Every year 30,000 tonnes of active insecticide ingredients are sprayed onto fields across the US. Some farmers are more dependent on insecticide than others; in the Midwestern US the amount of cropland in each county treated with insecticide varies from 0 to 60%. Some of this range is explained by the type of crop: fruit trees are more vulnerable to pests, for example. But much of the variation in insecticide use is not so clear.

Tim Meehan and Claudio Gratton from the University of Wisconsin-Madison, US, used Census of Agriculture data to investigate the relationship between landscape and insecticide use in the Midwestern US between 1997 and 2012. Using spatial regression techniques they investigated whether landscape simplification (decreasing the diversity of the landscape) was correlated to insecticide use.

Sure enough, they found a positive relationship between landscapes that had been significantly simplified and increased insecticide application. So why do pests increase when natural habitats are lost and landscape diversity decreases?

"Hypothetically, a simple landscape (which, in this case, means one that is dominated by annual monocultures) is a big target for mobile crop pests, and there are few barriers to moving around from field to field," explained Meehan.

To make matters worse the natural enemies of crop pests find it harder to live in these annual monocultures – there is little habitat provision for the winter months, and a limited diet of corn alone. "The current hypothesis is that when you increase the amount of habitat for pests and decrease the amount of habitat for their enemies, you might expect more pest problems and more insecticide use," said Meehan, whose findings are published in Environmental Research Letters (ERL).

Similar relationships are likely to exist in other parts of the world, though the degree of correlation will probably vary according to the ecological requirements of the crop pests and those of their natural predators. "Some pests rely on natural habitat for part of their life cycle," said Meehan. "In cases where these pests are the ones that cause the most damage, simplifying landscapes could possibly reduce pest problems."

In general the study indicates that retaining more diversity in the landscape could reduce reliance on insecticides. In particular, farmers might do well to encourage natural enemies of crop pests by keeping hedgerows, wetlands and small woodlands. And more diversity of crops could help too, with some perennial crops, or cover crops to provide autumn and winter habitat. The solutions will vary from region to region, but on the whole it seems that industrial-scale farming makes life too easy for crop pests.

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