Onions

From WormWiki

Onions, garlic, leeks, scallions, and other members of the onion family have anti-bacterial properties. Since the worms don't eat the food in the bin directly, but eat the microbes on the food (see What do worms eat?), these anti-bacterial properties have to be exhausted before the worms can go to work on these foods. As a result, members of the onion family can take a long time to break down, and may cause unpleasant smells before they are fully processed. These foods can be added with care, using extra bedding as a "buffer." If you want to add these foods, experiment with them to see how they work for you.