解析库 > 阅读经典300篇
Insect predators usually keep the number of aphids in crop fields low. However, sometimes the aphid population explodes in size, causing major damage. Much explosions happen when unusually cold weather keeps the number of aphids low in the spring. One possible explanation is that, with fewer aphids to feed on, the predator population also drop, and in summer when the aphid population starts to grow there are not enough predators to keep it in check.