Something eating our Penny Royal mint

We have Penny Royal as a ground cover. Until recently it was doing incredibly well. A few months ago we noticed one of the plants had been stripped entirely of it's leaves. For a while after that nothing more happened.
Recently whatever it was returned and went a little crazy. Most of the plants are now stripped to their stalks. We've spent hours looking over the plants for possible culprits but have found nothing that explains it. A single caterpillar here, or beetle there but nothing in the kind of numbers I'd expect to do this kind of damage so quickly.
I started looking on the net for answers. It seems that Penny Royal is a popular pest repellent which leaves me further perplexed. If it's so good at repelling insects what could possibly be eating it?
Assuming we can identify it the next question is what can we do to control it? We're against anything that might negatively affect the local insect, plant or animal life. And we're even happy to cede a percentage of the plants to whatever it is. But this total devastation is hard to accept.
Anyone got any ideas?
James
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You didn't mention where you were, but what about deer, woodchucks or other herbivores? Even if it's not part of their normal diet, animals are known to discover new things, or self-medicate.
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I think you should start looking over the neighborhood and not the plant. Does this only happen when you aren't home or at night? Do you have anyone near by making sachets? Start sniffing your neighbor's breath for the scent of penny royal tea, usually what the leaves are used for. I'd set up surveillance, park your car around the block, tuck some branches in your hat and be vewwy, vewwy qwiet ;)
Val

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We had exactly the same problem. No idea.
I recommend going out at night with a torch. I'll bet you find dozens of caterpillars on the mint. We spent a few night going out and picking them off and our mint is slowly recovering. I still don't know what species of caterpillar they are, I found the same perplexing pest-repellant info on the web too.
James
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snipped-for-privacy@gmail.com wrote:

"If you see skeletonized leaves on your mints (peppermint and pennyroyal seem especially affected), or webbing and small brown specks (caterpillar feces) on your rosemary, sage or thyme, then it's time to start looking for the culprit. Before determining how to rid your garden of pests, it's important to identify exactly what the problem is. Look for very small caterpillars (usually 1/4 inch or less) that are light green in color, and usually wound up in thick webbing. Once you have determined that this is indeed the problem there are several options for control. If you only have one plant that is affected, removing all of the webbing, dead leaves and caterpillars may provide ample control. For more substantial damage the best control may be to use an organic microbial pesticide called Bacillus thuringiensis, or Bt for short, which is a pathogen that is ingested by feeding caterpillars and destroys their stomach cells. Within several days the caterpillars will cease feeding on your plants and die. One trade name this product is sold under is Caterpillar Killer. It's important to note that it will also kill butterfly caterpillars, so only treat affected plants. Bt breaks down rapidly in light, so it is best to spray in the late afternoon so the caterpillars have maximum feeding time. Don't forget to follow the label directions carefully (the label is the law) and never use more pesticide than recommended. Happy hunting!"--Rose Loveall-Sale, Horticulturist
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