Answer: Frogs would start eating some other kind of insect or else they would migrate to some other place.
Step-by-step explanation: The population of frog is dependent on the dragonflies for their food and if the number of dragon flies are reduced due to the effect of pesticides then there are two options available for the frog.
First, the frogs can shift their preferences from dragon flies to some other insect.
Secondly, the frogs can migrate to some other place to fulfill the need of food.