Where Do They Come From??

Here is a picture of this years garden. We have a small strawberry patch, and we planted lettuce, carrots, broccoli, zucchini, cantaloupe, tomatoes and onions. The kids are all very excited. They love to eat things out of our little garden, for that matter so do the neighbor kids and some of their parents as well. The other morning I woke up to find this....

One whole row of broccoli eaten down to the nubs (that row was to the left of the row Jacob is pointing at and on the other side of the lettuce). You can't really see it but that doesn't really matter because it isn't there to see anymore, and half of the next row eaten (the row Jake is pointing at). At first I thought a rabbit had jumped the little fence and that he really liked broccoli. For the life of me I could not figure out why he did not eat the lettuce. Well later in the day I noticed little green worms all over the existing broccoli plants. Now we have never tried broccoli before, this is a first and we have never seen these little worms before. So here is my question; where do they come from? Now this question goes for tomato worms as well. In another month I am sure we will have those as well. I mean really do they just crawl around for miles sniffing the air trying to find tomato plants. Are their little eggs in the dirt just waiting for tomato or broccoli to be planted and then poof they hatch and start eating my plants. I just do not understand this, but year after year they show up. They certainly do not live here in the winter. So there it is my thought inspired question for the day.