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Exploring the power of plant diversity

Intercropping offers potential for controlling crop pests.


March 5, 2008
By Carolyn King

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Growing wheat or barley in the same field with canola is not very practical currently, but in the future it could become an alternative to monocultures in some situations. Like several earlier studies, an intercropping project underway in Alberta is showing that increased plant diversity in a field can help in controlling crop pests. It is also finding that the wheat/canola intercrops can provide similar yields to monocultures of the two crops.

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A canola/barley intercrop. Photo Courtesy Of L. Dosdall.

Crop diversity in a field can improve pest or weed control in many ways. For example, the various crop species may have different types of root systems, different plant heights, different nutrient needs and so on, so that together they can capture most of the available light, water and nutrient resources, and few resources remain to support weed growth. Or, perhaps one of the crop species provides physical barriers to the movement of a certain insect pest, or gives off compounds to repel that pest, or provides habitat for predators or parasites of the pest.

“The simplicity of most monoculture agricultural systems really seems to bring out the worst in insects that are there,” notes Jeremy Hummel, who is working on the intercropping study for his PhD at the University of Alberta in Edmonton. “Agricultural systems that involve intercropping, such as in the tropics, tend to have less problems with insect pests than monocultures do. And natural plant systems, which usually have many different plant species and even many different varieties of the same species in the same area, rarely have as big a problem with mass insect invasions as do agricultural systems.”

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An earlier Alberta study had investigated the possibility of tapping into the pest control benefits of plant diversity by retaining a small number of late emerging weeds. The concept was to decrease insect damage in canola and reduce herbicide inputs, without significantly reducing crop yields or increasing weed seeds in the seedbank, explains Dr. Lloyd Dosdall, an entomologist with the University of Alberta and Alberta Agriculture and Food.

Due to very dry growing conditions during that study, weed populations and root maggot damage were both unusually low, so the results were not as clear-cut as the researchers had hoped. “In plots where we had the most weeds, we consistently had less root maggot damage. However, we couldn’t say definitively to farmers, ‘Use this herbicide rate and you’ll get this amount of weedy background that might interfere with yield a little bit, but with less root maggot damage and the cost savings with less herbicide, you’re going to be better off’,” says Dosdall.

Nevertheless, Dosdall felt that the idea of increasing plant diversity to improve pest control had potential. He says, “The logical next step was to plant crops together.”

That is what Hummel’s three year study involves. He is comparing monocultures of canola, wheat and barley to canola/wheat and canola/barley intercrops, with various proportions of each crop in the intercrops. He is assessing the effects of the two cropping systems on crop yields, insect pests, weeds and other factors. The study is funded through the Alberta Canola Producers Commission and is being done in co-operation with Agriculture and Agri-Food Canada.

In 2005 and 2006, Hummel conducted field studies at Ellerslie, Lacombe and Fort Vermilion, and in 2007 he is conducting greenhouse studies. “Just setting up the intercropping was an experiment on its own!” laughs Hummel. Both seeding and harvesting of the intercrops required some innovative thinking.

“Before seeding, the fertilizer was cross-banded across the experiment – everything was fertilized to canola production standards,” he says. “Then in the seed drill, we put the wheat in where you would usually have the granular fertilizer for side-banding, and the canola went in where the seed goes. So the two crops were basically side-banded together, a half inch to an inch apart.”

Hummel used imidazolinone tolerant (Clearfield) varieties of canola and wheat to have a herbicide option in case the intercrop did not provide effective weed control. There are no imidazolinone tolerant barley varieties.

To address harvesting challenges, Hummel began by selecting canola, wheat and barley varieties with similar times to maturity. “Then at harvest, we waited until both crop species in the intercrop were as close to their optimal harvesting maturity as we could get, and then we straight combined them together. Sometimes the canola was a little bit green and sometimes it was too ripe,” he says. He adds, “A practical intercrop harvesting system will have to be worked out eventually by someone. But we figured we had enough on our plates with what we were doing, so we’d leave that to the next person to solve!”

