Ecology of Interactions between Weeds and other Categories of Pests

Introduction The term pest includes pathogens, weeds, nematodes, mollusks, arthropods, and vertebrates. One category of pests does not occur in the absence of pests in other categories, yet most IPM systems are designed around a single category of pest. For full implementation of IPM it is essential that all categories of pests be managed in an integrated manner. Allowances for the interactions that occur between the categories of pests be incorporated into the IPM decision making process. Interactions between pest categories can be diagrammed in the form of a 'pest hexagon' (Figure 1). Not all interactions between all categories of pests are of equal significance, but due to the trophic position of weeds in the ecosystem weeds have the potential to impact all other categories of pests. Weeds presence in agricultural systems can vary from essentially none in a highly managed monocropping system, such as that used for cotton in the San Joaquin valley, to very diverse mixture of species in small field cropping systems interlaced with hedges, fence lines, stream banks, and woodlands. Such contrasting agricultural systems provide very different habitats for all other organisms present in the systems. At the farm level there is a large difference in habitat for all organisms between fence lines that are weedy and those that are maintained weed free. Similarly, within orchards there can be large habitat differences between those managed with a weed-flee floor versus those managed with vegetation cover. There are several different mechanisms by which interactions can occur between categories of pests, including" 1. Trophic relationships 2. Alteration of habitat/environment 3. Physical factors 4. Control tactics 4.1. Non-chemical 4.2. Caused by pesticide use.