The biggest consequence is that nobody knows the consequence
. There have not really been very good studies on captive box turtle diets. This is largely due to the fact that the diet you feed the turtle today may not affect the turtle for a year or longer, depending on their age. Reptomin is supposed to be formulated for aquatic turtles and not box turtles. Reptomin is made up of largely fish meal. I don't imagine box turtles run across many fish in the wild. Having said that, I love Reptomin. I feed it to my box turtles frequently. They love it also. I sprinkle some calcium powder as well as multivitamin powder on it every now and then. It has a calcium to phosphorus ratio of 2 to 1.8 (nearly 1:1). You want at least 2:1 for most food items. I'd say that Reptomin should only be used as one part of a VERY varied diet. I keep a log of what I've fed my young turtles in order to make sure I feed enough variety. You're on the right track with seeking insects.
If you'd like to catch insects, I have some tips. I do this quite often. I have a herd, and feeders can be expensive. Also, I really think that insects caught from pesticide-free areas are more healthy than farm raised individuals. I'm cautious with slugs and snails as they are notorious parasite vectors.
You can have a nearly neverending supply of earthworms by constructing a compost pit. Put in grass clippings, leaves, hay, sticks, vegetable scraps. Make a nice pile, soak it well, and cover it with an old sheet. Dark sheets will help speed up the process by warming in the sun. After a few weeks, it should be loaded with earthworms. Also, on cool rainy nights, I walk my neighborhood with my dog, a poncho, and a flashlight. You can find areas where earthworms will crawl across the road. Again, try to find a pesticide-free area if you can determine that. You'll find that there are certain "hotspots". These are areas where you know with almost certainty that when the conditions are right, you'll find all the earthworms you can handle. It takes a little while to locate the spots, but on a rainy night I can tell my wife, "Going worming, I'll be back in 15 minutes." I'll usually collect 50 or so worms, more if I have the time.
Ok, I'm getting really long-winded here. I'll try to make it short. Buy a butterfly net. Go into any field in mid to late summer and catch grasshoppers. Use the same net to scoop up mats of weeds in any clean pond for dragonfly larvae...my turtles LOVE these things. They're very abundant and slow moving on land. Break apart old rotting logs. You may find white grubs. They can be very large, but most should be yearling box turtle edible. If you learn to recognize the holes they make, you can find lots of them. I always keep a pile of rotting logs in the very back of my yard for this purpose. Keep your front and back porch lights on in the evening. Make sure you have a good landing site near them...like a screen door. After several hours of dark, go out and collect all of the beetles and moths that your heart desires. I call them junebugs, but they are reddish, round beetles that come in two sizes, adult box turtle size and juvenile box turtle size (1.5 and 0.5 inches). I can catch hundreds of these things in this manner beginning in June. I don't know where you live, but they are at least up and down most of the Eastern US coast. One of the best things about them is that they won't fly when it's daytime. They'll try to burrow to escape, but a hungry box turtle won't give them time to do that. They only keep for a day or two at room temp., and I've been overzealous on more than one occasion and end up having to freeze dead ones for mixing in with adult box turtle chow. I'm a bug collecting nut. I keep mason jars with homemade screen lids with me at all times. I'll try any bug, unless I know it to be noxious or poisonous.
You can breed your own feeders. Some are very easy. That way, you can make sure to feed them really high calcium veggies from day one. If you don't want to breed them, you can just buy them as needed. There are more and more feeders becoming available. I'm sure you know that variety is the key. There are mealworms, butterworms, waxworms, earthworms, phoenix worms, crickets, tomato hornworms, silkworms, superworms, roaches, pinkies, maggots, and more added all the time.
Your turtle is at the age where it should start sampling fruits and veggies. Offer these, even if they're not eaten. Even just one bite can be beneficial.
Nobody really knows how often we should dust the feeders. At least two times per week is considered a requirement.
Sorry for the long post.