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The manure is a possibility, if the soil where the manure is rotted (composted) is infected with flea beetles. They would travel with the manure.

I like neem oil for treating pests. It is safe for people, pets and beneficial insects, but very effective on "bad" bugs. Here is more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

As long as the leaves look stiff and not wilted, this is fine. Sometimes plant leaves grow faster than the leaf spines and the result is they get a somewhat wrinkled appearance.

If the plants look wilted though, try giving them more water.

The soil should be kept fairly moist and watering, especially container plants of arugula, should be done regularly. Typically, it needs to be given at least an inch or so of water each week, with container plantings watered daily or as needed to maintain moisture. For more information on growing arugula plants, this article will help: https://www.gardeningknowhow.com/edible/vegetables/lettuce/growing-arugula.htm

To know for certain if the soil is low on nutrients or the pH is off, you would need to get a soil test. Once you have the results, you can go from there. This article will help with additional growing information: https://www.gardeningknowhow.com/edible/vegetables/lettuce/growing-arugula.htm

Here is a link with more information on how to control and rid the arugula of pests.
https://www.gardeningknowhow.com/edible/vegetables/lettuce/lettuce-pest-control.htm

Arugula.

https://www.gardeningknowhow.com/edible/vegetables/lettuce/growing-arugula.htm