It is well established that brown plant hopper outbreaks in rice are caused by the use of pesticides. So why do farmers and their advisers spray even more when there is an outbreak?
According to this post on the Ricehoppers blog, it might be because plant protection services (in Vietnam) operate like fire brigade services, equipped for rapid response and control. And because, for the people leading these services, it is better to do the wrong thing (spray) then to be perceived as not acting (and perhaps lose their job).
Perhaps, like modern fire brigades, plant protection services will be able to shift their emphasis to prevention. And, like modern fire-ecologists, learn to let the occasional outbreak run its course.