"Bug" in this case is not a colloquial term but the correct name
for one of many species of insects within the order Hemiptera. Once
you know what a bug is, it's a term that can be used as
definitively as "bee", as in, "No, it wasn't an ant, it was a
Milkweed bugs (Oncopeltus fasciatus) feed on plants in
the genus Asclepias, including common milkweed and
butterfly weed. Like the monarch butterfly, it's full of rather
noxious substances from having fed on milkweed sap, and its color
announces its distastefulness to birds and other predators.
The damage milkweed bugs do is not serious although it can
disfigure a plant during years when their numbers are high.
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