The best product to use in a horse barn with hay is Essentria
. Essentria is a naturally based insecticide that kills spiders on contact and also helps to repel them. The only downside to using Essentria, is that it does not leave any residual and therefore needs to be reapplied every few days or week to get maximum effectiveness. Any residual insecticide could not be sprayed onto the hay and could be somewhat toxic to the horses. Essentria can be sprayed almost anywhere. Essentria can have a strong odor of peppermint and wintergreen but it goes away fairly quickly. Removing the horses before application is highly recommended. Let it dry and ventilate before allowing the horses in back inside.