Pest control services provided by naturally occurring, beneficial insects save growers in the United States around US$4.5 billion per year.
Add that to the just over US$3-million benefit they provide through pollination of crops, and these are pretty valuable critters to have on the farm.
New research at the University of Manitoba is trying to find out more about how the landscape, different crops, and movement patterns of predator and parasitic insects affect the degree of suppression of crop pests.
Researchers already know that most predator insects such as lady beetles and lacewings find places to complete their life cycles outside of crop fields.
“Many insects find residencies outside the field for most of the season,” said entomologist Alejandro Costamagna of the University of Manitoba in a recent presentation to the Manitoba Agronomist Conference in Winnipeg.
“Some are overwintering sites, moderate microclimates, alternative prey or hosts, or host plants, and nectar or pollen sources are very important for parasitoids to have enough energy to find their prey.”
Several studies in different areas of the world have shown that the more diverse, natural habitat farmers have around their crop fields, the better level of pest control they can expect. A German study found that there was less crop damage by herbivore insect pests in crops in areas that had a higher percentage of non-crop areas than in areas with fewer non-crop areas.
“Having non-crop habitats in the landscape within one to two kilometres of the field explained most of the parasitism and reduction in herbivory, said Costamagna. “Complexity in the landscape will increase, in general, responses associated with natural enemies. They are more abundant, more diverse, and exert more control of pests.”
Costamagna has been conducting research into suppression of soybean aphids in Manitoba using common predator insects such as lady beetles and green and brown lacewings. Soybean aphids are a huge problem in the U.S. Midwest, and although there have been a few outbreaks in Manitoba over the past 10 years, they are a sporadic issue.
Even though levels of these natural enemies are generally quite low in Manitoba fields, the study found they still did a good job of controlling small colonies of soybean aphids. When natural enemies were completely excluded, using exclusion cages on some plants, aphid populations doubled in one week and came close to threshold levels. The researchers also found that the level of control significantly interacts with the landscape, and the intensity of predation was not the same across all the fields.
More cereals, fewer aphids
When there are more cereals on the landscape more predation on aphids was observed, but the opposite is true for canola — the more canola on the landscape, the lower the predation on aphids. This is fairly understandable given that the study found green lacewings the most effective natural enemy in reducing aphid numbers, followed by lady beetles and brown lacewings.
“It makes perfect sense with what we know about the biology of the main predators,” said Costamagna. “The more cereals you have the more green lacewings and lady beetles you will have. By the time soybean aphids arrive, there will be populations of lace-wings and lady beetles moving out of the crop areas looking for natural habitat and that’s when they will find the few aphids in growers’ soybean fields and keep the levels below thresholds.”
These are services that are costing growers nothing, so preserving areas of natural habitat around crop fields that are sources of natural enemies is important to keep receiving this free benefit, and further research is being done to fully understand how far natural enemies range, and where and when they develop their populations.
“We know these natural enemies don’t reproduce in the field,” said Costamagna. “These natural enemies in their immature stages can’t fly so they are selective about where they lay their eggs, and will only do that in a crop if there is a high population of aphids. The larvae can really knock down the pest population but it’s usually too late from a management perspective, and growers will be already losing yield. It’s the adult stage when these insects are most beneficial, and we know the adults are coming from other habitats, so we need to understand what habitats they need. We know they need cereals but there might be other habitats, like natural vegetation, hedgerows and alfalfa, that are also important.”