Backer rod and caulk, remove the baseboard and do the same there.

Thanks. Is there a video that shows how to do something similar to what you're describing?

This video is for cracks but gives you the premise.

You will probably have difficulty getting to the wall under the baseboard heater, and if insects are in the wall already they'll find another place to come in. So in addition to what was suggested, go outside and make sure that the soil/groundcover is 8" (min 4") below where the wood framing starts. That can reduce a lot of insects and if moisture is being held too high by groundcover. Then, spray a repellant around the house perimeter once or twice a year per directions.

Here are more pics. The outside wall is on a patio and I live over a parking garage if that helps. It looks like there were two previous gaps in the outside wall that were filled up. One by expanding foam and the other by this rubber piece(the grey sheet looking thing at the base of the outside wall).