Depending on the size of the field diatomaceous earth would be a good organic option for more immediate results.
There is also the manual labor option of walking through the field killing them in the early morning (predawn/dawn) when they are the most dormant.
Otherwise you can take steps to prepare next year by luring more insects that will eat them. Of course If you aren't adverse to chemical treatments, I'm sure there is a list of insecticides that will work. Although I would never recommend the use of chemical treatments.
RE: Pests in rice