What we call blister beetles out here. The scourge of southwest gardens. There are lots of different kinds, but these little speckled gray ones especially love potato leaves. They usually come at the driest, hottest time of the year. (now) A couple of weeks ago I saw a couple, which I always call the advance scouts. I haven't had any for a few years now, but am always on the lookout for them.
Came home yesterday evening and there were a few dozen on the potatoes. I found yesterday that spraying with alcohol kills almost instantly. I don't think it damages the leaves because it evaporates in just a few seconds. Hope I'm right. They're very tricky. If you reach for them to hand pick, or they see your shadow over them, they simply go limp and fall down into the foliage where you just can't find them.
Once in awhile, a really big black version of BB comes, and they especially like tomato leaves. If you think a hornworm does fast work, it doesn't compare to the big black blister beetles. (bbbb's) I need to watch out for them later. One thing I've noticed is that both versions only come one time. Thank goodness.
Both kinds in hordes can strip plants in a few hours.