One way to protect plants is to use fungi that cause diseases in insects. Fungi are simple organisms, like mushrooms and toadstools, that get their nutrients from the environment around them instead of making their own nutrients through photosynthesis.
Sometimes they're edible. For example, humans eat mushrooms in soups, omelets, pizzas and salads. But other mushrooms are really dangerous to eat.
Different kinds of fungi can be dangerous for insects too, and the insects don't even have to eat them to get sick. If the fungal spores (tiny cells that help the fungus reproduce) contact the insects' cuticles (their exteriors, sort of like our skin) they can enter the insects and make them really sick.
Fungi that kill insects are called entomopathogenic [EN-toe-mo-path-oh-JEN-ic], which is basically a fancy word for "insect-killing." Sometimes weevils like to eat things that are bad for them, and that's a trick that scientists can use to keep them from damaging plants.