Insects on leather horse gear?

Leather is organic. It acquires dust and debris in it's travels, oils from the horse, your hands, what ever it's been in contact with, where ever it's been.

It's a delicious buffet to all kinds of tiny livestock. The critters you found are cleaning the plate and leaving behind their own waste. eew

There aren't any bugs that specifically attack leather, per se. Even roaches are grazing on the detritus when they mar the suface of leather.

Your tack is easily cleaned with a bucket of tepid sudsy water and a soft brush. You need to be out of direct sunlight. Go over the items to your satisfaction, then rinse thoroughly.

At this point, I would use LEXOL on all the leather. and allow it to dry, out of direct sun.

Now, the place that you had them stored comes under scrutiny. If it's a tack box or closet, give it a thorough cleaning and follow it up with a spritz of bug spray. I'd find some cedar shavings, and put the bags in the tack box/closet to discourage their return.

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You may want to look at what you are cleaning your tack with. Use of oils and such that are of animal base could have attracted the bugs. Spray ALL leather tack with glycerin, then rub clean. Use a toothbrush to get to the areas you can not reach.

This might be a dumb question, but are you certain you're seeing living, moving insects? It would be more likely that you are seeing the beginnings of mold, which on tack tends to look whitish.

If it turns out to be mold, wipe your tack down with white vinegar after cleaning it.