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return to Farmers Independent Weekly
February 10, 2005
by Neil Holliday,
Department of Entomology |
How lygus bugs affect buckwheat yield
Manitoba produces about 60% of all the buckwheat grown in Canada. Much of the production is exported to Asia where one of its major uses is the manufacture of soba noodles. Buckwheat is a short season crop that can be planted late in the spring, begins to flower in late July with flowering and seed development continuing from that time until harvest. Harvest may be in late September or early October if severe frost does not intervene.
Buckwheat is frequently heavily infested with lygus bugs from late August onwards. A single sweep of a sweep net could capture 10 or more bugs, but before our study began, the significance of this high infestation of bugs on buckwheat seed yield was unknown.
In 2001, Ph.D. student Ayman Mostafa began his study on the impact of lygus bugs on buckwheat crops. His initial work showed that the particular lygus bug in buckwheat is the tarnished plant bug, Lygus lineolaris, and that the enormous peak late in the season consists of winged adults. Early in August, there is a much smaller peak of flightless nymphs. The patterns suggest that small numbers of adult bugs arrive in the buckwheat crop in mid- to late July. Females lay eggs in the stems of the buckwheat plants, and the eggs hatch into nymphs in early August. The nymphs develop into adults in about three weeks.
 The large size of the August peak of adults relative to the small peak of nymphs suggests that the adult peak comes not only for nymphs that developed in the field, but also from immigrant adults. We believe that adults migrate into buckwheat in late August from early-maturing crops such as canola and cereals which, as they mature and are harvested, become unsuitable for the bugs.
In five trials in which we manipulated the number of lygus bugs in buckwheat with insecticides, we produced significant yield benefits from an insecticide application at the end of July. The application at this time reduced populations of nymphs to virtually zero. Yields in plots sprayed at this time were from 12 to 78% greater than in unsprayed control plots, with an average yield benefit from the late July insecticide application of 41%. Insecticide applications aimed at the late season large adult peak population successfully reduced insect numbers but provided no consistent yield benefit. Thus, the producers’ initial concern about the large numbers of insects in late August is not of economic significance, but the inconspicuous population of nymphs in early August is highly detrimental to buckwheat seed production.
Using caged plants, we studied the way in which yield loss occurs, the most vulnerable stage of the plant and the most injurious stage of the insect. We compared the flowering stage with green seed stage and ripe seed stage. The flowering stage lasts about two weeks and is in early August, and the subsequent two stages each last three–four weeks. We found that significant yield loss occurs only when nymphs are present on the plant at the flowering stage. Adults at any stage, and nymphs at the green seed and ripe seed stage did not produce significant yield loss.
There seem to be two mechanisms by which yield (that is the total weight of seed) is reduced by lygus bugs. Most important is the effect of nymphs feeding on flowers and flower buds which causes buds and flowers to drop from the plant . Nymphs feeding at the flowering stage in cages caused a 74% reduction in total yield and about 90% of yield loss can be linked to reduced seed set, attributable to increased bud and flower abortion. A secondary effect occurs when bugs feed directly on seeds, causing the seed to shrivel. This effect is also primarily attributable to nymphs feeding at the flower stage, but is not as restricted to this plant and insect stage as other types of injury.
From the combination of the field and laboratory studies, we have gained a good understanding of how lygus bugs affect buckwheat, and we are now working to encapsulate this information into an economic injury level that will allow producers to determine whether they need to spray insecticides and when. Our results are being used as a basis for a minor use pesticide registration, to provide producers with the tools to manage the bugs. This work was funded partly by the Manitoba Buckwheat Growers Association, with matching funds from Manitoba Rural Adaptation Council and from NSERC.
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