(Sorry Betty, I do know what a heat pump is, and how it works, as well as a heat strip but for some reason, I'm having a tough time figuring how you're having this problem. Well, anyway, I have some useful info, I believe.)
As John 2000 says, you do need proper venting in the crawlspace. You need to remove brush, plant growth, and such from anywhere near your vents. If there are bushes and plants up against the house wall outside, they actually 'obstuct' air from flowing alooooong the outer wall and, hence, into the crawlspace. Typically, if your crawlspace is insulated from the room above (which I hope it is), you need 1 vent per 250+ sq. ft. (I'll leave the math to you to convert your cubic inches to square feet.) Also, I think you said the crawlspace seems dry. If the humidity is high enough (usually due to the space being warm and lack of air flow and vents), you'll have this type of problem.
FWIW, I have a 450 sq. ft. crawlspace that's 3 ft high. It has 2 vents. It gets some water in a heavy rain but in a few days it's gone. (It has a concrete foundation too.) It's insulated from the room above it. There are no bushes or plants anywhere near the 2 vents I have. Fortunately, I don't have a problem. Not that it's any consolation to you but even given certain conditions that might promote mold/mildew, I (knock-on-wood) don't get any.