Pillbugs, which feed mainly on decaying organic matter, congregate in areas of high moisture and are commonly found under mulch. They are said to occasionally feed on young plants and their roots with damage that is usually considered minimal and not significant. However that is said in respect to small populations, populations in the thousands might be a little more harmful. How has your orange tree been struggling? I know that California is experiencing a drought so I am sure that has not been kind to your tree.
Here is some articles on sow (pill) bugs that will help you: