This appears to be the Bagrada bug, Bagrada hilaris, although these your bugs seem to be green while B. hilaris is more black in photos I've seen. Bagrada bug is native to Africa, but has been found in India and was recently introduced into California and South America. It can feed on a wide range of weeds, but prefers and causes much damage to cruciferous crops such as broccoli, cabbage, mustards, and cauliflower.

Control can be difficult to achieve because the bugs can feed on many non-crop weeds and adult bugs fly away when disturbed, which can make chemical control difficult. Best practices in the US currently include removing cruciferous weeds near crop fields, destroying crop residues post-harvest, and multiple applications of foliar insecticides. Additionally, the use of insectice-treated seeds seems to help.

I've linked some fact sheets below that may be of some help:

http://ipm.ucanr.edu/PMG/PESTNOTES/pn...
http://entnemdept.ufl.edu/creatures/V...
https://cisr.ucr.edu/pdf/uc_ipm_bagra...
http://www.annualreviews.org/doi/abs/...