During May 23 and 24, a total of 102 Monarchs were sighted along a 5 mile stretch of the Trinity River north of Trinity Center in the Trinity National Forest.

Many were migrating north along the river at a rate of about 5 per hour and 21 individuals were tagged. Many eggs and a few first instar larvae were also found on Showy Milkweed.

I believe the monarchs are following and being funnelled into the Trinity River valley and likely use other valleys in the area as well. I think they likely originate from areas around Sacramento-Napa where ex-cluster females laid their eggs for the first post overwintering generation. As they travel north, some clearly 'drop out' and begin breeding (like the resident population along the Trinity River) while others continue flying into Oregon, Washington and British Columbia.

I discovered the Trinity River area as a good location for Monarchs at the end of May a few years ago (by accident!) and have monitored it annually ever since. Numbers since 2012 have never exceeded 20-30, so seeing ~100 this year was a welcome surprise.

Based on the migrants we saw and a migration rate of 30 miles/day, I predicted they would begin turning up in northern Oregon and southern Washington in 7-14 days and that’s what happened this past weekend (May 30-31)! I think we will get reports from central and northern Washington in the next 2 weeks then British Columbia by the end of the month. I am trying to get a group of monarch people in southern Oregon to tag Trinity monarchs next May. This could potentially yield some interesting data!

Details of monarch butterfly breeding and migration biology in the Pacific Northwest (Oregon, Washington, Idaho, British Columbia) are poorly known. Low populations of monarchs and people in the region have discouraged studies on breeding populations and the tagging of wild individuals to learn more about monarch migration.

Washington State University in association with Walla Walla Penitentiary established a long term research program in spring 2012 aimed at identifying migration routes and biology of monarchs in the Pacific Northwest.