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I live in England and grow mine in a greenhouse, but I grow Marigolds below the tomatoes and never see either white or green fly. I can't speak for outside? it might work..

Hello,

This appears to be a fungal issue. I have a recipe that I recommend, which is safe, and natural. This will not harm your plants. This recipe is a broad range pest control for most insects, fungi, and bacteria.

The recipe is as follows: First, you will want to prime the soil with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.

Already answered:

https://www.domyown.com/monterey-houseplant-insect-spray-p-11260.html

Your question is a bit like asking what causes flies or soil fungus. They are part of the complex eco-system and a healthy system keeps the numbers in check. The occasional fly doesn't bother us but when they congregate they are a real nuisance, just like aphids. Aphids reproduce rapidly and don't have to work to find food. Once they have located a host plant, they insert a tube to allow the plant's sap to flow into their mouths. Sap is under pressure so the aphids don't even have to suck it out. This is why they seem to move very little. Specialized bacteria convert the sap carbohydrates to proteins, allowing the insect to get everything it requires from a single source. Plants that are stressed for any reason from lack of sufficient water to nutrient stress are magnets for aphids. Stressed plants produce glutathione and this chemical seems to spur aphid growth and reproduction. A healthy, well-maintained garden is your best defense. Avoid use of pesticides as they kill beneficial insects that keep aphids in check. Consider putting up a finch feeder as finches are one of the birds that eat aphids.

These little creatures commonly are referred to as garden centipedes, but they're not centipedes at all. They're garden symphylans, distant cousins of centipedes. You will want to remove this soil for an indoor terrarium. Replace the soil will a good quality potting mix.

https://www.gardeningknowhow.com/plant-problems/pests/insects/garden-symphylan.htm