Proper water management (irrigation, drainage) is required to grow a healthy crop. When the soil is too dry, the wilting plants will be more vulnerable to attacks of some sucking insects (e.g. aphids and thrips). Alternatively, too much water will also weaken the plants and the humid environment will favor the development of diseases.
With adequate irrigation or drainage the farmer will try to create an ideal condition for the crop. Furthermore, he/she can use irrigation or drainage to manage pest populations.
For example to control whorl maggot and case worm in rice fields, the farmer could drain the field for a few days. In the drier environment these pest usually disappear quickly. Draining the rice field is also an option to reduce outbreaks of sheath blight.
Irrigation can also be a tool to directly control certain pests. Small pests such as aphids can be washed off the crop by overhead irrigation, for example in the cultivation of beans.
Mulching is another way of maintaining humidity in the soil. The damp conditions created by mulching may make the environment less suitable for certain pests such as thrips, while the higher humidity may be more favorable for parasites of insects.