We have a positive identifcation! It's a Western Boxelder Bug, Leptocoris rubrolineatus (also referred to as Boisea rubrolineata). Thanks to Christina and Bill for their fantastic bug identification skills. From bugguide.net:
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Superclass Hexapoda (Hexapods)
Class Insecta (Insects)
Subclass Pterygota (Winged Insects)
Order Hemiptera (True Bugs, Cicadas, Hoppers, Aphids and Allies)
Suborder Heteroptera (True Bugs)
Family Rhopalidae (Scentless Plant Bugs)
Genus Boisea
Species rubrolineata (Western Boxelder Bug)
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Superclass Hexapoda (Hexapods)
Class Insecta (Insects)
Subclass Pterygota (Winged Insects)
Order Hemiptera (True Bugs, Cicadas, Hoppers, Aphids and Allies)
Suborder Heteroptera (True Bugs)
Family Rhopalidae (Scentless Plant Bugs)
Genus Boisea
Species rubrolineata (Western Boxelder Bug)
Synonyms and other taxonomic changes
listed as Leptocoris rubrolineatus on many websites and also as B. rubrolineatus
Food
Feed on Boxelder (Acer negundo), other maples (Acer spp.), and ash (Fraxinus spp.). Adults may sometimes also feed on fruit.
Life Cycle
As true bugs they have incomplete metamorphosis - egg hatches to nymph which goes through several molts to become a reproductive adult.
Remarks
Particularly noticeable in fall when they may invade homes in search of a safe place for winter hibernation, and in spring when they emerge again.
See Also
Eastern Boxelder Bug, Boisea trivittatus.
