Answer: I'm only guessing without seeing the pest, but you may be dealing with Mexican Bean Beetles. Mexican Bean Beetles can be formidable foes. There will be one to three generations per year, so destroying egg clusters during the growing season will help reduce subsequent populations. Organic solutions include planting nasturtiums, garlic, marigolds or rosemary along with your dahlias. Be sure to clean up plant debris at the end of the season so adults won't have a place to hide. You might also try growing a variety of plants to attract predacious wasps and assassin bugs, who dine on Mexican Bean Beetles.
Finally, if possible, move your dahlias to a different garden bed this year. This will help them avoid attacks from overwintering insect pests.
Q&A Library Searching Tips