Lygus – race between insect and crop development

Quite high populations of lygus are being found in parts of the Peace and southern and central Alberta (finding as many as 80 lygus per 10 sweeps with approximately 25 to 30% of those being adults with the remainder a range of instar stages). Sweep net results also seem to be variable. For instance, if fields under irrigation are sampled after the pivot has watered then lygus numbers are significantly lower than if sampled before the pivot watered. Seems the application of water might drive the insect deeper into the crop canopy. Sweeping net sampling is still the best method of determining the potential economic impact of lygus bugs in canola. Thorough scouting on a field-by-field basis must be done to adequately gauge the insect threat. Take a close look to confirm it is a lygus instar and not alfalfa plant bug (since the two can look quite similar). Alfalfa plant bug nymphs will be larger in size and have more angular body shaped (lygus are more compact with rounded body shape). Click here for a photo of both of these insects.

Lygus juveniles usually become numerous at the time canola is mature (so there tends to be a late increase every year). However, this flush seldom requires spraying because the crop is often advanced and the bugs feed on the immature pods at the top of the raceme that do not contribute as much to yield. In a year like this one, if canola is taking longer to mature it may be more susceptible. Lygus development is also dependent on temperature. Research has shown that it would take about 3-4 days for each instar to proceed to the next stage when temperatures are at 25⁰C with cool nights. So if there are mostly young nymphs with no spots on the back (1^st to 2^nd instar) it would take about a week for them to damage the crop. Growers who are finding high numbers of adults and late-instar stages with crops that are still three weeks from cutting may need to spray and may consider spraying earlier. Each field should be scouted and assessed individually since crop staging and lygus staging both need to be determined to assess the risk. Regular scouting will help capture the population of young instars as they mature and move up higher in the crop canopy. Click here for more information on proper sweep net technique.

With lygus, it is the pod that needs to be protected. Research has shown that late-instar nymphs can feed on young developing pods but if pods are mature with ripening seeds then only adults are able to cause damage. Once pods become leathery (a rule of thumb is about two weeks before swathing), then even adult lygus cannot cause damage and control is not economical.

 Economic control thresholds for lygus bugs are based on adult and late instar counts. When canola prices are $12 per bushel and application costs are $12 per acre, the threshold is 11 adults or late instar bugs per 10 sweeps. Before spraying, consider the pre-harvest interval for insecticides. This late in the season, a product with a long pre-harvest interval (e.g. 21 days or more) should likely not be used. It is vital that all of Canada’s Canola is Export Ready. Click here for more information on lygus. Click here for more information on product pre-harvest intervals.