The whole idea centers around the fact that part of the thrips' life cycle is exposed and might be vulnerable to predators. In both the nymph and adult stages, thrips are fairly well protected deep within the closely spaced onion leaves where it is difficult for any kind or predator to go. Occasionally a small pirate bug manages to find a thrips (yes the singular of thrips is also thrips in case you were wondering) but for the most part, the thrips are able to freely feed on the onions without being molested.
One stage, however, is not so hidden. It is the pupal stage. When the nymphs have eaten and grown about all they can, they leave the protection of the leaves and drop to the soil where they change into an adult. In most agricultural situations, the thrips can find a small crack in the ground to hide without being bothered. If, however, an environment could be created (say a mulch covering of sorts) to bring in more predators, the thrips population might be reduced. The predators could eat the pupae and break the thrips life cycle.
With that thought in mind, I found a nice section of young onions last fall and built a bit of compost around a few dozen plants. It consisted of only a long board with a bit of grass clippings beneath - not much - but it was what I had available. I anchored these boards around the onions using thin wood stakes and left the plants pretty much alone. I kept the mulched onions and a couple of rows of adjacent (un-mulched) onions fairly free of weeds and adequately watered through the winter and spring. Then last week I decided to go out and see what the thrips populations were like.
What I found was what I had expected. The onions growing by the mulch had on average about 2 thrips per plant. The plants growing without a mulch had on average about 13 thrips per plant. As this was not a completely randomized and replicated trial I didn't run a statistical evaluation but I expect that the differences were real (the numbers were pretty tight). Somebody with more land and more time might want to do a more complete study but I think my hunch will bear itself out: thrips don't do as well when mulches are present because more predators exist to break their life cycle.
And in fact I took a look under my improvised mulch (the boards) and discovered several predators. There were small ground beetles and a couple of species of rove beetles. I don't know if they actually ate any of the thrips pupae (I didn't make any observations) but I suspect that some of them did.
Of course a conventional farmer, with acres of onions, will not have the wherewithal to mulch everything. They will need to continue using conventional methods to control the thrips. But there are better ways for the small farmer. Grass or stones might work well, as might other kinds of mulch. Try some out and let me know what you find. Your onions will be happy you did.