Insect pests thrive in less diverse cropland

A lack of biodiversity in fields can result in a reduction in predators that feed on pests and an increase in food resources

In any natural landscape, biodiversity thrives in the interplay of prey-predator relationships and the soils and vegetation that support them.

But how does that environment translate in agricultural fields? How do croplands and monocrop production influence populations of crop pests and pest predators and how does that, in turn, dictate pesticide use?

At the University of California, Santa Barbara, scientists found less diverse croplands led to greater variability in pesticide use and higher peak pesticide application, underscoring the fact that pest insects benefit from larger farms and less diverse crop production.

Researchers analyzed 13 years of data from Kern County, one of the state’s top agricultural producing regions.

“There are two main pathways by which less diversified production can benefit pests directly,” said Ashley Larsen, assistant professor at UC Santa Barbara.

She said one way is through a reduction in predators that feed on pests. The other is through an increase in food resources.

Larsen said it is well known that many natural enemies of pests decrease with less ecological diversity. But it was less clear whether the reduced natural enemies translated into reduced pests, the use of pesticides, or influences on yields.

Larger farms and larger fields have sparked much of the decreased diversity, she said. She acknowledged that farms have gotten larger to take advantage of economies of scale.

However, she said that the potential benefits from ecosystem services could reduce the cost of inputs with possible market benefits through revenue streams, such as farmers’ markets and other regional outlets.

Larsen and her colleague Frederik Noack, assistant professor with the Food and Resource Economics Group at the University of British Columbia, studied Kern County’s records from 2005-17 focusing on field size and the amount and diversity of the croplands. With larger fields and increasing croplands, they found there was an increase in the use of pesticides.

“We find increasing cropland in the landscape and larger fields generally increase the level and variability of pesticides, while crop diversity has the opposite effect,” the authors wrote in their report.

They said as the field size increases, the area of the field gets larger than the perimeter. Conversely, smaller fields have proportionally larger perimeters, which invites more spillover from nearby beneficial predators like birds, spiders and ladybugs that eat agricultural pests. In a smaller field environment, predators can keep pest populations more under control.

With the centre of the smaller field closer to its edge, the ecological benefit of predatory insects thriving in the perimeter extends further into the area of the smaller field. In this way, they extend their reach protecting crops across the broad spectrum of the field as a whole.

However, large crop fields are more vulnerable to pest damage because the ecological interplay at the boundary has less impact in the field’s centre.

“As we simplify landscapes, there is less diversity of habitat available to support enemies with heterogeneous resource needs,” said Larsen. “Think of birds needing nesting habitat or insect predators that need overwintering habitat. As you have more continuous cropland there is less habitat to support these organisms.”

In addition, said Larsen, if agriculture is simplified there is little to stop a serious outbreak of one type of pest. A pest in a monoculture not only benefits from a reduced number of predators but has unlimited food resources which allows them to increase rapidly in population.

“With regard to environmental health, pesticides in general and insecticides specifically have negative impacts on a range of (valuable) organisms beyond their target pests,” she said.

“Reducing environmental contamination stemming from insecticide use is of broad interest to scientists, policy makers and the public. The challenge, of course, is that diversifying cropland and maintaining non-crop habitat are costly for farmers and they don’t reap all of the benefits of those actions.”

Even so, she said, there is robust evidence that smaller fields support more biodiversity, including beneficial species like birds, spiders and ladybugs.

“However, the evidence gets a bit murkier when we consider outcomes that directly impact human welfare, such as yields or insecticide use,” said Larsen. “A very recent meta-analysis suggests that yields more often than not benefit from diversification.”

Going forward with the research, she sees a number of gaps to be filled.

“Studies that capture stability of pests directly, or tie pest abundance over time to pesticides are sorely needed, as are ecotoxicological studies that better assess the suite of pesticides used in high value, diverse cropland.”

Specific to their own research is to further explore the impacts of field size, farm size, and on-farm management for pesticide use as well as for natural enemies of pests such as birds.

These characteristics, she said, are under the control of one farmer so are easier to target with agri-environmental programs in contrast to landscape characteristics.

The research study was published recently in Nature Sustainability.

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