Citrus

From WormWiki

Citrus peels contain a compound called limonene that is poisonous to worms. This compound will breakdown in the bin like anything else, but if citrus is added to the bin in large amounts, the limonene may cause difficulties to the worms before it breaks down. As with other challenging foods, the solution is in moderation -- add citrus in small amounts and see if or how it works for you, and proceed based on what you learn.