This iconic caterpillar was spotted on some roadside milkweed southeast of Corvallis, Montana. Monarch caterpillars feed exclusively on milkweeds (Asclepias spp.) and the chemicals from them make both larva and adult extremely distasteful to predators, as advertised by their bright “warning colors.” 

Diane wondered if it was late in the season to see one. As far as their lifecycle in western Montana, it can vary a lot. Adults can arrive / migrate through anywhere from mid June to early to mid August. Eggs laid in June will produce butterflies that will travel further north before starting their fall migration / return. This individual was likely an egg 3 or 4 weeks ago and if it manages to complete its life cycle will be one of the adults that will head to their overwintering areas and then begin the migration again next spring. This will most likely be along the coast of California …but it could be central Mexico. We don’t really know for sure where Monarchs in western Montana are coming from. It could be a mix.

For more information about Monarchs in the Bitterroot Valley, we invite you to check out the Bitterroot Monarch Project, run by wildlife biologist Maggie Hirschauer, and the Western Monarch Advocates (WMA) Interstate Website.  

Size: 1st instar, 2-6 mm; 2nd instar, 6-9 mm; 3rd instar, 10-14 mm; 4th instar, 13-25 mm; 5th instar, 25-45 mm

Photo by: Diane Bressler on 9/8/21 near Corvallis, MT