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I have those. Do they feed on any genus in the Onagraceae? I have evening primrose (Oenothera sp.) in the garden too.
The time to worry is when you get the colonial feeders: http://www.dailymail.co.uk/news/article-2195935/The-hungry-caterpillars-Thousands-moth-larvae-weave-giant-web-shrubbery-protect-importantly-food-predators.html Chris
Usually it is Forumists who terrify me with "bug" pictures but today this is self-inflicted :"The World's largest aquatic insect, a species of Dobsonfly, has been found in China." That's the good news - since it means unlikely to be found in a garden anywhere near me!
You have peacock butterflies in Canada John?