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Study suggests marginal land boosts beneficial insect population

A new study suggests that restoring permanent non-cropped areas to marginal farmland can be hugely beneficial for a diverse group of arthropod insects, including many pollinators and pest predators.

Arthropods are a huge phyla of animals with exoskeletons and jointed limbs: beneficial arthropod insects include bumblebees, parasitic wasps and beetles.

Uncultivated pasture, field and road edges, hedgerows and riparian areas aren’t considered profitable land and can’t be cropped. But, these areas are actually “resource rich” for arthropods, writes Aleksandra Dolezal, a PhD candidate at the University of Guelph, in a new publication on the importance of marginal areas for beneficial insects.

“Typically, these areas are critical habitat for many life-history stages of arthropods, providing stable food resources, nesting sites and refugia for overwintering,” she writes. “As agricultural landscapes transition towards larger crop fields on bigger farms, non-crop habitat is under pressure.”

Dolezal’s supervisor is Guelph ecology professor Andrew MacDougall. The impetus of the research, says Dolezal, was a tour of an Ontario farmer’s operation when Dolezal was set to begin her Masters research. As Dolezal and MacDougall walked along a field edge bordered by natural prairie, they noticed the insect diversity in the area and paused to wonder whether the prairie grasses were a “source or sink” of beneficial insects.

MacDougall and Dolezal opted to set up a study testing the impact of restored marginal farmland on 13 conventional farms in Norfolk and Elgin counties in Ontario, and comparing arthropod richness and abundance on three habitat types – forest, crops and restored prairie. 

The researchers’ hypothesis was that richness and abundance of arthropods in restored areas might not be very different from crop fields, because the latter has been the dominant regional landscape for decades. But this wasn’t the case, they write.

There was a rapid response from native pollinators to newly created habitat. “Indeed, many taxa were detected in restored areas that had only been planted one year earlier, even though Prairie species in this region typically take three to five years to reach full maturity,” they add.

They found that restored prairie had twice the abundance of arthropods compared to the other two habitat types. 

Aleksandra Dolezal.

Insects collected for Dolezal’s study.

Research results
Dolezal says there are gaps in the research that the study sought to fill. Often, studies that look at insect ecology rely on one-time sampling for field data. Dolezal wanted a higher level of “spatial and temporal resolution,” she says.

The team did sampling over seven weeks, between May 18 and July 1, 2017. Researchers wanted to sample before insecticide application began – typically in early August. They used yellow pan traps and sweep netting and classified insects into groups, focusing on herbivores, predators, parasitoids and pollinators for the purposes of the paper.

Dolezal and her collaborators identified a total of 12,590 arthropods from 183 families on 13 farms: a total of 52 families of herbivores, 52 families of predators, 24 families of parasitoids and 10 families of pollinators. 

Crop fields supported many pests, but had lower richness than prairie; prairie had higher abundance and richness despite being relatively recently restored (within the last decade) and comprising a small percentage of the farm (an average of 9.6 per cent, says Dolezal). In fact, there was no change in arthropod abundance based on the year the natural prairie was restored. 

“These prairies were so small, [just] nine per cent of the total farm area,” she says. “But, the farms we sampled had comparable insect richness to conservation areas.”

The researchers also wanted to understand what aspects of the restored habitat best explained its attractiveness to beneficial insects, so they looked at four factors, including plant biomass, tissue quality (foliar N), plant cover and canopy structure (for example, the degree of exposed bare ground). They found that plant functional cover was the most important factor; plant biomass and tissue quality didn’t have an influence on arthropods.

The researchers compared their data to data housed in an online database that records insects people have observed in the area.

The research points to the importance of even very small restored marginal farmland for supporting a diverse community of beneficial insects, says Dolezal.

“Having a prairie buffer is most helpful to the spatial distribution of insects,” she adds. “Predators will be closest to the edge of prairies, and they dwindle in the interior. Plant them right next to your crop.”

There is support for this kind of restoration. Dolezal points to the work of ALUS (originally an acronym for Alternative Land Use Services), a Canadian charity that aims to promote natural ecosystem services on farms. Norfolk County, where some of the study’s sampling was performed, has the oldest continuously operating ALUS program in Canada, according to its website.

ALUS provided field sites for sampling, says Dolezal. “Without the farms to sample, I wouldn’t have a project. ALUS also provided support in terms of a platform to talk about my research findings and they will use my technical expertise to help build better natural infrastructure projects,” she says. 

ALUS can help share establishment costs for land restoration projects, and provides annual payments for farmers over contract terms “to recognize the value of your time as you maintain and manage your projects,” according to its website. 

ALUS hosts many research sites across Western Canada, particularly in Alberta.