I'm pretty sure that this is the Acadian Hairstreak (Satyrium acadica). I would be absolutely sure except that the underside of the wings of this butterfly look more light brown than grey (a feature of the Acadian) to me. Pretty butterfly in any case--and to identify a hairstreak at all is new for this novice butterfly watcher.
Like so many other critters, this butterfly was found feeding on common milkweed (Asclepias syriaca), which is just now coming into bloom. In Ontario common milkweed is one of twenty-four species on the noxious weed list--a kind of hit list. The Ministry has this to say:
Milkweeds are an important component of the native and naturalized vegetation communities of Ontario as they are an important nectaring plant for many species of Lepidoptera, such as the Monarch butterfly. (From Milkweed Species in Ontario)
Then on their FAQ page they add this:
Why is Milkweed on the List of Noxious Weeds in Ontario?
Common milkweed can be a very difficult weed to control in many field crops thereby causing significant reductions in crop yield and quality. This can have a considerable negative impact to a grower's net economic return. In the last 10 years, new herbicide technologies have greatly improved the control of common milkweed in field crops. However control of common milkweed around field borders is essential as it minimizes seed spread into fields and therefore reduces the reliance on herbicides for "in field" control.
Common milkweed when consumed in large quantities is poisonous to livestock. Therefore minimizing populations in actively pastured land will greatly reduce the chance of any adverse health affects to livestock.
For more information on Milkweed and the Weed Control Act, refer to the article entitled: "Milkweed Species in Ontario".
Which is yet another example of us humans getting hold of the wrong end of the stick it seems to me. Especially in the current climate in which we are engaged in international efforts to protect the Monarch Butterfly, which depends on milkweed as the foodplant for its larvae.
Here's what they have to say about Monarch protection at the Royal Ontario Museum's Species at Risk site:
Protection: There is no formal protection for this species in Ontario. Three key management strategies have been identified to protect the Monarch Butterfly. Milkweeds, the larval foodplant, should be taken out of the noxious weed acts in Canada; native wildflower habitat should be protected and encouraged; and migration stopover sites should be protected from disturbance.