Sunday, June 12, 2011

Onion Thrips Don't Like Mulch

Onion thrips don't like mulch. I know this because of a little experiment I conducted just recently. You see I live in an area of California were onions are regularly stripped of their chlorophyl by this little creature (that is smaller than an aphid and quite thin) and a lot of money is spent every year trying to keep it under control. So late last year as I was considering this sad state of affairs, I thought of a solution that might just be helpful to small farmers.

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.


  1. I randomly found your profile (and subsequently your blog) on when I searched "handicap". My daughter is also handicapped and I wanted to see if anyone else had used similar experiences in their profiles. I am loving your blog for many reasons: 1. I am from California (we moved to Houston in 2008), 2. I am an obsessive gardener and 3. because I always enjoy religious views on science or vice versa. If this is too "stalkerish" for you, I apologize. But I have to ask, do you compost in some sort of fancy bin, or do you have a pile? My compost pile is 2.5 years old and it has yet to produce anything other than smelly rotting clumps that double as maggot havens. Don't answer if you are weirded out in any way. PS If you like bugs you should check out Houston. There are approximately 1 million bugs per resident here (OK that isn't an official number).

  2. Caitlin thanks for the note and sorry for the delay in responding. I'm obviously not a blogger every day. My compost pile is something I have a lot of fun with. Actually I have three piles: one that is the last of last year's leaves and kitchen scraps, one that is from this spring and a new one that only looks like a pile of old salad and lawn clippings. I don't use a bin but I do have them in a place that gets a little moisture from the sprinklers and I cheat by adding a bag of manure ($1 a bag at Home Depot) once in a while. The worms really come in with the that little extra. I turn everything over with a pitchfork every few weeks. I hope you're weathering the California summer well enough.

  3. Hi Sam, thanks for responding. I don't blog everyday either and frankly I am wary of anyone who does. Thanks for the tips on composting. I am thinking that 2 piles might be the best for our situation and I will add some manure per your recommendation. We love worms here too and tried vermicomposting with moderate success. Unfortunately the worms were always cooked to death in the summer. We still haven't thought of how this could be prevented so our worm friends are on hold for now. I WISH we were weathering the California summer, but alas we are in Houston, Texas where the heat index is usually above 100 degrees. We miss California's weather and fertile soils but Texas is the place for us now and with 3 growing seasons and an average 50 inches of yearly rainfall, I can't complain too much!

  4. Caitlin, I wish you all the best and don't worry too much about the dry summer. Your pile will eventually break down.