Biological pest control

While plagues of insects pests can be a problem for agriculture, other insects can literally munch on the pests and in doing so protect crops.

Marcel Dicke’s study office is infested by insects: Scale models, posters, toy spiders, art objects and jewellery. It illustrates the fascination of this professor of entomology for his profession, which has had a place at Wageningen UR for long. The scientific attention was initially focused on the damage caused by insects in agriculture. Many different types of insects partake in the rich harvest of agriculture: Caterpillars, aphids, grasshoppers, beetles, you name it. Scientists are constantly developing new methods to contain their gluttony and prevent pests. And they are receiving help from an unexpected source: Other insects.

Certain insects are in fact able to selectively control another species. Biological pest control of insects by insects has made great leaps forward. We know the successes: Parasitic wasps, which lay their eggs in caterpillars or on the larvae of, aphids and whiteflies, are extensively used for insect control in greenhouses. In open cultivation, too, insects can do wonders, the best example being the suppression of a scale insect in citrus cultivation with ladybirds. This solution dates back to the late 19th century and is still effective today.

When implemented in time, biological pest control lasts longer than chemical control

The Wageningen research has shown that plants themselves play an important role in the effectiveness of the biological pest control. Plants attacked by herbivorous insects can use parasitic wasps and predatory insects for protection as a kind of bodyguard, for example responding to an attack by producing odours that attract the bodyguards. Such tritrophic relationships appear to be common in plant-insect interactions. Plant varieties that cooperate more closely with bodyguard insects can be selected, and this can lead to crops on which biological pest control is more successful.

Plants interact with a community of insects at different trophic levels Plants harbour a community of herbivores and their natural enemies that form a complex of interacting associations. Plant defensive traits directly or indirectly interact with each of these organisms and thereby affect the composition of the insect community. Plant direct defense is exploited as plant resistance, plant can have an indirect defense by stimulating the effectiveness of biological control agents.
Plants interact with a community of insects at different trophic levels Plants harbour a community of herbivores and their natural enemies that form a complex of interacting associations. Plant defensive traits directly or indirectly interact with each of these organisms and thereby affect the composition of the insect community. Plant direct defense is exploited as plant resistance, plant can have an indirect defense by stimulating the effectiveness of biological control agents.

From incident-based to system

Until recently, both farmers and scientists were lagging behind the facts. To solve this, Dicke and his research team promote taking a systems approach instead. What do biological control agents require from their environment in order to combat pests as efficiently as possible? Based on scientific knowledge and extensive experiences by growers and biological control companies, a pest management system has been developed for greenhouse-grown vegetables which – thanks to biological control – is largely pest-free. The system is pretty well understood and the control of new pests can therefore be quickly included.

In open cultivation, things are somewhat trickier. The biological control agents can fly or crawl away, and are more likely to fall prey to their own natural enemies. The quality of an effective biological control system revolves around accessibility and balance. Leaving field margins intact and growing flowers, for instance, creates a favourable habitat for the bodyguards from which they can invade the crops to be protected.

At the same time, it is important to ensure that the introduction of new insects does not disturb the existing system. If a beneficial insect protecting one plant attacks the bodyguard of another plant, the problem may be worse than the solution. Biological control is always a search for balance. And this requires a systems approach.

A systems approach connects fundamental and applied science. This results in the design of intelligent farming systems in which growers know exactly what is needed at any given time. Dicke expects that agriculture without chemical insect control will be possible in the long term. It will be achieved via a systems approach which effectively integrates biological control and breeding and cultivation measures so that they reinforce each other. In the protection of, for instance, cabbage against certain insects, such an approach can even be the only way – the diamondback moth, for instance, is resistant to almost all pesticides.

An obstacle for many forms of insect control is that not every farmer knows enough about the mechanisms behind this systems approach. For instance, the time that insects need to do their job properly can be relatively long. The pest does not disappear overnight when the bodyguards are introduced, as is the case with chemical control. The perception is then often one of ineffectiveness or an insufficient impact, especially among farmers who cannot afford a bad harvest. When implemented in time, however, biological pest control lasts much longer than chemical control. If farmers can move beyond the reflex of quickly spraying with chemical pesticides, more chemicals could be removed from agriculture than we currently think possible.

Hyposoter ebininus laying eggs in a Pieris rapae caterpillar
Hyposoter ebininus laying eggs in a Pieris rapae caterpillar

Stress

We have long known that insects play an important role in nature and agriculture. It is no coincidence that a fierce debate is raging about the apparent disappearance of the honeybee, one of the most important pollinators [LINK: 4.1. ecosystem service]. Seventy percent of crops cultivated for human consumption depends on pollination by bees. Dicke’s team would like to start research on the mechanisms that cause stress to the honeybee. Why do some honeybee colonies have such high mortality rates today? The answers range from chemical pesticides, the Varroa mite and a fungal disease to the uniformity of cultivation and intensification of land use, which may make bees hungry and vulnerable. The relative contributions of these possible causes require a critical analysis of the underlying mechanisms."

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