Research unveils secrets to protect canola from lygus bug

Three years of research by Agriculture and Agri-Food Canada (AAFC) has confirmed lygus bug monitoring and control at canola's early pod-ripening stage best protects yields and avoids economic losses from the insect.

The lygus bug is a small, oval-shaped insect that uses its needle-like mouthparts to suck plant sap from canola flower buds and seeds, reducing the plant's yield and quality, says Dr. Hector Carcamo, an entomologist studying the insect. High populations of lygus bugs in recent years have made the insect a serious economic pest of canola, with annual yield losses estimated at 10 to 35 percent in infested areas.

"Lygus numbers fluctuate highly depending on weather and are difficult to forecast, however, based on surveys in weeds conducted in southern Alberta and above average moisture earlier in the spring, the risk for 2003 is considered moderate," says Carcamo. "Monitoring during July is the key to making control decisions."

Previous and ongoing research shows the early pod ripening stage, which typically occurs from mid to late July in southern Alberta, is the most effective time to assess lygus bug risk and apply chemical control if necessary, he says. "Sampling lygus bugs at earlier stages is ineffective in predicting yield losses in canola, partly because this is when many lygus bugs reside lower in the plant canopy where they are not captured by sweep nets."

A typical economic threshold is 15 to 20 bugs per 10-sweep net sample, but this varies depending on canola prices and control costs, says Carcamo. While chemical control remains the only current control option, AAFC researchers are studying ways to optimize timing and frequency of sprays for maximum control. They are also developing other potential tools such as biocontrol.

One major study recently completed by Carcamo in southern Alberta and by Jennifer Otani of AAFC Beaverlodge in the Peace River area, sought to pinpoint the best time to apply insecticide. Results obtained over three years confirm that monitoring and control at the early pod-ripening stage best protects yields and avoids economic losses to the producer from lygus bug feeding.

"The early pod stage is when the older juveniles and adults are feeding on the seeds in the upper plant canopy where they can be sampled, so this is the optimal time to get an accurate measure of the risk and determine the need to spray insecticides," says Carcamo. "Lygus numbers from surveying at earlier stages can be misleading and spraying at earlier stages does a poorer job of protecting yield, unless other insect pests warrant it, for example the cabbage seedpod weevil in southern Alberta."

The study also confirmed results from a previous study led by Carcamo and Otani's colleague Jim Jones, which showed that lygus bug activity at early stages can actually help the plant defend against early pod damage.

"We found canola plants can compensate for lygus damage during the early flowering stages," says Carcamo. "Having a few lygus bugs causes the canola plant to produce more branches and pods, which helps compensate for damage inflicted during the bud to early flower stage. Even adding very large numbers, such as 10 bugs per plant between bud to early flower stages, didn't cause any yield losses."

These findings demonstrate why research into how insects interact with plants is essential to knowing when control is warranted, says Carcamo. "As an industry, we're learning to get past the bias of wanting a 'clean field' with little or no insect pests. Having a few insects is often beneficial. In this case, it can be very good at an early stage because it triggers the plant to produce more branches. This is typical of the type of interaction we're exploring in other entomology work."

Other research by Carcamo, Otani and colleagues has examined geographic variation of the lygus complex, the effect of cutting alfalfa on lygus bugs in canola and the overwintering biology of the lygus bug. Potential biocontrol options are also being investigated as part of a major project that involves several research centres across the country and the international biocontrol institute, CABI-Bioscience. "The overall idea is to improve our forecasting and continually work toward better strategies for sustainable lygus bug management," he says.

Agriculture and Agri-Food Canada's Lethbridge Research Centre has a mandate to promote innovation for growth, maintain security of the food system and protect the health of the environment.

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