If you see pin holes in your leaves but no cats (and you've done a thorough search), it's likely predators (wasps, particularly, around here!) are eating your tiny cats. In very warm weather, I've found eggs will generally hatch within 4-5 days. Have you checked the underside of your leaves? Monarchs will oftentimes lay their eggs there instead of on leaf tops.

I'm resigned to bringing in any monarch eggs I find and raising the cats indoors; it's been brutal out there for these little guys. I do a plant check every morning, as the wasps are quite thorough and efficient.

Eggs easily are seen when you view leaves' undersides which is where
97% of eggs are laid. .
At least with tropical (A. curassavica), stem allows 270 degree twisting,
very useful in the viewing when combined with bending the stem.

I find that it is easier to look instead for fresh nibbles,
then to take the tiny hatchlings before the wasps do.
I remove the nibbled parts so as not to be mislead by them thereafter
(evenly torn leaf does not look nibbled).

No pinholes ... nothing was eating it at all. I do have one very small muncher now that wasn't there before:

I'm assuming it is the same little one that created both spots. This is the first I've seen of any effect of any kind on any of the leaves except for a slight curl on one leaf which on further inspection had no insect involvement.

I just left everyone on the plant last year (yeah, even the aphids ... they won't get to stay if they move in again; there were just so many so fast last time), and they didn't seem to have any issues. Maybe I'm just impatient ... everything seems slow this year, so maybe the monarchs are too.

Back to the pale bug, this is it:

I guess it's a bit thinner than I remembered it being, but that's the bug. It also has a friend that's similar but more brown, which I guess could be a different stage of the same bug, I think there were two green ones previously.

Additionally, today there was this visitor:

I shooed this one off the leaf.

Both bugs scurry around the leaf/stem when I was trying to take pictures of them.

Are they friend or foe?

Okay, my mum confirms it ... I'm just really bad at seeing eggs. She says she saw some little clear-white eggs on the top leaves the other day. I'll have to see if she can point them out to me. D'oh!

Forget "pinholes", because caterpillar-munched holes
quickly progress to bigger than pin size.

As you see, caterpillars of this age /size do not perforate
a mature leaf, rather they delaminate it.

Your pale green insect is a leafhopper nymph. Adults become flying insects. If numerous they are considered garden pests.

The other insect is pictured at a tough angle for ID; it is some kind of a plant bug nymph, but not a leafhopper.

Yes, four, that is the brand new caterpillar found today ... it's in the picture above the left nibbles ... it's the first I've seen of anything affecting the milkweed and wasn't there when I said the plant was pristine. No other leaves have this.

Thanks, Logician ... looks like they eat plant sap, so they'll be dispatched, either to a different plant or all together. I was having trouble getting a picture of that one since it moved around the leaf anytime I got near it or touched the plant to steady it.

This behavior alone is enough to ID as a leafhopper; they all sidle away to the opposite side of stem or leaf when disturbed.

We've had an absolute infestation of leafhoppers for several years running, I found out this past spring. My three salvia greggii Mesa Azure come back beautifully every spring with a flush of lush green foliage and blooms galore. That's the first couple of weeks, then it's downhill from there: leaves start to look pale, blooms come to an almost-halt, and the plants limp their way through summertime.

See the completely mottled, pale leaves? They're supposed to be solid green.

Good news for you, Blueberry, is that the leafhoppers 100% stayed away from my milkweeds which were planted close by. (I've since torn out all the salvia and replaced them with a different type -- so far, so good.)