Actually, it’s an error on my part: this is a Large White. But they still turn our brassica crops to filigree lace, rendering the plant exhausted and useless.)
What you do is slightly squash a pregnant (or not) female, so she dies, but is not dismembered utterly. then, you place her carefully near some brassicas and watch the sex-crazed males come along and shag her. Then they go away and die of course.
Result: less f*****g caterpillars, and more kohl-rabi, broccoli and the like. The purple thing surrounds a glob of unfertilised oocytes (I checked under the scope):-