In the early 20th century, people brought the western mosquitofish to other places because they thought it would eat mosquitoes and their larvae, but this did not work. Sometimes there were more mosquitoes than before: The western mosquitofish ate, outcompeted or killed other fish that were eating mosquitoes. It sometimes killed fish that were better at controlling mosquitoes than it was. It also ate some of the insects that had been eating the mosquitos and larvae. It also ate zooplankton, which left more food for the mosquito larvae.