The buckwheat effect
A new study at the Harrington Research Farm in P.E.I. is examining how to make buckwheat crops an even more effective and practical option for potato growers looking to control wireworm. Photo by AAFC.
Growing two years of buckwheat in a potato rotation is effective at managing wireworms, one of the toughest pest problems for potato growers. But how does buckwheat actually affect wireworms, and can we make buckwheat rotation even more effective and practical for growers? A rotational study is underway on Prince Edward Island to answer these questions.
The P.E.I. study is part of a major project to investigate different strategies for dealing with wireworms, led by Christine Noronha, a research scientist with Agriculture and Agri-Food Canada (AAFC). The project, funded by AAFC, involves AAFC scientists across the country because wireworms cause serious problems in many regions and many crops.
Wireworms are the soil-dwelling larvae of click beetles. Canada has about 30 wireworm species of economic importance. In most species, the beetles lay eggs in the soil in the spring. A few weeks later, the larvae hatch. The larval stage lasts about four or five years, depending on the species. Then the larvae pupate and the adults emerge from the soil in spring.
In potatoes, wireworms tunnel into the tubers, reducing marketable yield. The tunnels can also be entry points for potato pathogens.
Noronha has been conducting wireworm research in P.E.I. for over a decade. Her rotational studies show growing either brown mustard or buckwheat as a cover crop for two years before growing potatoes can reduce tuber damage by about 80 per cent or more. Brown mustard is known to release chemicals into the soil that control wireworms, but the reasons for buckwheat’s effect on wireworms are not yet known.
AAFC research scientist Aaron Mills is leading the new P.E.I. buckwheat agronomy study. It is taking place at the Harrington Research Farm and runs from 2014 to 2016.
“Christine Noronha has done an excellent job at figuring out how to control wireworm with buckwheat; it’s now another tool in the toolbox for controlling wireworm. A big part of this new buckwheat study is to find out how and why buckwheat affects wireworms,” Mills explains.
“We also want to establish local protocols for how to deal with buckwheat in the rotation, so growers wouldn’t be as intimidated by the potential for buckwheat to become a weed problem, and to establish some protocols for managing buckwheat as a grain as well.”
Better understanding of how buckwheat impacts wireworms could potentially lead to advances that enhance wireworm control. For instance, one possibility is that buckwheat is releasing compounds into the soil that kill or suppress wireworms directly or perhaps affect other components of the soil ecosystem in ways that make conditions less favourable for wireworms. If such compounds are released when buckwheat foliage decomposes in the soil, then the most effective wireworm control strategy might be to disk the plant into the soil as a green manure. Or it may be best managed as a mulch, in which case flail mowing as a green manure may be best. If the compounds are released by the roots, then perhaps it might be better to let the buckwheat crop grow for a longer period before terminating it. If buckwheat produces compounds that deter other insect pests or pathogens, in addition to wireworms, then it could provide even greater benefits in the crop rotation. In the longer term, perhaps buckwheat lines could be selected that have higher levels of those compounds, or perhaps a bio-insecticide could be developed using the compounds.
A key aspect of successfully including buckwheat in a crop rotation is to ensure volunteer buckwheat doesn’t become a weed issue. “Traditionally buckwheat has been grown as a weed control measure, and some growers are concerned about growing it because it can become a weed itself if you let it grow too long and go to seed,” Mills says. He explains buckwheat grows very well in P.E.I.; the seedlings emerge and grow quickly, enabling the crop to outcompete weeds. As well, some research indicates buckwheat releases chemicals into the soil that inhibit the growth of certain plant species. Those characteristics make buckwheat great at fighting weeds, but can also make volunteer buckwheat a problem for the next crop.
Although letting buckwheat go to seed is a weed risk, growing buckwheat for grain would be an important opportunity for growers to obtain some income from growing two years of buckwheat for wireworm control.
