Back in the Dirty ’30s, farmers planted shelterbelts in a desperate bid to slow those drying winds and keep them from blowing our precious topsoil east. Now, at almost 100 years old, these wind-breaks have grown into stands of mature trees that continue to serve that purpose well.
Not only do these shelterbelts curb wind erosion, they also reduce evaporation and help trap snow that melts into the spring, adding soil moisture for that season’s crop.
The current generation of farmers, however, is looking at shelterbelts differently. Since conservation tillage is already holding the topsoil down and trapping the snow, why not remove the trees and put the otherwise unproductive land under crops?
This would also clear the way for machinery to run unfettered from one side of a section to the other, and make it easier for an aerial sprayer. Yet new thinking from the biological control people suggests there may still be good reasons to maintain those old tree belts.
“Shelterbelts and those kinds of perennial habitats around crops provide resources for beneficial insects, including natural enemies and pollinators,” says University of Manitoba insect ecologist Alejandro Costamagna. “A lot of them can’t overwinter in fields that harbour summer crops because it’s just bare ground. They need to overwinter in perennial habitats.”
The key word here is “perennial,” a permanent and stable refuge for insect predators during the off-season. A field of wheat may be very productive for a short period and predatory insects may have a fine time in there for a few months. But it’s fleeting and, once the crop is in the bin, the remaining stubble provides almost nothing for the ground beetles and parasitoid wasps that hunt there. They need a local refuge where they can spend the winter and be ready to move into the fields the next spring. Otherwise they could die off.
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On the other hand, native prairie offers a cornucopia of different plants that provide shelter, nectar and food year round. Although the populations of predator and prey insects may fluctuate, there’s an overall stability in the system. In this way the resident population of predators keeps a growing populace of plant-eating insects in check.
Those old shelterbelts are the last holdouts of stable habitats in a surrounding ocean of ephemeral crop fields, and research in other parts of the world suggests those groves are even more valuable than we know.
“There are some examples from Australia where they show that natural enemies from shelterbelts move into cotton,” Costamagna says. “They marked predatory beetles with rubidium and found that they move from these shelterbelts into the cotton and attack several pests.”
It’s a matter of timing. We’ve been farming for several thousand years and one of the first things we learned is how well pests can exploit a large stand of virtually identical plants. Under the best of circumstances, a crop gives them a limitless supply of high-quality food with no predatory enemies, so their populations can boom completely unchecked. On the other hand, if there’s a nearby pocket of generalist predators such as lady beetles, spiders and assassin bugs, the initial invasion doesn’t get the chance to boom and the numbers remain manageable. The predators may not eradicate them completely, but at least the pests are kept below the economic threshold.
Costamagna says he found this in his graduate studies on soybean aphid. The aphid isn’t native here and it doesn’t survive the tough Canadian Prairie winter very well. Its numbers bump up in the spring however, when the southerly winds bring in a new supply from the United States.
Once here, the aphid can do a lot of damage if unchecked. “We didn’t think that natural enemies were effectively controlling these aphids, which are from Asia,” Costamagna says. “We assumed that natural enemies here couldn’t control this pest.”
But their studies actually showed that when you put the aphids in exclusion cages (a box that separated the aphids from predators) the aphid population took off and very quickly overwhelmed the system. Under natural conditions, aphids are very poor at escaping predators so they’re a ready source of food for many of the more voracious types. Their principal defence is rapid reproduction.
The exclusion cage experiments showed two things. The first was how quickly those aphid numbers increased when there were no natural enemies in there with them. The second was the difference in aphid populations in the surrounding environment. It turned out that the native predators were effective at controlling aphid numbers as long as they got an early start.
“These aphids had always had the potential to become a pest and really knock yield down. In any experiment we did in any season and in any field, if you excluded the predators, the aphids became a pest,” Costamagna says. “But most of the time we don’t actually see that, so we think these natural enemies are controlling these aphids.”
We don’t know the environmental cue that triggers the aphids to boom out of control, but having the predators in place and ready before the aphids blow in may be the critical factor for keeping them in check.
That’s what makes these perennial habitats so important. They provide that pocket of habitat where the predators can live until they’re called into the fields again. Nor is it only for overwintering. Shelterbelts are also providing some relief from the summer heat.
“Insects can’t regulate their temperature well so when it’s very warm, parasitoids will move into wooded areas during the day and they will survive a lot better in there,” Costamagna says. “If you have areas like that near your crop, it’s more likely that you’ll have higher populations of these parasitoid wasps that can move when the conditions are better, find their prey and kill them.”
Now, more then ever, farmers are pulled in several different directions. They need productivity but they must also consider sustainability. If maintaining shelterbelts can provide both, then farmers can save time and inputs without hurting their profit margin. Those old shelterbelts can do this by providing a frontline defence against insect pests that requires no time investment.
Besides, those pests will never develop genetic resistance to predators. Still, these are not simple systems and we need to know a lot more about them.
“The first step is for farmers to understand that not all the insects are damaging, and I think the people working with farmers in that capacity are doing a good job on that,” Costamagna says. “And the second is before you spray, make sure that you actually have the pest you think you have, and that you have it in high enough numbers that it’s damaging.”