Q. What does a venus flytrap eat?
In the wild, Venus' flytraps (Diomaea muscipula) grow in swamps where the soil is deficient in nitrogen and other nutrients so they use flies and other insects to provide extra nutrition. If you are growing Venus' flytrap outdoors (USDA Zone 7 - 10, or as low as zone 5 if providing additional winter protection) then you will probably not need to feed them yourself. Plants grown indoors (for example, in a terrarium) will benefit from feeding.
Do not fertilize venus fly traps. It is better to feed the plants directly. There is some controversy among experts as to the best source of food. Some say that only live insects should be used. Most agree that human food ,such as hamburger or cheese, should not be used. Since live insects are hard to catch (although some, such as crickets, can be bought from pet supply stores, many growers use other sources of food. Betta fish pellets, blood worms and mealworms are possibilities. If purchased as freeze-dried products, they should be re-hydrated before use. Slugs and caterpillars, collected in your garden, can also be used. It is important to use only small pieces of food (no larger than one third the size of the trap). Finally, do not overfeed the plants; about two pieces each growing season is sufficient. Do not feed during the dormant season. Also, do not trigger the plants to close without offering them food. The closure mechanism uses lots of energy, which the plants can ill-afford to waste.
For more information on growing Venus flytraps is, refer to the NYBG guide Carnivorous Plant Care: Venus Fly Trap.
The International Carnivorous Plant Society is also a good source: https://www.carnivorousplants.org//