Q.rhubarb eaten by insects
After sowing and well watered and manured, I discovered leaves eaten. What causes this to happen?

Slugs, snails and beetles are typically the usually suspects. Snails and slugs typically come out at night so you may want to go out with a flashlight at nightfall and see if you can spot them. Deer and rabbits typically leave rhubarb alone unless they are desperate.
For more information on snails and slugs, please visit the following links:
https://www.gardeningknowhow.com/plant-problems/pests/insects/kill-garden-slugs.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/organic-snail-control.htm