Answer: I wonder if you are describing whiteflies. These are tiny white insects that suck the juices out of plant leaves. They will leave tiny yellow marks, called stippling. If you tap the leaves, the whiteflies will use rise in up a "snow cloud" effect. If that sounds like your problem, here are some methods of control. I always start with the simplest method first, and if that isn't successful, move on from there. A strong blast of water from the hose often works. Spray underneath leaves where they hang out. Do this daily if you notice insects.
Whiteflies are attracted to the color yellow. You can purchase or make yellow "sticky" traps from yellow cardboard smeared with petroleum jelly. They fly to it and get stuck.
Soapy water sprays are another possibility. Use 1 teaspoon to 2 tablespoons of liquid detergent soap per gallon of water. Use regular, not concentrated soap. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. Spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Spray early in the morning before the sun heats up. Next on my list would be an insecticidal soap spray.
The insecticidal soaps are made from plant-derived fatty acids and target soft-bodied insects. There's really no way you can target the bad guys without fallout on the good guys. If you can regularly monitor and tolerate some damage to your plants, over time Mother Nature strikes a balance, with the beneficials coming in to control the bad guys. Healthy vigorous plants will withstand insect attacks best, and it's really the best thing you can do to prevent insect problems.
Without knowing exactly what the insect is, it's hard to answer some of your questions. The insects shouldn't prevent you from eating the parsley, nor should most of the control methods. Read the label on any insecticidal soap products carefully before using. Some insects do propagate in the soil. They may not care for the aroma of the other herbs, but it's a bit unusual that they haven't moved on to all the plants in the pot. You are right that there are beneficial predator insects that will eat other insects, but timing is everything! Sometimes, it takes a while for the predators to build up sufficient populations to control the bad guys, so it's not always possible to do by just setting the plant out for a few days. I hope this information helps!
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