Agricultural scientists select and breed food and fibre plant species with the ultimate objective of optimising productivity to feed and clothe the world’s growing human population. Recently a strong focus emerged in plant breeding, namely that of developing cultivars with their own inherent tools to combat plant pests and diseases.
The well-known Bacillus thuringiensis (Bt) genetics that are cloned into maize is an example of such technology that makes life easier for producers. It is becoming necessary to put a greater focus on these technologies, because as plants become progressively higher yielding, their natural resistance to pests and diseases wanes significantly. What we gain in yields, we unfortunately lose in natural resistance against pests and diseases. Plant pest and disease management has long surpassed the concept of ‘control’. Modern crop production is more about having healthy and productive plants than fighting pests and diseases with a range of chemical solutions.
Food and fibre producers deploy modern concepts such as minimum till, disease and pest genetic traits, soil health, moisture retention and biological pest management along with synthetic pesticides to gain optimal yields from the same crop fields that were low yielding less than two decades ago. Yet, the pests and diseases are still competing with the combine harvester for the producer’s crops and it requires clever footwork to outwit them on the farm.
Vulnerability of plant pests to pest management technologies
Pest management is a costly exercise for producers, not only because of the cost of pesticides, but also due to the dispensing costs, labour, and wear and tear of equipment. It should be the objective of the producer to spend as little effort and pesticide as possible to manage plant pests as early as possible in the pest’s life cycle to avoid paying the price of late-season corrective action against mass pest outbreaks.
Plant pests are mostly invertebrates such as insects and mites, while rodents are the only vertebrate pests worth paying attention to. Insects and mites start their life cycles off as eggs and go through various developmental stages or instars until they reach adulthood. The advanced larval stages are in most cases the most damaging to plants, especially in the case of Lepidoptera (moths and butterflies), while some insects such as thrips and whiteflies gnaw their way through crops in their adult stages. Larvae in their youngest stages are the easiest to control – as they grow older and moult into adults, larvae pose bigger control challenges.
Young larvae, which are caterpillars or worms in the case of moth pests, are much more sensitive to insecticides than older larvae. Once insects like the Coleoptera or beetles reach adulthood, their strong chitin exoskeletons are solid defences against contact insecticides of a chemical nature. Even biological insecticides such as the Metarhizium fungi are more effective against younger stages of insects than full adults.
These young larvae are voracious feeders and will succumb to stomach insecticides very rapidly because their intake of the toxins is vast. Their metabolism is also rapid, which results in very quick action of the insecticides at receptor sites in the larvae’s systems.
Early intervention against plant pests prevents large-scale physical damage and successive bacterial, viral, or fungal infections. There are simply too many reasons for early-season pest management to wait for late-season outbreaks. Another aspect is that the early-season insecticides are often the ones that are ‘softer’ compared to ‘harder’ chemicals that are required to knock out adult insects. As such, these are more compatible with natural predators that assist with pest management.
Scouting is key
Producers are well versed in the crop-pest complexes that they can expect in various crops. The fact that the African bollworm is a threat to many a crop, does not mean that it will emerge in every crop on every farm in every season. It is the producer’s own paramount defence to be vigilant for signs of the typical plant pests prior to planting, directly after planting and during the crop’s lifecycle. It is troublesome that many producers are caught off-guard by pest outbreaks. The infamous African bollworm wreaked havoc in canola some four years ago. By the time that producers became aware of the infestation, the larvae were already close to pupation – meaning the battle against the bollworm was lost. The question is then: ‘How can producers detect plant pests early enough to control them before they run amok in crops?’
Scouting is the only answer to prepare for early lifecycle attacks or pest outbreaks. The nature of the term seems very military, but in practical terms it is much more encompassing than simply being on the lookout for signs of pest infestations. Technology provided weather forecasts on smart phones, miniature weather stations to deploy on farms, drones with infrared and ultraviolet detectors and insect traps with pheromones. All of these technologies must be embraced by producers as a means of forecasting insect pest outbreaks.
Weather patterns
Moist air, even before any precipitation, is a tell-tale sign that insect populations will gather momentum and hatch to send their legions into crop fields. The warmer the ambient temperature, the better for the insects. Cool, moist weather seldom brings out the insect hordes, but once the ambient temperature exceeds 16˚C, eggs hatch and insects flourish. It is important to keep a close watch on wind patterns, because moths can be airborne by strong winds that can bring pest species from far away into crop areas. Producers should recall the fall armyworm invasion of 2016 when this exotic pest arrived under the radar in the early summer of 2016 – courtesy of northern winds that brought them down from the tropics.
Visual scouting and inspections
There is no better way of detecting the enemy than laying eyes on it. A producer should be checking crop fields at least twice a week for any signs of pests that may be arriving or have already established themselves in the crop. Plant leaves must be checked visually and if any unusual objects are observed, pull out the loupe (small magnifying glass) and study it. It may be an egg parcel of a moth or beetle species or worse, thrips or whitefly that have already established a population in the crop. Leaves should be thoroughly inspected on the upper and lower surfaces because some pests like mites often take refuge under leaves.
Back in the old days yellow glue boards were used to trap insects. Although that tool is still available, a late afternoon drive-by in and around the crop fields should give the producer early warning of pest invasions, especially moth pests. Had producers been scouting for moths in the early summer of 2016, the fall armyworm would not have caught them off-guard.
Pheromones are unique chemicals that serve various functions in invertebrate biology. Some pheromones lure males to females, others trigger alarms in insects in the face of imminent threats and others simply function as social attractants, like the pheromones produced by dung beetles when they find fresh food and alert their mates to the presence of a feast. Agriculture uses sex pheromones to lure males to traps; these pheromones are quite species-specific, meaning one pheromone may only attract one species of moth. Suppliers of these pheromones give guidance on the thresholds at which the pest species requires intervention. Traps are placed in and around crop fields with the species-specific hormones. Early morning checks will tell the producer whether the pest species require control actions.
Early pesticide application is better and more cost-effective than late-season corrective action
As mentioned earlier, it is strongly advisable to tackle pest infestations at the earliest possible stages. Biological remedies are usually better suited to early-lifecycle control, because juvenile insects are much more susceptible to such products than adults. Chemical insecticides are also much more effective against the younger instars, not only because of their soft outer skeletons, but also because they do not burrow into leaves or cobs at such an early stage of their lifecycles.
Take as example the brown locust. The current brown locust outbreak started in January 2019 and turned into one of the largest ever experienced in South Africa. It turned into a cataclysmic outbreak in the summer of 2022 due to a number of reasons. The first reason is the depopulation of the Karoo and Kalahari and subsequent insufficient reporting of outbreaks over most of the area. The second is that emerging swarms were not controlled mechanically or by insecticides during the hopper instar phase. These hopper bands aggregated into massive flyer swarms that covered thousands of hectares which necessitated aerial application of insecticides to bring them under control.
The message to take home is that plant pests must be detected and brought under control as soon as possible. It is cost-effective and prevents the mass pollution of the environment when large quantities of insecticide must be applied to rescue crops from mass pest invasions.
