Authors: Jocelyn Smith and Andrea Hitchon, University of Guelph Ridgetown Campus
The time to start scouting corn for corn rootworm has arrived! There are a few important objectives for scouting for rootworm injury including: scouting can help protect yield, plan control measures for next year, and detect potential cases of resistance to Bt traits. The Canadian Corn Pest Coalition is interested in any cases of suspected resistance to a Bt trait and these should be reported immediately to Jocelyn Smith, Tracey Baute, or Andrea Hitchon for follow-up.
Resistance to some Bt traits has been confirmed in areas of the US Corn Belt, and is suspected in some Michigan fields close to the Canadian border. In order to detect problems early, fields planted to Bt hybrids need to be scouted this season.
Scout high priority fields first:
It is a good idea to scout all corn fields, however the following risk factors make scouting more critical:
- continuous corn production
- repeated use of same Bt trait
- high beetle population last year where a lot of egg laying likely occurred
- lodging which often occurs in fields with high rootworm pressure – root digs are needed to determine if damage is from insect pressure or other factors such as high winds
Scouting for root damage:
Rootworm larvae are presently feeding under the soil on corn roots and will begin to emerge as beetles around the end of July and continue into late summer. Root digs can be completed to assess root damage towards the end of their feeding period, which this year is likely during the 3-4th week of July at the earliest. Roots should be dug up, washed, and then examined for evidence of insect feeding. Look for brown scarring, tunneling, and root pruning. In extreme cases entire nodes of roots will have been eaten by larvae. The Iowa State node-injury scale (0-3.00) can be found here.
When to report potential cases of Bt resistance:
- Root feeding on Bt hybrids that is greater than expected. Follow-up is needed if the Iowa node injury score is higher than 1 in a single traited Bt product or > 0.5 in a SmartStax hybrid
- Lodging in a Bt field which cannot be explained by factors such as wind
- Exceptional numbers of adult beetles present in a Bt field
We will follow-up on all in-season reports with a field visit. During this visit we will dig roots to assess larval damage, ensure that Bt plants are expressing Bt trait(s), and if needed collect adult beetles for further testing in the lab. In order to test beetles we need to collect beetles before egg laying has occurred – for this reason, we ask that you report cases where Bt traits did not perform adequately as soon as possible. Finding out a the end of the season is too late. This will ensure the best response and help ensure the continued effectiveness of Bt hybrids for Ontario growers.
Scouting for adult beetles:
Adult beetles can be found feeding on the above-ground parts of corn, especially on silks. If the average number of beetles exceeds one per plant during the month of August, OMAFRA recommends controlling rootworm in the following growing season (i.e. crop rotation, planting a Bt hybrid or using soil insecticide). Continue to monitor silks for silk clipping throughout pollination; unless silks are clipped to ½ inch at 25-50% pollen shed adult beetles are not of economic importance. Late-planted corn can act as a ‘trap crop’ for beetles to move to after silks in early planted fields have dried up and turned brown therefore increasing the risk of rootworm problems in the following year.
If you have questions or concerns regarding the performance of Bt traits please contact:
- Jocelyn Smith: 519-674-1500 x63551, firstname.lastname@example.org
- Tracey Baute: 519-674-1696, email@example.com
- Andrea Hitchon: 519-674-1500 x63128, firstname.lastname@example.org
Registered control options for rootworm are provided here.