pests in straw mulch

I have been using straw mulch, but unfortunately it seems to attract pests that eat my seedlings, especially my tomatoes. The pests seem to be too small to see, so I don't know what they are. I have tried using diatomaceous earth with very little success. Any advice?

You sure it's pests and not mold?

whether mulched or not, I tend to "ring" most new transplants, including tomatoes, with a cut open can or plastic jug in the form of an open-ended cylinder that can be sunk a little into the actual soil around the plant and the upper edge is a few inches above the soil and mulch. This protects the small plant especially from cutworms but also to some extent from other ground pests like slugs, earwigs, pillbugs, etc. Most of these things are exacerbated by mulch around the plants when they are small. In some climates, such as where I live now in CA, this problem is so severe that I've given up mulching my annual vegetables altogether, instead opting to trench or "hugel" the organic matter below ground.....

I did a similar thing about 4 years ago (burying the organic matter). At the time, the idea was to make leaf mould under the ground and let the worms cycle it to the surface. The soil was very sandy loam, had been tilled mercilessly, and desperately needed organic matter.

4 years later and the worms are doing their job, although the process has been slower than I expected. I dug down the other day and the leaves have converted wonderfully (I buried 180 garbage bags of leaves in 500 sq feet of garden beds).

However, one thing I have noticed is that over time, grass has sent "runners" into the leaf mould and I am now dealing with what appears from the surface to be a spontaneous explosion of grass in my grow beds. This is only an issue in my outer beds which adjoin the lawn, but it's something to bear in mind when burying organic matter.