What makes fields suitable for natural pest control again? UvA biologists figured it out
Hoverflies and other natural enemies of pests can protect agricultural crops from aphids as an alternative to pesticides, provided there are enough bushes, hedges and flower borders around farmland. UvA biologists demonstrate this with a mathematical model. “This is the first step to make the vague idea of biodiversity functional for agriculture.”
Since World War II, many hedges, woodlands and flower borders have disappeared from farmland. Predatory insects such as the hoverfly can therefore no longer survive well there, making crops increasingly dependent on chemical pest control.
Chemical pesticides provided a solution – with the added drawbacks – but as more pesticides are gradually phased out in the EU, alternatives are becoming increasingly important. Researcher Paul van Rijn and PhD student Laura Mansier (IBED) used a mathematical model to calculate what is needed to make natural pest control effective in agriculture again.
Why did you choose to study the hoverfly?
PvR: “Hoverflies are a very important natural enemy of pests in crops such as wheat, potato, sugar beet and onion, which are common in the Netherlands.”
LM: “And the hoverfly plays an important role in keeping aphid populations in check, which are a pest for many crops.”
What is the minimum requirement for hoverflies to survive throughout the year?
PvR: “In the study, we show that at least four landscape elements are needed for the hoverfly. In our model, these are flower borders and woody elements, such as hedges and trees. And in addition two agricultural crops that grow early and late in the season, for example winter wheat and potatoes. If all four habitats are present then the hoverfly can reduce aphid density much further than if one element is missing, by up to 95 per cent.”
But does the aphid not disappear altogether?
LM: “No, and that’s not ideal either, because then the population of the hoverfly also collapses, which in turn causes the aphid to break out again on a neighbouring crop. Precisely a low quantity of aphids on your crops ensures that you maintain the hoverfly population over several years and thus have a low population of aphids on several crops that do not cause damage. So a small amount of pests is better than nothing.”
It has long been known among ecologists that a biodiverse landscape can prevent pests. Why does that require a mathematical model?
LM: “That’s a good question. You would say that you could also go out into the field and count the aphids and hoverflies in the bushes, hedges and flower borders, but it proves not to be that easy.”
PvR: “Indeed, in the field, counts of the predator insects and preys are made sometimes two or three times a year, but this makes it difficult to map the population dynamics through the year. With the mathematical model, we were able to do so. With it, we saw that during the year, aphid densities in one habitat went down faster than previously thought. As a result, the hoverfly population is endangered. With the model, we can see that it takes both a late and early crop to provide the hoverfly with prey throughout the year.”
How drastic is it for farmers to switch to natural pest management?
PvR: “One of the prerequisites for effective natural pest control is that you don’t spray pesticides. And sometimes it takes several years for natural enemies to recover sufficiently from pesticide use before natural pest control works optimally. So that does require patience from a farmer, and good monitoring of pest and natural enemy.”
“I do notice increasing interest in sustainable pesticides, also because pesticides are going to be banned one day. In the Hoekse Waard, where biologists have been researching how landscape elements stimulate natural pest control since 2005, more and more farmers are becoming convinced of the importance of flower borders and are stopping spraying. There, we have shown that it works, and then they are willing to accept it. But in a lot of places, we are not that far yet.”
LM: “There is often also the fear that flower borders will cause new pests again. But that is not necessary, as the aphids in the flower borders are usually different species from those in the crops. You can control that by sowing the right mixtures. On the other hand, farmers also show interest when we share our results. Then they ask for lists of flowers they can best place to support predatory insects for their crops. There is now also a handy tool for that.”
The use of pesticides in the EU has not yet been abolished. Will your model help with that?
PvR: “Initially, I don’t expect it to have much impact, because the availability of good alternatives is not currently used as an argument for abolishing pesticides. But I do hope it will have an impact on use in practice, simply because farmers see that it works.”
“At the same time, this research is just a very small step in a bigger project. We do now have clarity in the Netherlands for winter wheat and potatoes under which conditions natural pest control is effective. But we need to do the same for other crops. That way, you can make the ‘vague’ idea of biodiversity functional for agriculture. So not: we are going to make the landscape more diverse because then pest control will probably improve. But: what requirements do natural enemies have and how can you best design that environment? That also requires species diversity but it is not ‘random’. I can imagine that many farmers want to wait and see that before making the switch.
