寄生蜂是调节寄主害虫种群数量的一个重要因子。
Parasitoids is an important factor in regulating insect population dunamics.
油菜品种和栽培方式间害虫种群数量存在显著差异。
Significant difference can be found in pest's population quantity among different rape varieties and cultivate patterns.
而增加对害虫天敌的种类或数量，来打破自然界的相对平衡，使之不利于害虫种群的增长，从而减少和防止它们的为害。
We can break the balance of nature by increasing the category and quantity of pests' natural enemy so as to limit their growth and thus prevent the harm.
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