Return to Canola Council Home
Lygus bug and cabbage seedpod weevil economic thresholds are based on sweep net counts, and proper counts depend on using the standard sweep net and technique. (See the videos below.)
The standard sweep net has a 35” long handle and 15” (38 cm) diameter net.
Lygus can show up in all areas of the Prairies, but even if you don’t typically have lygus or cabbage seedpod weevil, a sweep net can be useful to identify the presence of beneficial insects, of diamondback moth larvae and other insect pests. Alberta’s government site has info on where to buy them.
Large Ziploc bags, especially the breathable ones, are a handy companion for the sweep net. Flying insects can pop out of the net quickly, making it harder to do an accurate count. Carefully dump all contents into a Ziploc bag, then count insects through the perforated bag.
Sweep net results can be highly variable throughout a field. Extension entomologists recommend a minimum of 5 sampling sites, in a W or X pattern throughout the field, with 10 sweeps at each site. New analysis of cabbage seedpod weevil (CSPW) sweeps indicate that two pairs of sweep locations – a total of 4 sets of 10 sweeps – provides around 97% accuracy as compared to much larger sample sizes. The first two locations for 10 sweeps must be separated by 50 m or more from each other, and the next two should be in a completely different area of the field, again separated from each other by 50 m. CSPW as they first move into a field can accumulate in high numbers along field margins, so make note of this if it occurs as this edge effect will not accurately predict the insects numbers across the whole field over time.
Time of day: While it doesn’t seem to matter for CSPW, time of day, temperature and wind speed can make a difference in lygus counts. You may want to scout two days in a row, unless average numbers are well above threshold.
Here are three videos that can help with scouting and sweep-netting tips for cabbage seedpod weevil and lygus.
