You are correct when you guessed that the Japanese Beetles were feeding on your dahlia’s buds. The insecticide you applied have controlled the Japanese Beetle infestation for the time being. Continue to monitor for Japanese beetles and remove the beetles from your plants. The best time to do this is early in the morning or early in the evening. If you pick more than a few every day reapply the insecticide.

Do not use Japanese Beetle traps as the can draw more beetles to your yard. The traps attract beetles using synthetic female sex pheromone and a blend of chemicals with a strong floral odor. Researchers developed them to monitor for the presence of Japanese beetles so that management strategies could be implemented.

More information can be found in this article from the University of Minnesota Extension. Japanese beetles in yards and gardens | UMN Extension

Thank you for your question and have a great gardening season.