Ask-a-Dave's-Gardener: What caused these little holes in my chard?, 2 by LAS14

LAS14 wrote:
Last year and early this season, I had problems with leaf miners in my chard. This year I stopped them with vigorous harvesting and Spinosad. But from mid-summer on, they\'ve been plagued with something that makes tiny little holes. Leaf miners made paths and patches. These are too small for slugs or snails. I\'m very familiar with those pests. The harvesting/Spinosad approach doesn\'t seem to be working.

Any advice?

tia
las