ‘Why do we only recover 5.7%?’ It turns out, we don’t have many EPNs in our soil naturally. This is due to years of applying nematicides and not putting the good ones back.”

Farmers know, when preparing an orchard, you plough and then fumigate it. This is to kill everything in the soil and to prevent issues like apple replant disease. But, according to Addison many farmers forget the EPN element in this.

“The steps to setting up an orchard haven’t been kind to EPNs. First, you fumigate the soil to sterilise and protect it from nematodes that attack roots, then you’re irrigating and adding fungicides and chemical nematicides to it. After a while, the insecticides you’re spraying on the trees wash into the soil. This process repeats for 40 years. Now the biodiversity is our orchard soils is depressed, a casualty being EPNs.”

Malan and company’s research aims to turn this around. “Nematodes are soil organisms. Their job in nature is to keep the balance and control flares of insects that suddenly appear. If the nematodes aren’t in the soil, they can’t perform this task.

“Our goal is to mass produce EPN and apply them to orchards in the same way you would any chemical — and gain the effect of chemical, but in an eco-friendly manner.”

Malan explains this process as taking a soil organism (the nematode), multiplying it, and creating a spray-on nematode solution, and spraying it on the tree.

“The advantage of creating spray-on EPNs is targeting the pest that lives on the tree —the destructive codling moth is an example of this. The EPN can successfully wipe out the codling moth because the pest has no natural defences against it as it has never been exposed to it. We found this is an effective method in the annihilation of codling moth.”

This method has also been impressive in tackling codling moth in bins. According to Addison the control strategies used to manage codling moth in orchards, should also include bin treatment.

“Once apples contaminated with codling moth end up in the wooden fruit bins, they find a crevice, spin a cocoon, and overwinter happily. These polluted bins go from orchard to orchard and the pest keeps spreading. Cleaning these bins is also expensive, and when you’re a cooperative with 100,000 bins, it adds up. Our mission was to find an affordable and effective method to combat this, and we have.

“Research done on bin treatment using an EPN solution has been encouraging. Dipping bins in EPN solutions worked because it was a direct hit to the pest.”

Jeanne de Waal and Deidre Odendaal, both recruits of Malan, proved EPNs to be effective in destroying codling moth when applied to wooden bins.

The most effective EPNs were Steinernema jeffreyense, and when used with an adjuvant, Heterorhabditis zealandica.

“The reason local is lekker, is that our EPNs can survive here. They’re used to the Western Cape’s hot summers and winter rainfall.”

Where there is soil, there will probably be nematodes. And some locally sourced and described nematodes have strange hometowns. “During our first survey we had nothing to work with. We used to take samples from everywhere we went,” Malan recalls.

“Many of the EPNs we found in our samples were new to science. Of all the Steinernema, there are about 100 described species in the world, and we’ve described eight. Of the Heterorhabditis there are only 20 known and described species, and we described two. We found these species in samples in unconventional places like a graveyard in Knysna and vacation sites.”

One of these new species came from a sample taken in a backyard in Noenieput, in the Northern Cape, near the Namibian border. Malan named this Heterorhabditis species H. noenieputensis.

“One reviewer said I should change the name, but I said ‘nooo’. Noenieputensis is very popular, the growers know and ask for it.”

Now, Malan says, they are in the final stages of their project: the commercialisation of EPNs. “Getting the formulation right is elementary for mass production. We’ve been working with SU’s engineering department and their fermenters. And we’re very excited because we’ve bred nematodes on a large scale.”

For Addison it is important that growers know the value of EPNs. “I think every time you use nematicides, you should be putting nematodes back into the soil. Using EPNs is like using a probiotic during a cycle of antibiotics. They reset the balance.”

With the formulation and mass production figured out, and commercialisation on the horizon, it seems a new EPN cavalry could fight the war on pests and soil biodiversity in South African orchards.

Image: Entomopathogenic nematodes destroy a pupating fruit fly.

Supplied by Antoinette Malan.