Saprotrophic wood-decay fungi are the primary cause of decomposition of wood, causing billions of dollars of economic damage each year.[40] Fungal decay, while useful in composting, is destructive of timber exposed to the weather, and in the case of dry rot caused by Serpula lacrymans, also of timbers in largely dry houses.[41] Some wood-decay fungi such as the honey fungi, species of Armillaria, are parasites of living trees, attacking their roots and eventually killing them, and continuing to decompose the wood when they are dead. Honey fungus is a serious horticultural pest, as it can spread from tree to tree by long strap-shaped rhizomorphs in the soil.[42]