Field data collection included such aspects as crop emergence, weed species and numbers, crop and weed biomass, soil organism types, and insect species and activity. Insect activity measurements included assessing flea beetle damage, monitoring root maggot egg laying, quantifying root maggot damage, setting up insect traps on posts to monitor insects at the top of the canopy, and placing pitfall traps flush with the soil surface to collect predatory beetle species.

This year’s greenhouse experiments are measuring predation on root maggot eggs and larvae by two important beetle species in the intercrops and monocultures. Hummel notes, “Different species of beetles are affected differently by the intercrops. Some beetles prefer open canopies that you might find in a wheat crop, for instance, where there is more light penetration to the soil surface. Other beetles, including a lot of the larger beetles, prefer closed canopies, like the intercrops with a high proportion of canola, because the birds can’t see the beetles so they don’t get eaten.”

Preliminary results
Hummel is currently analyzing the mountain of field data collected in 2005 and 2006. He says, “Just to give you an idea, there were about 13,000 beetles just from the pitfall trap samples in 2005. I identified each one of those beetles down to species level so we can get a picture of the beetle community.”

So far, Hummel has been focussing on the canola/wheat intercrops compared to the canola and wheat monocultures, and on root maggot and predatory beetle activity. He outlines some of the preliminary findings:

Crop yield
“Generally the yields of the intercrops and the monocultures are pretty close.” However, some of the intercrop plots significantly out-yielded the monocultures and a few significantly under-yielded the monocultures, in some site-year combinations.

Crop competition in the intercrops
“Depending on environmental conditions, it looks like either the wheat is really strong in the intercrop or the canola is really strong. It doesn’t look like both are strong at the same time.” Hummel suspects that under drier conditions wheat tends to dominate, while in moister conditions canola does better.

Root maggot damage
“Intercrops do effectively reduce the amount of root maggot damage to canola compared to a pure canola stand. And as the amount of wheat in the intercrop increases, the amount of damage to the canola roots decreases.”

Root maggot eggs and larvae
“Although root maggot egg laying decreases as the proportion of wheat in the intercrop increases, other factors are also influencing root maggot larval populations. One factor may be predatory beetles feeding on the eggs and larvae.”

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One of the predator beetles that feeds on root maggot eggs. Photo Courtesy Of J. Hummel.

Predatory beetles
“The canola monoculture had a different beetle community than the wheat monoculture, and the beetle communities in the intercrops ranged between those two. The beetle community in the canola monoculture was more dominated by this one big, nasty introduced beetle species, called Pterostichus melanarius. It is a generalist predator that preys on anything it finds, including other types of predatory beetles, but it is too big to eat root maggot eggs. The wheat monoculture and some of the intercrop plots with higher proportions of wheat had more of the smaller beetle species that would be preying on root maggot eggs and larvae, which says to me that they could have been diminishing the populations of root maggots in the intercrops.”

It is promising that the intercrops and monocultures typically had similar yields in this project. However, for intercropping to become a practical option, Hummel says, “Ideally, we would want the intercrops to over-yield as compared to the monocultures because intercrops have somewhat higher production costs. For instance, you may not be able to harvest both crops at their optimal stage for harvesting, so you might have to put some money into drying the canola if it’s a little too moist.”

He adds, “If the increase in the yield of the intercrop was enough to offset the added harvest cost, then there would be a definite benefit to intercropping just from a yield standpoint, and that’s ignoring any benefits from controlling insects, diseases or weeds that the intercrop is giving you.”

Whenever Hummel talks to producers, he gets a lot of questions on the practical aspects of intercropping. “Producers say, ‘The results might be nice – the pest insects are limited, but what do we do with that information?’ And I tell them that they’re right, we don’t have a quick and easy way to grow and harvest intercrops yet. But ‘yet’ is the key word.”

He emphasizes, “We’re doing this research to lay the scientific groundwork for intercropping. If the science shows that intercropping is a viable agricultural practice in the Canadian prairies, then people can start to invest money developing the systems to plant, grow and harvest intercrops more efficiently.” -end-