There is a review concerning the effects of carbon dioxide on insects
in:  Annual Review of Entomology, 1989, 34, 97-116 Paragraph concerning
mortality pp 107-108.  Insects are in fact very resistant to CO2 With
regard to bees, A.  Deyme (in Apidologie, 1977, 8 (3), 217-228) tested
different concentrations of CO2 (to 75%) on bees of different ages.  No
bees died and the 2-day-old bees were the most sensitive .