Fortunately for us, everything that eats insects loves aphids. They are sweet. They are dense. They seldom defend themselves. They are the living example of “the best defense is a good offense”. Their strategy is simple; just reproduce as fast as you can and you will hopefully survive.
This pre-amble explains to some extent what you see in your crops. When a Green Peach Aphid flies into your crop, it lays its eggs in its favourite plant. The flying stage is “diploid”, like humans, some are male and some are female. This allows for sexual reproduction which is the fastest way to select for the appropriate characteristics, such as temperature range, host suitability, and chemical resistance. Once they have mated and reproduced, they shift to “haploid” stage which is asexual. At this stage, they are all female and the ten little babies surrounding this wingless adult are actually clones, all perfectly adapted to your crop and chemicals. In the haploid stage, the Green Peach Aphid can complete its lifecycle in about a day. If you had a good scalpel and a steady hand, you could dissect one of those “little babies” and find another 10 of them in their abdomen. This explains why you suddenly discover a plague. Over one weekend, in ideal circumstances (your protected greenhouse is ideal), 1 aphid can become 10,000 or even 100,000 by the time you take a walk on Monday morning.
Before we go any further, I mentioned that they seldom defend themselves. Instead, they produce honeydew; such a delicious, sweet excrement other insects, namely ants, make caring, nurturing and defending aphids their lifes’ work. Ants “farm” aphids. While man was struggling to get out of his hunting and gathering rut, ants had already figured out agriculture. Ants guard their flock. Ants remove predators and parasitoids. When predators or parasites become overwhelming, they pick up the prettiest aphids and move them to a safe place. If you are going to make a dent on the aphids, you are going to have to get through the ants first. You are going to have to kill the ants. Most of the species of ants that we work against have a “hive” system of governance. If you can kill the Queen, the hive disperses. Most of the ant strategies involve tricking the worker ants into taking a toxic snack back to the Queen and hopefully killing her. I will let you figure out how you can get rid of the ants. Maybe in the future, we may be able to rear and sell Ant-eaters but, it is a bit of a stretch for an insectary.