Annual crop fields typically are simple habitats dominated by a few plant species where pesticides play a major role in managing weed and insect infestations. Recently, there has been significant interest in the potential to reduce reliance on pesticides by manipulating plant species and communities to benefit natural enemies of insects and weeds. Such efforts aim to enhance natural enemy impact by providing appropriate food, shelter, and hosts, and efforts typically are accomplished by manipulation of plant species, populations, or communities. Habitat management is generally viewed as an important factor in maintaining stable insect and natural enemy populations in agricultural systems and may have a similar function in increasing weed seed predation. Crop and noncrop habitats provide resources to natural enemies either directly through floral nectar and pollen, indirectly by increased host or prey availability, or through emergent properties of the habitat such as by moderating the microclimate. These critical resources for natural enemies can be provided in agricultural ecosystems at several scales: within fields, at field margins, or as a component of the larger landscape. Because individual natural enemy species may require quite specific resources at different times and spatial scales, not all attempts to manipulate habitat diversity are equally effective. We review the role of plant resources, including weeds, in supporting natural enemy communities and provide case studies of how varying plant diversity at different spatial scales can influence the effectiveness of biological control in agricultural landscapes.

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.

Weeds and native plants should be considered when endeavoring to manage and control plant pathogens of cultivated plants. Whether as a pest itself, vector of a pathogen, or reservoir of a pathogen or its vector, weeds can significantly influence disease incidence. The relationship between these factors plays a critical role in determining disease incidence and impact. Weeds can interact with pathogen management in several ways, including provision of weed biological control, parasitic weeds can directly serve as vectors of plant pathogens, weeds can serve as reservoir alternative hosts for pathogens and vectors, weeds may be obligate alternate hosts for some pathogens, and herbicides used for weed control can interact with plant pathogens. A recent concern is the advent and deployment of plants genetically engineered for pathogen resistance, raising the question of “super weeds” resulting from genetic drift of genes from crops into surrounding weed populations, the impact of which has yet to be determined.

The literature relating to the impact of other pests on weeds of agroecosystems is minimal. A great deal of literature discusses the effect of organisms used for biological control of weeds; however, pest organisms used as biological control agents are not the subject of this paper. The objective of this review is to present what is known about the impact of insect, pathogen, and nematode pests on weeds; to outline some of the gaps in our knowledge; to present concepts from the ecological literature that might provide insight; and to discuss implications for integrated pest management. The limited data that are available suggest that weeds require fewer resources to survive in the presence of the pest complex than the crop and that weeds would potentially have a greater ability to survive, compete, and reproduce in a competitive environment compared to the planted crop. We suggest that three categories of weed response to polyphagous crop pests may occur in agricultural fields: susceptible weed species or biotypes that host the pest with severe effects on growth and fecundity and therefore are of limited concern in terms of competition for resources; tolerant weed species that host the pest without severe effects on growth and fecundity, resulting in effective competition with the crop and larger pest populations; and resistant weed species that do not host the pest but compete effectively with the crop. We propose the hypothesis that the weed community in many agricultural fields is dominated by plant species that are tolerant or resistant to the endemic pest complex, particularly the soil pest complex, because of constant selection pressure from these pests.

Weeds are alternative hosts for plant-parasitic nematodes and have long been recognized for their ability to maintain nematode populations targeted for suppression by various management strategies. The impact of weeds as alternative hosts depends largely on nematode feeding behavior, which is determined by the level of host specialization required for the parasite to feed successfully. In general, the more specialized feeding adaptations are associated with greater crop damage, more diverse nematode management options, and greater negative impact from weeds. Besides serving as alternative hosts, certain weeds can protect nematodes from pesticides and the environment, provide nematode suppression through antagonism, contribute to changes in future nematode biotic potential, or exert indirect effects through competition with crops or by the effects of weed control strategies on nematode populations. Shrinking nematicide options and increasing environmental concerns are making integrated pest management (IPM) a necessity for nematode management in many crops. A prominent similarity between most major weeds and plant-parasitic nematodes is that both are place-bound organisms that are passively dispersed. Weed–nematode interactions in agricultural production systems may be more intricate and complex than the simple function of weeds as alternative hosts. Their relationship may represent a normal adaptation resulting from the limited mobility of both groups of organisms and the obligate parasitism of phytophagous nematodes. The challenge that faces weed scientists and nematologists is to identify effective, compatible IPM strategies that address weed and nematode management collectively.

Plant model systems have contributed greatly to the dramatic progress in understanding the fundamental aspects of plant biology. Using model weeds will also help facilitate focused funding and research in the weed science community. Criteria for developing model weeds require attention to weedy characteristics that impart economic losses and a wide geographic distribution, attributes that present the potential for political and scientific support. Expressed sequence tag (EST) databases for model weeds are the most practical approach to identifying new genes and obtaining data on the gene expression underlying weedy characteristics. Weeds such as Canada thistle, eastern black nightshade, johnsongrass, jointed goatgrass, leafy spurge, waterhemp, and weedy rice are proposed as model systems.