Robert F. Norris

Interactions between weeds, arthropod pests, and their natural enemies in managed ecosystems

Abstract Interactions between weeds and arthropods occur frequently. This review covers the topic of weed/arthropod interactions, and provides the reader with access to literature in the subject area that is scattered in weed science, entomological, crop production, and ecological journals. We first analyze the current status of weed and arthropod management in the context of multidisciplinary integrated pest management (IPM). The remainder of the review is organized according to the mechanisms driving interactions. The first section deals with interactions driven by trophic relationships, and is subdivided into direct and indirect trophic interactions. Direct trophic interactions occur when pest or beneficial arthropods feed directly on weeds. Indirect trophic interactions occur when arthropod feeding damage to crops impacts weeds through alteration of ecosystem resource availability, or through weeds serving as hosts for alternate prey for beneficial arthropods, or via tritrophic interactions. The second mechanism driving interactions is considered in relation to alteration of the physical habitat by the presence of weeds, such as alteration of temperature within the plant canopy. The third major mechanism driving interactions is based on control tactics for the two types of pests. These are considered from the aspect of direct physical effects, such as tillage, and from the aspect of interactions resulting from the use of pesticides. The latter is divided into direct effects of herbicides and insecticides on non-target pests and beneficials, and on interactions that result from alteration of host plant physiology by pesticides. A conclusion section attempts to place the impact of interactions into an IPM framework, and to indicate where multidisciplinary research involving weed and arthropod management should be focused in the future.

Interactions between weeds and cultivated plants as related to management of plant pathogens

Abstract 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.

Spatial arrangement, density, and competition between barnyardgrass and tomato: I. Crop growth and yield

Abstract Field studies were conducted to determine how the spatial arrangement of weed populations influences interspecific competition. We studied the influence of regular, random, and clumped distributions of barnyardgrass on growth and yield of direct-seeded tomato planted at different densities. Increasing aggregation increased intraspecific competition in barnyardgrass. At the same time, interspecific competition experienced by tomato from barnyardgrass decreased. Differences in the amount of shading of the tomato canopy by barnyardgrass contributed to yield loss differences for the various spatial arrangements. Clumped barnyardgrass caused significantly less average shading than barnyardgrass in regular or random arrangements. At a typical planting density of 10 tomato plants m−1 of row, yield losses ranged from 10 to 35% (1993) or 8 to 50% (1994) when competing with a clumped arrangement of barnyardgrass. At the same tomato density, yields were reduced from 20 to 50% (1993) or 11 to 75% (1994) for the regular and random arrangements for the same barnyardgrass densities. Predicted single-season economic threshold densities for barnyardgrass at a typical tomato planting density of 10 plants m−1 would be one barnyardgrass plant per 25, 19, or 15 m of crop row, respectively, for regular, random, and clumped spatial distributions. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; tomato, Lycopersicon esculentum L. ‘Peelmech’.

Spatial arrangement, density, and competition between barnyardgrass and tomato: II. Barnyardgrass growth and seed production

Abstract Barnyardgrass was grown at densities of 0, 0.25, 0.5, 1, 2, 5, and more than 50 plants m−1 of tomato crop row in either a regular, random, or clumped pattern. Tomato was established at 0, 5, 10, or 20 plants m−1 of crop row in a regular pattern. Crop density and weed density or spatial arrangement had little effect on phenological development of barnyardgrass. In the absence of tomato, barnyardgrass was estimated to produce over 400,000 seeds plant−1 when not subjected to intraspecific competition (0.25 plants m−1 density), decreasing to about 10,000 seeds plant−1 when weed density exceeded 50 plants m−1 of row. Differences in seed production between plants in the regular and random spatial arrangements were minor, but the clumped distribution resulted in 30 to 50% reduction in seed production at weed densities between 1 and 5 plants m−1 of row. Tomato reduced barnyardgrass seed production. The magnitude of the reduction depended on both tomato density and barnyardgrass density. In the absence of tomato, barnyardgrass produced over 200,000 seeds m−2 in 1993 and over 500,000 seeds m−2 in 1994 at 5 plants m−1 of row. Production was almost 700,000 seeds m−2 when the weed density exceeded 50 plants m−1 of row. Barnyardgrass seed production at the single-season economic threshold density in tomato was sufficient to maintain the seedbank at a level that would mandate high levels of weed control in subsequent crops. Because of the high fecundity of barnyardgrass, our experiments suggest that stopping seed production is the best long-term management strategy for the weed. Spatial arrangement of the weed, at the scale used in these studies, would not be a factor in establishing long-term management guidelines based on weed population biology.Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; tomato, Lycopersicon esculentum L. ‘Peelmech’.

