Isopods tend to have a high amount of metals:
"A word of caution - isopods are heavy metal bioaccumulators. Therefore feeding wild caught pillbugs is less preferable to using those you have raised yourself, as you can easily make sure you are using a quality organic soil/sand mix for the base and feed them healthy foods. Culturing / breeding them instead of using wild caught ones also reduces or removes possible parasite issues." (sandrachameleon)
Terrestrial Isopods Oniscidea /Porcellio (pillbugs, woodlice, rollie pollie...) - Chameleon Forums