Research in biological control is shaping the next agricultural revolution. By Anna Mouton.
The so-called green revolution of the sixties gave the world mind-boggling advances in agriculture. Crop production boomed thanks to the development of new varieties and cultivation methods in combination with efficient irrigation and the use of chemical fertilisers and pesticides. The rise of modern societies has been — and continues to be —powered by the green revolution. But for how long?
Chemical pesticides are a great illustration of why continued innovation in agriculture is essential. Pesticides are key to mitigating losses caused by pest proliferation under intensification. Pesticides have also introduced consumers to perfect fruit and vegetables —nowadays they will accept nothing less. Heightened awareness of phytosanitary risks makes pesticide use indispensable to global trade. Yet chemical pesticides are no longer viewed as the cure-all for insect troubles.
Insects have stepped up to the pest-control arms race and pesticide resistance continues to evolve. Chemical control is further complicated by ever-more stringent requirements from markets. The same consumers who demand blemish-free fruit and vegetables are concerned about the health impacts of pesticide residues. Regulators and retailers have responded by limiting the pesticides that growers can use and dropping allowable residue levels.
How will growers respond to these challenges? Fortunately the search for solutions is already well underway. Fresh Quarterly spoke to some of the scientists working on the next generation of pest control for a glimpse of the agricultural revolution to come.
The use of disease-causing microorganisms — pathogens — to control pests is well-established. Registered products range from granuloviruses of codling and false codling moths to a bacterium — Bacillus thuringiensis — that kills the larvae of moths that are pest insects. Two entomopathogenic fungi — Beauveria bassiana and Metarhizium anisopliae — are also commercially available.
Entomopathogenic fungi are fungi that cause disease in insects. Fungi reproduce asexually by spores. Spores are adapted for survival and can often remain dormant for long periods — think of spores as similar in many ways to the seeds of plants. Fungal spores lend themselves to formulation in products for the biocontrol of insects.
Researchers from the Department of Conservation Ecology and Entomology at Stellenbosch University have been hard at work to discover new strains of entomopathogenic fungi. Letodi Mathulwe is a doctoral student currently working on mealy bug control. She describes the process of searching for the next killer fungus:
“We collect soil samples from orchards. In the lab you put the samples in containers and on top you put larvae of wax moths or mealworms. We use the insect as a bait. After seven to fourteen days the insects will be killed by fungus that is present in the soil sample and we can harvest it from the insect to grow on an agar medium.”
Mathulwe isolated six different fungi for her master’s project and tested them against woolly apple aphids. She found that one of her isolates — Metarhizium pinghaense — destroyed up to three-quarters of treated woolly apple aphids. This is good news because woolly apple aphids are difficult to control and there are reports of increasing pesticide resistance.
“Entomopathogenic fungi don’t attack all insects, but they do infect multiple insect pests. I’m now using the isolates that I collected for my master’s to control the mealy bug and they’re working really well,” Mathulwe explains.
Metarhizium pinghaense is also the top performer to date in trials against false codling moth conducted by doctoral student Francois du Preez. “Some of the commercial fungi work really well,” he says, “but we need to also work with local South African isolates.” Local isolates are better able to cope with our hot and dry conditions.
Master’s student Steffan Hansen agrees. “You want to give the farmer the option to use a native fungus.” Hansen is studying the effect of entomopathogenic fungi on weevils. “Previous research found very good results with Beauveria bassiana. I’m throwing a new fungus into the mix — Beauveria pseudobassiana — which I isolated from a soil sample in the Karoo.”
Hansen anticipates that weevils will become a bigger and bigger problem as chemical-pesticide options decrease. “I feel the more biocontrol products you have, the better, because you’ll may get resistance with the fungi as well.” Using just one biological agent won’t control pests in the long-term — a mix of them in the right environment will be far more effective.
The spores of disease-causing fungi are usually sprayed on either the soil or the plants. Hansen says that some growers inoculate their mulch with entomopathogenic fungi. “Entomopathogenic fungi grow on organic matter. If the spores are in the mulch and the weevils crawl through, they become infected. That’s something we can test in my work.”
Prof. Pia Addison specialises in integrated pest management and is one of Hansen’s supervisors. She thinks there is potential to include fungal spores in bands placed around the trunk of trees. Weevils cannot fly — they walk up the trunk to reach the canopy and they could become infected when they come into contact with the bands. There is also research that suggests weevils could be lured to bands using aggregation pheromones.
