Field Borders Provide Winter Refuge for Beneficial Predators and Parasitoids
By Paige Embry
Plants are always at risk of untimely damage and death, often due to the weather—a late frost, too much rain, too little rain, excessive wind—but various living organisms can also cause devastation. For gardeners, damaged plants are maddening, but, for farmers, preventing or limiting that damage is necessary for maintaining their livelihood. The weather is beyond the control of farmers and gardeners, but they do have a long list of tools to use against many of their living foes, from crop rotation to pesticides to purchased predators and parasites. For some, a hidden benefit of the predator/parasite route is the morsel of satisfaction felt when some sap-sucking, leaf-chewing, wood-boring, or disease-bearing pest is taken out by a tiny predator or turned into a pantry for a parasite’s offspring.
Natural enemies of pests don’t have to be purchased. Wild predators and parasites can be encouraged to live near agricultural fields, and a new paper published in May in the open-access Journal of Insect Science shows that semi-natural field edges can provide winter refuges for natural enemies of pests so that they can be ready to jump into pest-destruction services early in the season.
The study took place in the spring of 2018 at five certified organic fields in Illinois that had grown soybeans the previous year. All of the fields had borders containing a mix of grasses and sundry herbaceous flowering plants. The scientists set up 20 soil emergence tents per site—10 in the field, 10 in the border—to capture overwintering arthropods as they emerged. The tents went out in mid-March, and the scientists collected their gleanings every two weeks until the end of April.
They found that the semi-natural edges had greater species richness and higher total numbers of natural enemies compared to the adjacent fields and that flowering plant cover tended to support more natural enemies (in total numbers and species richness) than grass cover. They also found that, at four out of the five sites, the fields and their adjacent borders contained different suites of natural enemies. The edge of the lone site with a similar suite had been mowed the previous year and contained more grass and fewer flowering plants than the edges of the other sites. “It was quite surprising the level of difference most of the sites seem to exhibit,” says lead author Scott Clem, Ph.D., who just completed his doctorate at the University of Illinois.
Having natural enemies like predaceous beetles, spiders, and wasps spending the winter near crops can provide enormous benefits. Clem and co-author Alexandra Harmon-Threatt, Ph.D., write, “If natural enemies are not present at sufficient levels early in the season, they are less likely to manage pest problems before they breach economic threshold in spring and ensuing warm months.”
Also, having a diverse array of predators and parasites means there is some redundancy in the system if one predator has a bad year; plus, it provides more avenues of attack on potential pests, hitting them in different micro-habitats or at different life stages. Clem says, “The type of species diversity really matters when it comes to pest management. … It also depends on the type of pest too. Pest insects that exhibit complete metamorphosis … might be more readily controlled because there’s multiple life stages that can be attacked.”
In areas with large amounts of agricultural land, semi-natural borders provide not just plant diversity but also areas where the cover is permanent and the land relatively undisturbed. These borders are havens for an array of useful arthropods that include pollinators as well as predators and parasites. Clem notes, “You don’t necessarily have to do that much work to improve biodiversity … especially in an Illinois landscape where it’s mostly monocultures as far as you can see.”
Journal of Insect Science
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.