Ecological notes: These population booms are (primarily) associated with female boxelder trees. But then again, a single boxelder tree is only mostly female or mostly male, meaning that on a single tree you can find both male and female flowers, just with one sex being in the vast majority (the fancy term is polygamodioecious). I heard the bugs before I spotted them. It's been so dry here that their rustling in the leaves made them sound much louder than they actually were. At first I thought (and hoped) it was a hibernacula of snakes waking up in the warm (60o) weather. Alas it was not.
Once I spotted the culprit (actually hundreds and hundreds of culprits) my interest was piqued. I'd watched some type of fly by the lake mating yesterday, with their rears touching and the boxelder bugs took the same posturing. The males are the larger of the two and have much brighter orange bellies. They more or less dragged the females over the rocks. Pairings seemed to last anywhere from 2 to 5 minutes or so.
Where: The bugs were lining the train tracks where North Ave crosses over (by the Donahue Sea caves). The whole stretch of the tracks between North and 127 is lined with boxelders, both male and female. The bugs were mostly along the rocks between the rail lines and the horsetails where it was the warmest.
Other notes: I didn't spot any eggs, but the females will deposit them in the bark of the boxelder. Once they hatch, these little guys have a gradual metamorphosis. They can't fly when they're young but develop wings as they get older (the young look like red aphids). Supposedly they eat boxelder seeds. I'd seen lots and lots of seeds this winter on boxelders that had little holes bored into them - I'm not sure if that was from these insects.