Interactions between weeds and organisms in other pest categories are inevitable. Weeds are plants and therefore ecologically are producers. All other pest organisms are consumers; they are herbivores or pathogens and can thus use weeds directly as a food source. Beneficial organisms are primary carnivores that feed on herbivores; weeds can support beneficials indirectly when they feed on herbivores living on weeds. Weeds can also serve to mask crop plants from herbivore pests; the mechanisms by which this occurs are still debated. Presence of a weed canopy modifies ecosystem microclimate and provides shelter for pests and beneficials that would otherwise not survive. Tactics used to control pests can have impacts on nontarget organisms in other pest categories. Changes in tillage for weed control can impact population development of other pests. Pesticides can affect nontarget organisms resulting in unanticipated changes in crop tolerance and pest control. Development of true integrated pest management programs requires a multidisciplinary approach that incorporates interactions between organisms in different pest categories.