It's hard to tell from just the photos what has happened to your barberry. It could be Verticillium wilt or some other disease. I'm including a link to information about what you could look for in few possible causes for the decline in the health of your barberry from the U of MN Extension. On that page, there is a link for Botryosphaeria canker where you can try to match the signs and symptoms you're seeing with those in the additional information.  The links for the other two Verticillium wilt and Nectria canker do not work, yet, but the U is currently updating its web pages and will be added soon. 

In the meantime, here is additional information about Verticillium wilt from the Missouri Botanical Garden website.  Likewise, the Missouri Botanic website  has information about Nectria canker.

No matter what the disease, it appears that the plant is a goner.  Knowing what caused it would help in determining replacement plants and maybe learn corrective practices that will ensure the success of other plants in your garden. 

If you prefer to get a definitive diagnosis, you could send in a diseased sample (look for a branch that exhibits some of the symptoms you've learned about in the supplied material) to the U of MN Extension Plant Disease Clinic the link here will provide you with information about their services, prices and how to submit a sample.

Good-luck!

Hello,

I would ensure that it gets an inch of water each week until it goes back into dormancy.  The plant has been stressed in some way this season and that stress can take a while to recover.  So watch it next spring as it leafs out again and make sure it gets an inch of water each week throughout next summer. Also make sure it does not sit in water.  You don't want to rot the roots. 

Good-luck!

Hello,

It appears to be a barberry looper. I think they have several life cycles each season which is why they're back. Here is a link to information about them: https://bugguide.net/node/view/33904

I've done some searching and can't find any research-based information on eradicating them but here's what you might think about. You can hose them off when you see them. Also, you might try to inspect the leaves frequently next year to see if you can find their eggs and hose those off.

Japanese barberry is listed as an invasive by the Minnesota DNR so anytime there's an enemy of it, that's a good thing. Those barberry loopers can help keep the wild barberries in check.

I hope this information helps.