If you begin to notice that another animal has been enjoying your marigolds before you've had a chance to, it's time to solve the mystery of the marigold munchers. What is interesting about your marigolds being eaten, however, is that marigolds are recognized for being a pest deterrent because of the smell they emit. In general, animals and insects find the smell offensive and are repelled, so you can narrow down which animals have been eating your marigolds.

...
Study the petals of your marigolds for signs of chewing.

Slugs

Slugs are known to be garden munchers and can quickly destroy your marigolds if you have a host of them in the garden. While marigolds deter garden pests, slugs are still drawn to them and enjoy feasting on marigolds. You can tell whether slugs have been eating your marigolds by analyzing the leaves and petals. If the leaves and petals look to be half-chewed in irregular patterns, this is evidence of the typical slug's feeding habits. You can also look on the ground for slug tracks, which look like linear grooves in the soil. It could be hard to catch slugs in the act of eating your marigolds, because they mainly come out at night or in damp conditions.

Birds

Birds have a tendency to peck at marigolds and other flowers. Blackbirds are one such bird that gravitate towards marigolds and will swoop down to peck at the petals. If it looks like your marigolds are missing petals, but they do not look chewed on, with holes in the petals and leaves, then it is probable that birds have been enjoying the marigolds with their beaks and flying away with their findings.

Grasshoppers

Grasshoppers can damage your marigolds. When grasshoppers intrude upon garden flowers, they will chew at the plants, from the petals to the leaves to the stems. Grasshoppers come out during the day and might relax on top of your marigolds, while eating them at the same time. This is because their jumping ability lets them access the tall marigolds and perch on top of them. You can control grasshoppers by introducing natural predators of grasshoppers into your garden.

Spider Mites

Spider mites are hard to detect due to their small size, but they are known for damaging marigolds. Spider mites are closely related to ticks and spiders, rather than insects, and have eight legs that they grow as adults. They feed on flowers by sucking the fluids out from the plant cells. As a result, you will see your marigold leaves and petals appear flecked with pale dots. An abundance of spider mites will turn everything yellow and cause your entire garden of marigolds -- along with other plants -- to die.