Holes are the symptom where the first question is not what to do but what made them, because the neatest holes in a pollinator garden are cut by a bee worth keeping. Look at the shape before anything else. Smooth circles taken from the leaf edge are leafcutter bees and need nothing at all. Ragged holes with dark droppings on the leaves below are caterpillars, which on a host plant is the point. Lacy leaves with the veins left standing are Japanese beetles, and slugs leave mucus trails and work overnight.
Most likely causes
Listed in the order they are worth ruling out. It is normal for more than one of these to be happening at once, so work down the list rather than stopping at the first match.
1
Leafcutter bees, and the plant is fine
Look at the edge of the leaf, not the middle. The cut is a smooth semicircle or oval taken out of the rim, as clean as if it were done with a punch, and it is always at the margin. The bee below is carrying the piece she just cut away to line her nest. Photo by Bob Peterson from North Palm Beach, Florida, Planet Earth! via Wikimedia Commons, CC BY-SACaught mid-cut: the bee braces on the leaf edge and shears out a clean half circle. The notch under her is the shape to match on your own plant, smooth-edged and always at the margin. She takes a few pieces and moves on, so the damage stays small. Photo by Herwig Winter via Wikimedia Commons, CC BY-SA
First on the list because it is the one cause here that asks nothing of you, and because the cut is unmistakable once you know the shape. A leafcutter bee takes neat discs from the edge of a leaf to build her nest.
What to check first
Look at the shape of what is missing rather than at the plant as a whole. Smooth, near-circular pieces taken out of the leaf edge, never from the middle of the leaf, is this and nothing else on this list.
2
Caterpillars, which on a host plant is the whole point
Look for the dark droppings as much as for the caterpillars. They are small dark specks left on the leaves below where the feeding is happening, and they stay there after the caterpillar has moved on. The holes themselves are irregular and eaten from the middle of the leaf as well as the edge. Photo by Schlaghecken Josef via Wikimedia Commons, CC BYThe droppings are the easiest sign to find. Scattered across the upper leaf surface like specks of dark grit, they sit below wherever the feeding is happening and stay after the caterpillars move on. Check the leaf below a chewed one before you look for the caterpillar itself. Photo by Schlaghecken Josef via Wikimedia Commons, CC BYUp close, the droppings are small dark cylinders rather than dust, and the caterpillars carry stripes and fine hairs. This is what the specks in the wider photo look like when you get near, which separates them from soil or seed that has blown onto the leaf. Photo by Schlaghecken Josef via Wikimedia Commons, CC BY
Ragged holes or whole sections eaten in from the edge, with dark droppings on the leaves below. Ranked second because on a plant that hosts butterfly or moth larvae this feeding is what the plant is there for, and that is worth establishing before anything else.
What to check first
Turn over two or three leaves near the damage and look along the underside and the midrib, then check the leaves directly beneath for small dark droppings. The droppings stay put even after the caterpillar has moved on.
3
Slugs or snails, working overnight
Compare these edges with the clean semicircle a leafcutter bee leaves. These holes are large and their edges are ragged and torn, they run into the middle of the leaf rather than sitting on the rim, and the leaves are usually wet. Look after dark or early morning, which is when the feeding happens. Photo by Schlaghecken Josef via Wikimedia Commons, CC BYHere is the feeding in progress on a different plant. The slug works from the edge inward and the holes it leaves are large with rough, torn margins. Find one at the edge like this after dark or early in the morning and the ragged holes elsewhere on the plant have their answer. Photo by Schlaghecken Josef via Wikimedia Commons, CC BYEarlier-stage damage and the other tell: the shiny, dried slime trail across the leaf surface. Holes start small and irregular and grow night by night, and the trail stays visible in daylight after the slug has gone. Photo by Schlaghecken Josef via Wikimedia Commons, CC BY
Large ragged holes that were not there yesterday, worst in damp spells and on the leaves closest to the ground. The animal is almost never present when you look, so what identifies it is the trail it leaves behind.
What to check first
Look for dried mucus trails, silvery lines across the leaves, stems or soil nearby. Extension guidance calls these the most characteristic sign of their presence, and they last long after the slug has gone.
4
Flea beetles, if the holes are tiny and there are many
Look at the size and the number rather than at the insect. These holes are small, round and scattered across the whole leaf, some punched through and some only pitted partway. The beetle itself is the dark speck near the middle, and it jumps away the moment the leaf is disturbed. Photo by Schlaghecken Josef via Wikimedia Commons, CC BYA second plant with the same pattern: dozens of small pale pits across the leaf, some open holes and some only scraped partway through, so the leaf looks sandblasted. That mix of pits and holes is the shot-hole look, and it shows up first on seedlings and tender leaves. Photo by Schlaghecken Josef via Wikimedia Commons, CC BY-SAThe insect itself, enlarged: a shiny black beetle only two or three millimetres long, here with a pale stripe down each wing cover, though many kinds are plain black. On the plant it reads as a dark speck, and it jumps like a flea the moment the leaf moves. Photo by Grey Smith via iNaturalist, CC BY
Dozens of very small round pits rather than a few large holes. The size and the sheer number are the identification, and the beetles give themselves away the moment the plant is disturbed.
What to check first
Brush a hand across the foliage and watch what happens. Flea beetles are small beetles that jump, and they scatter instantly, which nothing else on this list does.
5
Japanese beetles, in midsummer, in a crowd
The tell is what is left behind rather than what is missing. The veins stay standing as a fine lace while the tissue between them is gone, which is what separates this from holes chewed straight through a leaf. Hold a damaged leaf up to the light and the pattern is unmistakable. Photo by User:SB_Johnny via Wikimedia Commons, CC BY-SAThey arrive in numbers. Look for a crowd like this on one plant rather than a beetle here and there, because Japanese beetles gather where others are already feeding. Flowers and the leaves around them are the usual first place. Photo by Violet T. via iNaturalist, CC BYOne beetle at close range: metallic green at the front, coppery brown wing covers, about half an inch long, with small tufts of white hair along the sides. If the insect on your lace-eaten leaves looks like this, the identification is settled. Photo by Chris Ratzlaff via iNaturalist, CC BY
Leaf tissue eaten away between the veins so what is left looks like lace, with several metallic beetles feeding together in full daylight in July or August. Unlike everything above, you will usually see the insect itself.
What to check first
Hold a damaged leaf up to the light. Veins intact with the tissue between them gone, in a lace pattern, and beetles visibly on the plant right now, settles it.
Still not sure
This is exactly what your county Extension office answers, for free. They are the local horticulture experts for your area, they know what is going around this season where you live, and they will look at a photo.