As Lu says, after finishing up his collecting for the day, shaking off his hands and dumping the water out of the can he is holding into the grass, "once the water is gone, the larvae naturally die off." If you've got a container that you aren't using, but don't want to throw away, leave it upside down.
其實，要斬草除根，讓蚊蟲絕子絕孫，必須從清除積水做起。就像呂師傅，抓得差不多了，手一揚，把鋁罐的積水倒在旁邊草叢中，「水一乾，幼蟲自然死光光！」