Ecological bases of interactions between weeds and organisms in other pest categories

Abstract 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.

Growth analysis and canopy architecture of velvetleaf grown under light conditions representative of irrigated Mediterranean-type agroecosystems

Abstract Velvetleaf growth and canopy architecture were compared under a range of light conditions representative of competitive and noncompetitive environments typical of irrigated Mediterranean-type agroecosystems. Velvetleaf biomass and seed production exceeded those reported in the literature. Plants grown in full light produced 1,370 g dry weight and 44,200 seeds per plant and showed low relative variability. Velvetleaf grown with corn was reduced to 21 g dry weight and 349 seeds per plant, and had high relative variability for biomass and seed numbers. Velvetleaf grown with kidney bean, intraspecific neighbors, or under shadecloth had dry weights and seed numbers that were intermediate to plants grown in full light or with corn. Relative growth rate (RGR), net assimilation rate (NAR), and leaf area ratio (LAR) were assessed utilizing Richards functions, which were fitted to the primary biomass and leaf area data by weighted regression. RGR was highest for all plants early in the season, but declined later. Dynamics of NAR and LAR appeared to be correlated with increased self-shading, shading by neighbors, leaf age, and shedding of lower canopy leaves. Dynamics of specific leaf area corresponded with light availability such that the leaves exposed to full light were thicker than those exposed to shade. The branches of plants in all treatments had random azimuths and the foliage area density was concentrated along the perimeter of the plants canopy. Velvetleaf increased the canopy radius through extensive branching when exposed to full sunlight. Leaf area distribution and branching patterns resulted in leaf area indices of less than 1.0. Leaves maintained a perpendicular angle to the sun throughout the day, but this depended on whether leaves received a consistent directional signal from the sun and not necessarily on whether they received a high-intensity signal. When shaded, the allocation of dry matter went primarily to the stem tissue, which increased the height rather than the girth of the plants. There was a 10- to 20-d delay for allocations to seed in the case of shaded plants relative to those grown in full sunlight. In brief, velvetleaf had a wide range of growth and canopy responses to a variety of light availabilities and it should have little difficulty in becoming fully established in the irrigated agroecosystems of Mediterranean-type regions. Nomenclature: Corn, Zea mays L. ‘NC 4616’; kidney bean, Phaseolus vulgaris L. ‘Sutter Pink’; velvetleaf, Abutilon theophrasti Medikus ABUTH.

The century experiment: the first twenty years of UC Davis' Mediterranean agroecological experiment

The Century Experiment at the Russell Ranch Sustainable Agriculture Facility at the University of California, Davis provides long-term agroecological data from row crop systems in Californias Central Valley starting in 1993. The Century Experiment was initially designed to study the effects of a gradient of water and nitrogen availability on soil properties and crop performance in ten different cropping systems to measure tradeoffs and synergies between agricultural productivity and sustainability. Currently systems include 11 different cropping systems-consisting of four different crops and a cover crop mixture-and one native grass system. This paper describes the long-term core data from the Century Experiment from 1993-2014, including crop yields and biomass, crop elemental contents, aerial-photo-based Normalized Difference Vegetation Index data, soil properties, weather, chemical constituents in irrigation water, winter weed populations, and operational data including fertilizer and pesticide application amounts and dates, planting dates, planting quantity and crop variety, and harvest dates. This data set represents the only known long-term set of data characterizing food production and sustainability in irrigated and rainfed Mediterranean annual cropping systems. There are no copyright restrictions associated with the use of this dataset.