How plants tailor their defense responses to attack from different insects remains largely unknown.We silenced a mitogen-activated protein kinase (MAPK),MPK4,in Nicotiana attenuata (irMPK4 plants) to study its role in regulating resistance to two herbivores,the specialist Manduca sexta and the generalist Spodoptera littoralis.MPK4 activity did not alter after mechanical wounding,but increased after applying of M.sexta oral secretions (OS).