I have a Spineless Yucca plant that I bought several months ago and today when I was watering it I noticed worms in the soil. I can't find anything about it in a book I have. I've never seen this before. They are small black with white spots. They almost remind me of caterpillars. I don't know what they are. They are not on the plant but are crawling all in the soil. Any suggestions how to get rid of these and how they may have gotten in the plant? Thanks for any help.