Controlling weeds poses a special dilemma because once a weed infestation is identified, it is often already so extensive that containment is difficult and expensive. Biological control (using organisms such as introduced insects or diseases to suppress populations) is effective in slowing the spread of weeds but generally cannot eradicate the infestation. Mechanical control, e.g. pulling weeds or using machines to dig them up is effective with smaller infestations if done carefully to avoid spreading seeds. Chemical control can be effective in controlling weeds and stopping their spread especially when infestations are detected early. Cultural or ecological control, is the modification of the weed's immediate environment so that weed competition is depressed. Land managers generally take an integrated approach, using a combination of these methods which is referred to as Integrated Weed Management.