He tests whether limited host plant availability (food source), competition with other herbivores, predation, or some interaction of these three limits the population growth of aphids.
(He; tests; whether limited host plant availability (food source), competition with other herbivores, predation, or some interaction of these three limits)
He
tests
whether limited host plant availability (food source), competition with other herbivores, predation, or some interaction of these three limits