Lygus bugs: ID, scouting tips and thresholds

July 20, 2016 - Issue 18

The most vulnerable crop stage for lygus feeding is after flowering and when seeds are enlarged on lower pods. Lygus have piercing, sucking mouth parts, which they use to pierce pods and drain immature seeds. This is where the major yield loss occurs from lygus. In time, this puncture wound may turn black with sooty mold.

Lygus. Credit: Dan Johnson

Lygus. Credit: Dan Johnson

Identification: Three primary lygus species are pests of canola in Western Canada. Identification is not really important, but keep in mind that some regions may have dark brown lygus and other areas may have light brown or greenish lygus. If you have a mix of them in the sweep net, count all lygus species equally. Also include late-instar nymphs, the ones with the black dots on their backs. If you are seeing a lot of younger nymphs, check again every few days. Younger nymphs do minimal damage because they are unable to penetrate pods. In warm conditions, it may take only a week for lygus to grow from early to late instar stages.

Scouting tips: Lygus thresholds are based on sweep net counts, so you need to know the proper technique. An article on how to sweep net. Videos on scouting and sweep netting.

After sweep netting, look for sticky sap spots on pods, pedicels and stems before making the spray decision. Oozing on pods suggests active feeding. If sweep net counts exceed thresholds, but there is no evidence of pod damage, growers could decide to hold off on spraying. On the other hand, if heavy feeding is evident but the sweep shows low numbers, sweep net again at a different time of day or on a day with different weather conditions. In heat and wind, they may take cover.

Thresholds: Consideration of field experience and continued evaluation of thresholds suggest that 50 lygus per 10 sweeps (5 lygus per sweep) at late pod stages could cause a 2 bu./ac. reduction in yield, generally speaking. At $12 canola, a spray at this nominal threshold could save $24 worth of canola, but your time and expenses must be paid for from these two bushels. Also consider trampling of crop and killing beneficial insects. At late flower and early pod stages, the lowest action threshold to consider might be as low as 1-2/sweep, especially in a crop under drought stress. More on thresholds.

Take part in a lygus study: Lygus researchers in Alberta, Saskatchewan and Manitoba are looking for growers who plan to spray for lygus. This study aims to test in commercial canola fields the thresholds derived from studies with cages and in plots. The study would be carried out in fields that need to be sprayed for lygus bugs. If you plan to spray for lygus and want to help to develop better insect pest recommendations, please contact your provincial co-ordinator before you spray: Hector Carcamo in Alberta (403-317-2247 or hector.carcamo@agr.gc.ca), Tyler Wist in Saskatchewan (306-385-9379 or tyler.Wist@canada.ca), Tharshi Nagalingam in Manitoba (204-869-1215 or kstlk2001@yahoo.com)

Canola Watch