Why in agroecosystems insect pests can reach high numbers, while in natural ecosystems

Why in agroecosystems insect pests can reach high numbers, while in natural ecosystems their numbers are relatively constant?

Agroecosystems are defined plots of land on which certain crops grow. Simply put, these are ordinary agricultural fields. They are created by people artificially to grow the necessary plants. Due to the large amount of food, insect pests begin to actively multiply there. They are very comfortable there. Under normal natural conditions, there is not such a large amount of food concentrated in a certain area. Therefore, the fields often have to be treated with poisons to save the crop from pests.