You may have 'tobacco hornworms' (Manduca quinquemaculata - a sphingid moth) or 'tomato loopers' (Chrysodeixis chalcites - a noctuid moth). To the amateur, both are green, but have different sizes, markings and body features. Bacillus thuringeiensis sprays are safe and effective. New Spinosad treatments (e.g. 'Conserve SC', 'Entrust', ' Naturalyte', 'Monterey Garden Insect Spray' and 'Borer, Bagworm, Tent Caterpillar & Leafminer Spray') are probably effective and very safe, but you'd need to check the label. There are also registered chemical treatments, such as elemental sulphur (400 g/kg) + Cu as oxychloride (40 g/kg) + carbaryl (20 g/kg), or sprays of cyfluthrin, or pyrethrins, or fenthion. Just follow the label instructions for your Country, State or area. Then there are some generalist fungal control agents (e.g. Metarhizium spp. and Beauveria bassiana), but I'm not sure if they are commercially available to gardeners. Baculovirus agents against loopers are under experimental investigation. Good luck!

For an organic product, you can use BT (Bacillus thuringiensis kurstaki). It kills caterpillars (though they may munch for a day or so before dying) and won't bother anything else. BT is available in various garden products. [You don't want any BT product that isn't BT kurstaki. The other strains of BT kill other things but don't affect caterpillars.] (And BT kurstaki kills caterpillars but won't kill caterpillar-like creatures like the pine sawfly larvae currently infesting my dwarf mugo pines; I go on patrol daily. Isopropyl alcohol is marvelous stuff.)

Personally I have never seen leaf miners do that severe damage. Normally the leaf remain intact but with white lines squiggly all over the surface. But with a major infestation I suppose it is possible. Leaf miner larvae look like tiny white grains of rice. Which is why I asked what the white collection of spots near the bottom of the photo were. In the photo it sure looks like a mess of tiny larvae. Dave

*smacks head* Of course! We have a large and healthy slug population. I never thought about them going after cactus, but that makes total sense. I'll elevate it and breathe a sigh of relief that I don't need to drench it with something to kill pests.