Hansen emphasises the importance of field trials. There have been many laboratory studies on biocontrol agents, but he cautions that these are done under ideal conditions. “I’m going to produce the local isolates that work the best to see how viable the spores are. Then I will
inoculate different substrates — such as mulch — to evaluate the most efficient mode of delivery onto the weevils.”
The worm turns
“Nematology is a huge subject,” says Prof. Antoinette Malan. “It includes parasitic nematodes of agricultural importance. There are free-living nematodes that are good for soil health and there are plant-parasitic nematodes. And then there are the entomopathogenic nematodes that are parasites of insects.”
Malan has pioneered the concept of entomopathogenic nematodes as biocontrol agents at Stellenbosch University. She believes that nematodes have the edge over fungi. “A nematode can search out the insect, whereas a fungal spore lies there, and the insect must come into contact with it.”
Entomopathogenic nematodes occur in the soil. These tiny thread-like creatures are formidable hunters. They penetrate their prey and release bacteria that digest the body of the insect — essentially, they make the insect into a protein shake. The satiated nematodes then reproduce inside the insect corpse.
“I looked at how many nematodes are produced per false codling moth larvae,” reports Du Preez. “I inoculated them with fifty nematodes and on average, depending on the nematode species, I got about 8 000 juveniles out.”
Entomopathogenic nematodes have been shown to kill many of the key pests of deciduous fruit, including codling and false codling moths, fruit flies, weevils, and mealy bugs. Soft life stages such as larvae are most vulnerable to attack. “We know most pupae are quite hardened against entomopathogenic nematodes because nematodes need to find an opening to get in and pupae are very well protected,” explains Du Preez.
Nematodes work well for the soil stages of insects, says Dr Noma Stokwe. “Spraying on the soil is the easiest and it’s the most effective because soil is their natural habitat — that’s where the nematodes come from.”
Stokwe has researched the biological control of mealy bugs and woolly apple aphids. She highlights some of the challenges associated with above-ground application of nematodes. “One is desiccation — they won’t work if it’s too dry. So that is why sometimes you have to keep the trees wet after applying nematodes. Maybe you will also have to apply them very early during the morning or late in the evening when it’s not so hot.”
“Entomopathogenic nematodes are more susceptible to all kinds of variation,” affirms Du Preez. “If you spray entomopathogenic nematodes you need to wet your soil beforehand and then apply the nematode using nozzles that are clean of chemical residues. You have to wet it again afterwards. You have to spray in the evening so that the nematodes can go into the soil and be protected from ultraviolet light and desiccation.”
Entomopathogenic nematodes are already available as commercial formulations in Europe and the United States but there are currently no indigenous entomopathogenic nematodes registered for use in pome or stone fruit in South Africa. Doctoral student Murray Dunn has developed a technique to mass culture two local nematodes species using an artificial diet. Mass-cultured nematodes are essential for conducting field trials.
“We are on the brink of commercialising these nematodes,” says Malan. “We can easily breed them artificially and formulate them in massive numbers. The biggest thing holding us back is that you need a team of diverse specialists to take that final step. And we need a big funder — someone with the capital to buy large bioreactors for mass culture for commercial application.”
The world has moved on since the green revolution. Consumers and environmentalists are more assertive. Access to international markets has become essential and the benefits of green credentials are increasingly clear. Chemical pesticides sit uncomfortably in this new reality. But growers can rest assured that scientists are working to fill any chemical-pesticide gaps with innovative biocontrol solutions.
This is the new agricultural revolution — and it belongs to the entomopathogens!
What are people saying about this research?
Hugh Campbell | Hortgro Science
“Our biocontrol research is future-orientated. With the loss of chemicals and alternative measures, the key aspect is that the industry is working on long-term sustainable options. For us that’s a greater priority than delivering products. We recognise that product development is part of the long-term solution, but we believe that product development sits in the commercial world. Our mandate is not necessarily to develop and deliver products — it’s to deliver solutions. When the products that our growers are currently using are no longer available or don’t work due to resistance, we must should have another plan in place.”
Image: Flasks of nematodes being grown by Murray Dunn.
Supplied by Anna Mouton.