Answer: Slugs perhaps, but pillbugs don't eat live plant tissue. Another possibility would be cutworms, which, like slugs, are also nocturnal munchers. Try to go out at night with a flashlight to see if you can catch somebody in the act. Where you see damage, dig around the plant base with your finger. Cutworms are often found in the top inch or two of soil, curled in a 'C' shape. Pick them out and smash them. Try a "collar" around the plant stems to act as a barrier. Tear a strip of newspaper about 2" wide and a foot long. Wrap it around the stem of the plant placing the bottom end just below the soil surface. This will prevent cutworms from cutting off your tender plants. By the time the paper degrades away, the stems will be larger and stronger, and not subject to cutworm attack. You can try cardboard or paper towel rolls. Just take an empty paper towel or toilet paper roll and cut the cardboard into 3-inch long sections. Place it on the soil around the stem of your transplant, burying the cardboard about one inch into the soil.
Here's an effective way to get rid of slugs that won't harm you or the environment. Sink little plastic containers (such as butter tubs) into the soil so they are level with the ground. Fill them with beer. If you don't have beer, a mix of sugar, yeast and water also works. The aptly named slugs like to drink the stuff, and will fall into the container and drown.
Another method is to use barriers of diatomaceous earth or sawdust. Diatomaceous earth is basically tiny, fossilized silica shells made into an insecticidal powder. The slugs will crawl through it, be cut, and eventually die.
You can also create places for them to hide during the day, such as boards, garden pots, etc. over holes/depressions in the ground. They'll crawl under and can be easily collected.
Finally, rotate where you plant your zinnias (and all plants) each year so pests and diseases specific to that plant don't have a good chance to buildup in the soil over time. Hope this helps--I love zinnias also!
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