Patience is what is most required. You may be struggling with a form of root rot. Regardless, the dahlia is under stress from some cause and the insects are simply demonstrating its vulnerability. At time of planting you should prep your site by adding a composted manure, plus some type of slow-release fertilizer. What I do is incorporate either fish bone meal or Jobe's Organic Bulb food beneath the tuber with the backfill having been amended with the manure and some additional fertilizer such as Jobe's Organic granular 4-4-4. Due to the slow release nature of the organic nutrients, I don't worry about over fertilization. I do know that once the tuber initiates its roots, it does very well when it happens upon the food I've added at time of planting. A huge plus associated with the Jobe's Organic product line is in the mycorrhizae package they have added to the fertilizer. Mycorrhizae congregates along the roots and contributes significantly to the transport of moisture, nutrients and serves as a disease antagonist helping thwart possible site infection by pathogenic soil fungi. Frankly, it's worth every penny you might pay for the fertilizer as I have used it extensively throughout my garden, landscape & potted plants. Competitive brands such as the Espoma & Dr. Earth product line rival the Jobe's products, but, I have found them to be more expensive. Jobe's relies upon its proprietary mycorrhizae package to accomplish its goal and I have found it to be highly effective.