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.
Answered on October 22, 2018
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