The problem could be pests or it could be environmental. Yellow leaves can be a sign of over watering or not enough sun. It might mean a nutrient deficiency. You can find spider mites by taking a sample and looking with a magnifying glass for little black dots and indications of insect activity. Here is a link to a fact sheet on spider mites on Dahlias.
I recommend you take a sample to your local Penn State Extension and have them send it to the plant clinic for analysis. Also, pick up a soil test and send a sample of the soil in your dahlia bed. They will tell you if there is a nutrient deficiency.