The problem made me do something I should have done (and stuck to) a long time ago. I sat there and made a list of all the possible sources of the new invader. Below I list the three sources I found in my garden, plus a couple extra ones that are just as important. Considering each will give you control over each.
This is where I believe I messed up my pest prevention. I received two new plants, and I had no idea if the plants had been raised indoors and bug free or not. In addition, I just brought my new plants right in and put them next to some other small plants. Several weeks later, every plant I had was infested with thrips.
I have considered my foolish mistake, and this is what I do now...First, all new plants get sprayed with SM-90, dusted with a little diatomaceous earth, and placed in a separate room. Immediately after, I shower (just in case), although washing your hands and changing clothes should be enough. Whenever I visit those plants, I wash my hands and change clothes before I visit my other plants.
I continue this quarantine for two weeks, looking closely at the plants each day to check for any signs of bugs. I read at least one article that recommends a full 30 days! Either way, once you are satisfied you are bug free you can than put all your plants together once again.
Especially in good weather, it is easy to overlook your open windows, left open doors, holes in your screens, and any other holes there might be from the outdoors coming in. Pest prevention depends on having some separation from indoors to out.
Start by making sure there are screens without holes in all the doors and windows you leave open. Also, walk around your home once with a caulk gun. Check where your gas line comes in the house, your cable t.v., your air conditioning lines, et cetera. Now you should be in good shape.
Do you have an intake and/or exhaust for your indoor garden? This can be a direct pipeline into your garden. Make sure your intakes are covered well with screening. Your exhaust should have little flaps that close when you turn the fan off. Look around your house for other intakes, like your bathroom vent fan, your air conditioner, and your dryer. These are a lesser source, but still worth a thought.
There are so many problems with using outdoor soil in an indoor garden. You have to adjust its consistency, its water holding capacity, and you have to supplement it with the right additives without creating a nutrient lock-up situation. In addition, it makes pest prevention very difficult. If you must use outdoor soil, you can sterilize it in an oven at 200 degrees for about 45 minutes, but this can sometimes smell pretty bad.
Instead, I use commercial potting soil, perlite, vermiculite, and all my favorite organic amendments to mix my own soil. Perlite, vermiculite, and commercial potting soil are all sterile and pest free. Sometimes I will reuse the mix once by picking out all the roots and adding fresh worm castings, but I always avoid using soil from outdoors.
Visitors to your garden include anyone who could possibly have an unwanted hitch-hiker with them...especially you! If you have just spent time outdoors, wash your hands and change your clothes before visiting your indoor garden. Keeping your pet dog out of your garden area will help your pest prevention efforts here. If a friend shows up and wants to see your garden, well, it's your call. Just make sure you know where your friend has been!