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They can be treated with products containing Bacillus thuringiensis (Bt), a natural bacteria that kill worms/caterpillars without harming plants. Neem oil may be helpful as well. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/using-bacillus-thuringiensis.htm
https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm

It could be earwigs or slugs feasting on your plants, especially the young shoots and buds. Here is more information on these pests: https://www.gardeningknowhow.com/plant-problems/pests/removing-earwigs-from-the-garden.htm
https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm

Dahlias are normally dug up after the first initial frost in your area and then stored indoors for winter. Only in warm, southern regions can these tubers be left in the ground year round. They can be easily lifted with a garden fork or spade shovel prior to the complete die out of their foliage. Break up the clumps and separate the tubers, allowing them to dry out some before storing, usually about a week or two in a cool, dry area. Then, clip off the foliage, shake off any remaining soil and pack the bulbs in dry peat moss or wood shavings within a brown paper bag or cardboard box. Place them in a dark area with cool room temperatures, like a basement, until spring.

As for the mums, this article should help: https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/wintering-mums.htm

Here is a few links for you.

http://www.extension.umn.edu/garden/yard-garden/flowers/growing-dahlias/

https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/tips-dahlia-planting.htm

You can use dry sand or peat moss.

Here is a link with more information.

https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm

In spring you can move the tubers to a dark and warm location.
This should be enough to encourage eyes to develop.
Approx. 15 days before you are going to plant the tubers you can place them in plastic bags and add 1 teaspoon of water retie the bag and continue in the warm location.