Minecraft wiki entry for Cave_spider_farming, associated text: The disadvantage of using this method is that cave spiders will damage the iron golems as well, and eventually the iron golems will be killed. It is recommended to have an iron golem farm before trying to attempt this farm (otherwise, you will go through a lot of iron!). This can be mitigated by having the cave spiders land on magma blocks. You can also repair the iron golem by using iron ingots.