My sunfower seedlings keep losing all their leaves, and I don't know why.

The seedlings germinate after a short amount of time. They seem to grown fine and have good health, and suddenly I will go out and observe the top leaves are completely gone. At first I suspected cutworms, since the stems were cut off at the base, so I inserted toothpicks and collars.

However, now the leaves are being eaten during the day, leaving some plants with no leaves. For example, today I noticed that I had 3 healthy seedlings. I watered them in the morning. At 4:00 PM, noticed that two seedlings had completely lost their leaves. Two other seedlings lost their top leaves.

I am at a lost to what is causing the damage. Every time I plant new seeds, I seem to encounter the same problem.

Some of the seedlings are only 2 inches tall. Others are around 6 inches tall.