Maggots are commonly found on rotting food or dead animals. They're a clear indication that decomposition has set in. Of course this only applies to species of

Maggots are commonly found on rotting food or dead animals. They're a clear indication that decomposition has set in. Of course this only applies to species of flies that go for these food sources. Most fly larva (really most insect larva) look like "maggots", including for species like hoverflies that eat nectar. But these "maggots" are usually hidden in the soil or on the plant and don't show up in one place in large quantities. Thus there's an obvious association between maggots and rot.