Question

Why did corn borers increase after the fields were sprayed for beetles?

had more corn to eat
their natural predator was gone
were protected by the spray

Answer 1

The natural predators for the borers was gone

Answer 2

After the fields were sprayed for beetles corn borers increased because their natural predator was gone. The beetles had been keeping the corn borers at bay but once their natural predator had been eradicated the corn borers were protected and flourished.