ash fall是什么意思

用法例句

The occurrences and catastrophes of migratory pests connected with the habitude of their sources, the patterns of migrations, the taking-off and landing mechanisms in most inseparable associations.

迁飞性害虫的发生和灾变与其虫源性质、迁飞规律、起飞降落机制等有着不可分割的联系，而起飞降落机制是这些因子中作用最大的一个，也是决定危害或灾变程度最关键的因素之一。