blister beetles

Blister beetles feed gregariously on tomatoes, potatoes, beans, peas and many other vegetable crops.  They are ash gray or striped to blackish.  When crushed, they release a substance that can easily blister skin.  Care and gloves are needed to handpick beetles off plants into a bucket of soapy water.  Blister beetles generally move on after a few days. 
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https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Blister%20Beetles%20.pdf