I have only found them on this plant, which I believe is kochia. 

Hi Chea,

Sorry to hear about your outbreak of beetles.  You're right, they are a danger to horses certainly, and while they are more commonly a problem in contaminated hay, there's no way to know or ensure that a horse wouldn't accidentally ingest enough live beetles while grazing to become a problem.  It is a matter of your horse (will it eat beetles?) and your risk tolerance.

Several contact insecticides would work on blister beetles, however, dead beetles remain a problem for horses, so I don't know that trying to kill the beetles in place is a viable solution.  You might also find that some insecticides would preclude grazing after their use.

The safest course of action is likely to wait for the beetles to move on, which is usually fairly rapid--within a week or two, once the desirable plants they're after are gone.  They usually attack flowering plants (famously alfalfa, though your beetles seem to be after the kochia), so working toward limiting the number of broad-leafed plants in the field may be a long-term strategy you'd like to take.  

Sorry that there's not a great clear-cut solution for you, I hope this was helpful.

Cordially,

John