I'd say it's a type of shield bug. This site is goodhttp://www.gardensafari.net/english/shieldbugs.htm
After some research it appears to be a female Harpocera thoracica. You mut be later than Europe where they only appear in May to June! http://www.britishbugs.org.uk/heteroptera/Miridae/harpocera_thoracica.html
They were on my parsnip plants that I let go to seed.Thanks for researching them and commenting.