Ever since the term ‘biosecurity’ took a front seat on the global ecological train, consumer demand has increased for greater volumes of quality food that is free from pesticide residue. This has placed a fundamental focus on how classic or synthetic pesticides are used and on finding ways to manage their residues.
Chris Coetzee is a researcher and facility manager at BioScience Research in Cape Town, a facility responsible for research, development and registration of plant protection products and related technology in South Africa.
On the topic of residues, Chris says there should be no illusions about the way in which misuse and overuse of pesticides have resulted in pests and diseases developing resistance to established compounds, rendering them useless to growers.
“This is nothing new and it happens all over the world,” he says. “It has added to the drive for finding novel, effective and safer alternatives for synthetic pesticides in order to develop sustainable, integrated pest management programmes.”
This article outlines some of the advances made by researchers and the global crop protection industry in working towards this goal.
Cultural, physical and genetic control
Some of the oldest measures used by man to limit the effect of pests and diseases in crop farming, includes strategies such as physically killing or removing insects from crops, installing insect-proof netting in growing tunnels, or making use of geographic isolation and altitude to put distance between pests and crops. Another example is the use of physical stem barriers to prevent weevils from reaching crops.
“We have developed simple and very effective means of combatting insect pests in crops by studying and understanding insect behaviour,” says Chris. “Recent advances in genetics has led to the development of ribonucleic acid interference (RNAi) technology, whereby small ribonucleic acid (sRNA) molecules are used to silence gene expression in a specific organism. The first RNAi-containing products are already approaching the market.”
Cultural control can include planning and other strategies. For example, planting dates can be adjusted to avoid pest occurrence, as is currently being done in South Africa to minimise the impact of sclerotinia on soya beans.
Chris explains that short-growing, early-maturing cultivars are planted early in the season in areas where the disease is most devastating. “When the normal environmental conditions support sclerotinia outbreaks, the plants are no longer susceptible to the disease, thus mitigating any need for fungicide applications.”
Other controllable practices such as irrigation, fertiliser programmes and other agronomic practices can help create environments in which the pest is not able to flourish.
Some female insects use the secretion of a combination of specific chemical compounds to attract males for mating. These volatile chemicals are referred to as sex-pheromones. Scientists have developed ways of synthesising these pheromones to mimic their effect in order to confuse the males and disrupt mating.
“This is by no means a new technology,” says Chris, “but we are continuing research and adding new species to the list of pests that can be controlled this way.”
In addition to using insect pheromones for mating disruption, insects have also been placed inside lures to trap more insects. “Combining these pheromones with insect-killing compounds inside special formulations and devices, means they don’t come into direct contact with the food we eat.”
Insects can also succumb to diseases, often brought on by entomopathogenic fungi (EPFs). Because EPFs are natural mortality agents and environmentally safe, much research is being conducted around the world in respect of the use of EPFs for biological pest control.
Bacillus thuringiensis (Bt) is a bacterium that has been successfully developed as a biological insecticide. Chris explains that some Bt strains sporulate and produce special proteins called δ-endotoxins. “When these endotoxins are ingested by an insect, they interfere with the digestive system function, ultimately resulting in the insect’s death. These proteins are specific to certain insects and pose no risk to humans or the environment, provided they are properly used.”
Sterile insect technique
False codling moth (Thaumatotibia leucotreta) is an endemic pest that poses a serious risk to the major markets in South Africa exporting crops such as citrus, table grapes and stone fruit.
“Female moths fly around in orchards and lay up to 800 eggs on the fruit. The eggs hatch and the larvae infest the fruit, resulting in secondary decay, fruit drop and crop destruction,” says Chris. “However, females mate only once. By rearing millions of males in huge insectariums and sterilising them before they mate with wild females, the production of viable offspring can be effectively prevented.”
As the males do not feed on any crops, great numbers can safely be released in areas where natural populations threaten crops. “When the sterile males, outnumbering the wild males by at least ten to one, mate with the wild females, they produce eggs that do not hatch.”
Nematodes are parasites found in organisms ranging from plants to animals, humans and insects. These insect-killing or entomopathogenic nematodes (EPNs) are small, soft bodied, non-segmented, round worms. EPNs occur naturally in soil and hunt down host insects by following their carbon dioxide trail, vibrations, or other chemical clues.
Two families of EPNs, Heterorhabditidae and Steinernematidae, have been successfully developed as insect controlling EPNs for agricultural use in South Africa. These insect hunters enter the insect’s body and immediately secrete harmful bacteria. The infected insect dies within 24 to 48 hours, after which the EPNs continue to feed on the insect tissues and reproduce inside the body of the insect.
“It basically uses the host to produce more EPNs,” says Chris. “They are completely safe to handle and pose no risk to humans or the environment if used correctly. They also fit perfectly into most integrated pest management programmes and we even apply them using conventional application equipment.”
The use of EPFs, EPNs and related organisms are sometimes called microbial insecticides, or simply microbials, because they are microbiological in origin and action.
The use of predatory insects or arthropods to control pests, sometimes called macrobials, involves the deployment of predatory wasps, lacewings, ladybirds or other arthropods such as spiders or predatory mites to control a plant pest.
“It may be that the macrobial solution or predatory bugs are mass produced in a insectarium and artificially introduced into the area where the pest threatens the crop, or it may even involve bringing branches or weeds from an orchard where predatory arthropods are observed, and transferring them to a crop that is under pest attack.
“The grower may also specifically develop a crop protection strategy using the natural enemies already present in his fields or orchards, as a management tool to avoid damaging the natural populations of predatory bugs present in growing systems.”
Chris warns, however, that sometimes the use of an antagonistic compound to control one pest, may result in the disruption of the balance brought on by these healthy populations of predatory bugs which may lead to an explosion in numbers of yet another pest.
Plant disease agents
Plants are also part of a complicated system involving other living organisms, some of which attack plants, such as disease-causing bacteria and fungi. They are called plant- or phytopathogens.
Chris explains that plants serve as a source of food for these phytopathogens, but not all these microbes that live close to plants are bad. “A single soil sample may contain more than 30 000 taxonomic varieties of microbes,” he says. “This accompaniment of soil microflora assists plants in absorbing water and nutrients, and suppressing disease.”
The symbiosis between these micro-organisms and plants is based on an exchange of resources. In some instances, these beneficial micro-organisms can displace the bad ones or lead to the proliferation of others that produce toxins that may kill the plant pathogenic organisms or plant eating nematodes. These micro-organisms can even trigger the plant to induce natural resistance to the disease-causing ones.
“It is of utmost importance that the use of these alternatives to synthetic chemicals be researched to establish whether they pose any risk to the environment, the crop or ultimately the consumers of the food we grow,” says Chris.
When growers consider such a remedy, it is important to make sure that it is registered in terms of the Fertilizers, Farm Feeds, Seeds and Remedies Act, 1947 (Act 36 of 1947). He mentions that this piece of legislation governs all pest suppression methods, from EPNs, EPFs, mating disruption, plant growth promotors, plant resistance inducers, predatory bugs and other macrobials, including traps and related devices.
“The main aim of this Act is to ensure the protection of the consumer of the agricultural commodity, the farmer and his workers, the environment, as well as South Africa’s access to international markets.
“The South African Bioproducts Organisation (SABO) was established to engage with legislators, producers and growers to ensure that only products that comply with the requirements for user, consumer and environmental safety, are offered commercially.”
Chris concludes that SABO and its members subscribe to a code of conduct and that the products that are produced, comply with all the relevant safety and legislative requirements.