Based on the provided responses and the context of your question, the most plausible hypothesis to explain the unexpected results when beetles were introduced as predators to Leopard frog tadpoles and Malathion was applied is:
The Malathion killed the beetles, causing fewer tadpoles to be eaten, so the tadpole population grew.
This hypothesis suggests that the pesticide affected the predator population (the beetles), leading to a decrease in predation pressure on the tadpoles, which in turn allowed their population to increase.