Mealworms, which are actually the larvae of the darkling beetle, are not considered beneficial to plants. They are scavengers and will eat all types of organic matter, both live and dead.
Mealworms are most frequently associated with eating stored grains, such as flour, cornmeal and cereals, but they are also pests in the garden. These insects will eat all types of plant growth, including young seedlings.
The only possible advantage to having mealworms in your garden is that they will eat sticks, grass and dead leaves. This may seem helpful, but the fact that they also eat live plants cancels out that benefit.
Using mealworms in your compost heap is the only way they can benefit your plants. By eating and digesting your leftover kitchen waste, they can turn it into nutritional organic matter that will provide the plants in your garden with important nutrients.