“Right now I would say the majority of growers on the Island are growing buckwheat for wireworm control, but I think people are interested in growing it as a crop for grain as well,” Mills notes. “I think the main export market for buckwheat is for use in noodles in the Asian market, although it has been grown in places like New Brunswick to make flour for buckwheat pancakes. And there may be opportunities in the health food industry; for example, buckwheat oil is reported to have some bioactive benefits.” Buckwheat also attracts pollinators and is used for honey production.
Wireworms, organic matter, microbes and more
Mills’ study is comparing five different three-year potato rotations. Three of the rotations involve two years of buckwheat and are comparing three strategies for terminating buckwheat: disking, flailing, and desiccating and then harvesting it for grain.
“For termination through disking, we wait until the lower seeds start to drop off and then we use a set of disks to incorporate the aboveground crop material into the soil. For termination through flailing, we wait for the lower seeds to start to drop, and then go in with a flail mower and completely flail off the top of the aboveground biomass, leaving the residue on the top of the soil surface, almost as a smothering effect. And to manage buckwheat for grain, we desiccate the crop and then take it off with a combine,” Mills says.
He adds, “We wait for the lower seeds to start to drop off when flailing or disking because we’ve found that buckwheat turns to jelly when you flail or disk it – the remaining residue breaks down fairly quickly. So rather than ending up with bare soil going into the winter, we wait for the lower seeds to start to drop off to make sure there will be a little bit of cover going into the winter.”
For the three buckwheat treatments, Mills’ research team is measuring factors like buckwheat biomass and seed yield, as appropriate.
The other two rotations in the study are traditional three-year potato rotations used in P.E.I.: barley underseeded with clover and then potatoes; and barley underseeded with a grass and then potatoes. These traditional rotations have benefits for soil conservation, but they favour the spread of wireworms because the beetles prefer to lay their eggs in grassy areas.
The researchers will be gathering data on such factors as potato yields and the levels of wireworm damage in the tubers for the five rotations.
As well, they will examine effects of the different rotations on water quality, soil organic matter content and soil nutrient levels, so nutrient management specialist Judith Nyiraneza and water quality specialist Yefang Jiang are involved in the study.
“Although any green cover on the soil helps to reduce erosion, the study will be examining whether or not buckwheat is particularly good at preventing erosion or building up soil organic matter,” Mills says. “I think the jury is still out on whether buckwheat is a soil builder or not.”
Buckwheat’s effect on soil nutrients also needs to be clarified. Some previous research has suggested buckwheat may make nutrients like phosphorus more available to the next crop, while other sources suggest buckwheat may reduce levels of certain nutrients.
To better understand how buckwheat controls wireworms, Christian Gallant, a graduate student from Dalhousie University, is looking at the soil organism communities in the plots, especially the nematode communities, and also the fungal and bacterial communities. He will be examining how species diversity and populations change through the course of the growing season and with the different crops.
Mills explains the value of examining nematode communities: “Most people focus on the plant parasitic nematodes. Those nematodes feed on plant roots, and the damage from their feeding also opens up spots for pathogens like Verticillium to enter the plant’s root system. But there is a whole other side of the nematode community. They all have specific jobs to do; there are bacterial feeders, fungal feeders and predators that feed on other nematodes and even mites. By studying the overall nematode community, we can get a snapshot of how things are happening biologically in the soil.”
In addition, researchers will be testing the soil for buckwheat compounds that could be affecting soil organisms. “We’re trying to figure out if buckwheat is actually releasing chemicals into the soil that are affecting the soil pest populations, or if buckwheat is just a non-host for some of these pests,” Mills says.
With one field season completed so far, Mills is looking ahead to the results from 2015 and 2016. “By the end of 2015, we’ll have a really good idea of how the different buckwheat treatments affect soil organisms, nutrients and organic matter, [and water quality]. And by the end of the study’s third year, we’ll have an excellent idea of how everything affects potato production.”
In the meantime, growers are welcome to visit the Harrington Research Farm to see the plots and learn more about the study.
May 12, 2015 By Carolyn King
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