Despite growing concerns about side effects, chemical pesticides remain the primary method of controlling insect pests in agriculture.
A major concern is that pests tend to develop resistance to these chemicals. In addition, things tend to take a turn for the worse if these chemical pesticides also impact the natural enemies of certain pests.
A team of biologists at the University of Amsterdam has shown that pesticides often do not reduce average pest densities in these types of situations (commonly, it even increases pest density).
Adult hoverflies often live on pollen and nectar, such as this specimen on a daisy flower. The larvae of some species are natural enemies of aphids, important pests on many agricultural crops. - (Image Credit: Jackie Tweddle via Shutterstock / HDR tune by Universal-Sci)
Simulation of pests and their natural enemies
The Dutch researchers used various mathematical models to simulate the densities of pests and their natural enemies in order to find out how the higher pest densities occur after crop treatment,
The team simulated different chemical pest control methods, from once per season to regular application when pest densities exceeded a certain threshold.
The simulations show that when natural enemies of the insect pests are present, the average pest densities during a season increase when pesticides are used.
According to Dr. Paul van Rijn, one of the researchers, a possible explanation for this phenomenon is that the densities of natural enemies of pest insects decrease along with the density of the pests themselves.
The densities of the natural enemies decrease due to death from the pesticides; following that, a further decrease occurs due to the decreased availability of their food (the pest insects).
This hypothesis has been confirmed with simulations in which the natural enemies could tap into other food sources in addition to the pest insects: as long as the pesticides did not influence this alternative food, this did indeed lead to much lower pest densities.
Field experiments
A natural follow-up question was whether experiments confirm these theoretical findings.
The team performed a so-called meta-analysis of published field experiments to answer this question. The experiments in the analysis had to last longer than a few pest generations but had a maximum of one growing season because the increase in pest densities occurs on that time scale.
A flower-rich field border next to a crop field These edges provide food and shelter for natural enemies such as hoverflies, which control insect infestations in the crop and reduce the use of chemical pesticides. (Image Credit: angel217 via Shutterstock / HDR tune by Universal-Sci)
In addition, the experiments had to compare the densities of pests with and without pesticides. Enemies of the pests were found to be naturally present in the vast majority of these experiments, and indeed the mean effect of chemical control in those experiments did not differ significantly from no effect.
According to lead author Arne Jansen, associate professor at the Institute for Biodiversity and Ecosystem Dynamics of the University of Amsterdam, the average pest density decreased with chemical control when no natural enemies were present, which was the case in the minority of the studies.
The research shows that the presence of the natural enemies of pest insects often makes the use of chemical pesticides superfluous.
According to van Rijn, we could probably achieve superior natural control of pests by promoting the effectiveness of their natural enemies, for example, by protecting, stimulating, or releasing them in a targeted manner.
Moreover, pest control methods that utilize natural enemies will lead to more sustainable agriculture as the use of chemical agents can be reduced, leading to fewer unwanted side effects.
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