Dahlias suffer from two main pests-slugs will eat the soft growth and earwigs will eat the flowers-both can be dealt with quite easily
Slugs -put down some pellets or pick them off the plants after dark when they are feeding
Earwigs take an upturned flowerpot stuffed with straw or newspaper on the end of a cane-the earwigs can be found hiding in there -dispose as necessary-though they are not garden baddies
The yellow leaves are just old-is this plant in the garden or in a pot-you may be over -feeding and/or over-watering