I’ve also had infestations of thripes—Sevin dust seems to work well but below I’ve described a few things that I tried that also worked. Some of them were more work to apply. Lots more work in some cases.

Update 2009:  I’m going with the neem oil this year.  I’m using it for spider mites anyway and it does seem to work on thripes.  I’ve also put off planting chile plants for the second year in a row–thripes thrive on them and I’m hoping that if I take a couple of years off maybe I can get the bug numbers down enough to make a difference.  The sevin last year did seem to help a lot, but this year, I don’t think I need it–at least not yet.  So far the neem seems to be doing the job so I’ll keep at it.

2008 updates:

Neem oil (or insecticidal soap instead of the neem oil)–Neither product is harmful to humans, fish, lizards or anything except bugs. Using the spray seemed to kill and discourage ants, thripes and aphids. I tried diatomaceous earth but it didn’t bother ants at all and killed little else except for maybe the aphids. It is tedious to use because you have to reapply after every rain and sometimes more often, depending on your watering methods. I used a basting brush to apply the diatomaceous earth on the undersides of leaves, but if your garden is large, this could be beyond tedious and frankly, I don’t think it killed enough to be worth the effort. Ants laughed and crawled right across the stuff and I saw no effect on thripes either. It may have killed some nightworms that were eating my tomato plant leaves

Most insecticidal sprays don’t get rid of thripes because they spend a lot of time down in the soil. The sprays will kill the ones out on the plant, but the majority are in hiding and they will just keep coming out and attacking when you aren’t looking, so you will have to spray once a week and early in the season to keep this bug from getting out of hand.

Bad Thripe Infestation—A malathion spray killed whatever it hit, but it wasn’t killing anything by the next day–more bugs were back and the population seemed to be growing rapidly. I finally settled on insecticidal soap for seven days in a row. I actually bought the Safer brand, but you can also use two to five tablespoons of liquid Dial in a gallon of water (I used that in Houston—the amount of soap depends on what you are trying to kill).

The soap killed everything on the plant on contact, but it took about 5 to 7 days before the number of bugs died down to zero. This product is generally pretty friendly to the environment, but can “burn” the plants if overused or if used during the hot part of the year. I try to get control of the bugs before the temperatures hit 90—after that I have to be very careful if I spray insecticides at all.

For the first two days, I was spraying first thing in the morning and again in the evening. The bugs were so bad that the blossoms were half eaten and none of the chiles were setting. By the third and fourth day I was spraying only in the evenings and most of the bugs were gone, but I could still see thripes hiding in the blossoms. By day seven, I was spotting no bugs and the chiles were setting rather than dropping off. Usually once the bug population is under control I don’t have too many problems.