JDs are opportunistic feeders, so pretty much anything will be taken. Mussels and prawns are okay as treats but they contain thiaminase, so use in moderation. Bloodworms are popular, but there is some concern about their safety in terms of possible toxins they absorb from the water they're grown in; likewise for Tubifex, but even more so. Cockles are safe and nutritious, and you can buy them in many grocery stores. Cooked peas and frozen foods with cooked spinach in them are both especially good: besides providing useful vitamins they also contain fibre that keeps bloating at bay. If your garden is pesticide-free, you might also have a root about for earthworms; most cichlids, including JDs, go wild for them. But use small ones, and remove any that escape because they die underwater after a while, and when that happens and they start rotting, water quality will be harmed.
Cheers, Neale