Pressure is mounting to find alternatives to chemical crop protectants. How far are we in developing other options? By Anna Mouton
The current generation of table-grape growers is accustomed to chemical pest control. But synthetic pesticides only became mainstream after the Second World War. The initial success of highly toxic organophosphates and organochlorines led to rampant use and wild claims for these new wonder products.
“My father told me that when DDT came out, the marketing splurge was that it was going to be the end of houseflies and mosquitos,” says Dr Elleunorah Allsopp, senior researcher at the ARC Infruitec-Nietvoorbij. “Well, we all know who still has the last laugh on that account.”
Within a few decades, the harmful effects of synthetic pesticides on human and environmental health surfaced, sparking public pushback. Researchers started exploring ways to combine biological and chemical control and the concept of integrated pest management was born.
“The focus shifted from pest control, in the sense of eradication, to management, which is keeping pest populations below levels that cause economic damage without necessarily trying to annihilate them,” explains Allsopp.
Integrated pest management is more relevant now than ever as regulators and retailers respond to consumer demands for environmentally friendly production of residue-free fruit. So why isn’t it the standard practice everywhere?
Read MoreWhat are the alternatives?
Allsopp discusses two alternatives: resistance breeding and biological control. “Resistance breeding seems like a no-brainer,” says Allsopp. “We’ve got so many new gene-editing tools. We even have satellite markers for genes to aid us in conventional breeding.”
As with many great ideas, resistance breeding soon faced a reality check. Whereas it may be possible to breed cultivars with resistance to one or two pests, breeding for resistance to all pests is not feasible. At best, growers will eventually have the option to plant cultivars resistant to the most problematic pest(s) at a given site.
The other challenge with breeding is that consumers and pests share a taste for thin-skinned berries with high sugars and low tannins. “The very things we select as preferred qualities for consumers are the things that make life easier for pests,” says Allsopp. “So, we’re breeding for pest and disease susceptibility.”
Gene editing offers greater scope than conventional breeding, but consumers are even more suspicious of genetically modified grapes than synthetic chemicals.
Biological control is a catch-all term encompassing everything from natural chemicals to living organisms. It includes well-established methods such as pheromone-based mating disruption and insecticidal bacteria and viruses.
“We are also looking at new fields,” says Allsopp. “Entomopathogenic fungi and nematodes offer us another world of insect control.” Entomopathogens are organisms that infect and kill insects – SATI has funded at least 10 research projects on biocontrol, including entomopathogenic fungi and nematodes, over the past 15 years.
Another promising option is harnessing the protective chemicals that plants themselves produce to discourage insects. These work well in enclosed environments such as tunnels, but more research is needed to optimise them for the open air of vineyards.
Challenges in developing alternatives
In general, synthetic chemicals tend to be broad spectrum while alternatives tend to be specific. Where something like DDT can kill almost any insect, an insecticidal bacterium or virus may only target the larval stages of butterflies and moths. Or a parasitoid wasp may only attack the eggs of a specific pest while leaving other life stages alone.
Specificity is good for the environment – it saves beneficial and harmless insects from becoming collateral damage – but doesn’t provide insurance against the emergence of new pest problems. Climate change is altering the dynamic between plants and pests, as is the introduction of crops into new regions, leading to an increased economic impact of previously unimportant pests and more outbreaks of sporadic pests.
As a result, it’s doubly difficult to commercialise alternatives such as entomopathogenic fungi and nematodes. Firstly, developing and registering any new crop protectant requires enormous investment. Secondly, the resulting product will likely have a more limited spectrum, reducing the market opportunities and, therefore, the potential profitability.
According to Allsopp, one advantage of entomopathogenic fungi and nematodes is that most are not host-specific. “This is precisely why we are looking at developing those entomopathogens that work against several pests,” she says.
“We know we’ve got the potential products – local biocontrol options – but we have to take the work from the laboratory into the field and then develop the protocols for mass production. We must find ways to get products on the shelf for the farmer.”
Obstacles to adoption
Allsopp explains that integrated pest management is knowledge-driven and management-intensive. Growers must understand their environment and the pest’s relationship with its host plants. Continuous monitoring and adaptation are key – integrated management is always site- and situation-specific.
“Therein lies the rub,” says Allsopp. “For decades, growers got used to an annual spray programme listing the phenological stages of vine growth, the list of pests and diseases, and what you sprayed when.”
Nowadays, these spray programmes are provided by pest-control advisers affiliated with the agrochemical industry. Allsopp thinks there aren’t enough independent advisers with knowledge of integrated pest management.
Growers are also trapped between consumers’ contradictory dictates: fruit must simultaneously look perfect and have zero residues. Meanwhile, importing countries impose mandatory synthetic pesticide applications for phytosanitary pests and have zero tolerance for the presence of natural enemies on fruit.
Given the far-reaching implications of interceptions of phytosanitary pests, growers are understandably nervous about relying on biological control. Predators and pathogens never eradicate their pest hosts – that would be suicide. In addition, biocontrol agents need a burgeoning host population before their numbers increase.
“With alternatives, especially the biocontrol options, there’s a longer lag time to see the effect,” notes Allsopp. “It’s not the quick knockdown you have with chemicals, so timing is critical but challenging.”
Making integrated pest management a reality
The shift to integrated pest management requires readily available biocontrol options. “We must drive the commercialisation of local entomopathogenic fungi and nematodes,” stresses Allsopp.
Consumer education is another part of the solution. They need to be weaned off flawless fruit and learn to accept harmless blemishes.
“We spend a lot of time and money controlling little blemishes, like halo spots caused by thrips, because people don’t want to see the little angel kisses on their fruit,” says Allsopp, “even though they do not change the shelf life, the taste, or the quality.”
Crucially, Allsopp advocates more intensive training of future farmers and independent crop-protection advisers in integrated pest management while co-opting existing practitioners to spread the word among other growers.
“Asking growers to move away from synthetic-chemical, programme-based pest management to integrating alternatives is a complete paradigm shift.
“Researchers, who by their very nature are always exploring new ideas, don’t understand just what a massive change this is. We’re telling growers that the way they’ve always done things doesn’t work anymore – they have to do it differently,” says Allsopp.
