Lilacs, my first thought when I find a grub in the lawn is that it is a cutworm. Some cutworms overwinter as immature larvae but they are not white. The location in which you found the grubs make me suspect that rather than lawn pests, what you've uncovered may be fruit tree pests.

I can't seem to come up with any real good ideas. My best guess is that they are Oriental fruit moths. You can read more about them at the link below.

To read about fruit moths and all sorts of other pests in crops, the University of California has a real good starting point at http://www.ipm.ucdavis.edu/PMG/crops-agriculture.html

The problem you present with your little white grubs is that these are apparently larvae and they've either overwintered in your soil as larvae or, perhaps, as eggs. Your soil is now sufficiently warm for the eggs to hatch or the larva to become active. Many suspect insects either overwinter as adults or pupae or they do not overwinter in the soil or on the ground.

Other possibilities are coddling moths or leaf-roller larvae. If you have apple trees nearby, I'd think the darn coddling moth would be the best candidate. Your question should be pondered by others for an answer. But, one thing you could do is bring those little grubs indoors and allow them to pupate and emerge as adults and then identify those guys. Sounds like a fun project for the nearest elementary student, huh?

DigitS

Here is a link that might be useful: Oriental Fruit Moths

Not sure this is even what you have...but this is a link that explains the difference quite well about white grubs. One seems harmless and the other causes damage in corn. I get tons of the not so good guys but they never seemed to do any damage and do make great fishing bait. The site has great pictures and hints on how to tell the grubs types apart.

Here is a link that might be useful: White grubs

lilacs - it's real hard to try to pinpoint the "little white grubs" without more info. Digit has a great idea, but I would also want to find out as quickly as possible, so you might take a "sample" to the extension office and/or contact CU Boulder and see if there is a Professor up there who might have a few minutes that he/she could give you. They should also be able to tell you if it is something you need to worry about or if it is a "non-issue"
Good luck

B

My little white grubs don't match any of the pictures, which I suppose is a good thing. Mine didn't have legs or a visible head.

I should have thought to get a picture, but I wanted to get my plants in the ground.