Rationale. Changes in greenhouse gas concentrations and potential changes in climate will have a significant effect on crop systems and their associated pests, since the distribution and proliferation of weeds, pathogens and insects are determined to a large extent by climate. In addition weeds, like crops, are primary producers and will be affected directly by increases in atmospheric carbon dioxide. An understanding of these effects is crucial for maintaining agricultural productivity in the face of global change

What is known. In addition to stimulating growth and increasing water-use efficiency in plants, increasing carbon dioxide will stimulate photosynthesis and growth in most weedy species of plants and will reduce stomatal aperture and increase water-use efficiency in weeds. In addition, increasing carbon dioxide appears to reduce the efficacy of widely used post-emergent herbicides.

Many of the worst weeds in temperate systems originated from tropical regions, and their distribution is limited by low temperature. Depending on the extent of global warming, such weeds could extend their range northward. Increasing carbon dioxide also may increase the tolerance of certain weeds to sub-optimal temperatures, which also could expand their northward range. Partly because of increases in water-use efficiency under elevated carbon dioxide, weeds are expected to become more competitive in drier habitat.

Insect pests in agricultural systems are the second major cause of damage to yield quantity and quality, after weeds. Direct effects of carbon dioxide on plants can either benefit or harm insects, depending on their feeding habits. For example, pollinators could respond positively if more nectar is produced at flowering, with subsequent increases in fruiting. Conversely, higher carbon dioxide decreases the nitrogen content of foliage, which stimulates the feeding of certain insects such as the soybean looper and leaf miners.

Climate affects the range and distribution of insect pests through changes in minimum and maximum temperature, water availability, and other factors. Insects are sensitive to year-to-year differences in climate with large variations in population size and distribution. Consequently, absolute changes in any climate variable, the time rate at which the change occurs, and the frequency of extreme events can fundamentally alter insect ecology. Destabilization of insect habitats will result in increased migration as insects move to new locations to satisfy their ecological requirements. As a result, climatic change is likely to increase the range and distribution of agricultural pest infestations.

Almost no data are available concerning whether carbon dioxide could directly affect diseases or spread of fungi, bacteria, or viruses. Studies have shown that fungal infection increases with increased water content of plant tissues. Consequently, carbon dioxide-induced increases in water-use-efficiency could increase the occurrence of fungal infections. Overall, carbon dioxide enrichment could modify susceptibility to pathogen attack because of carbon dioxide-induced changes in the biochemistry or structure of the host plant.

The growth and rate of increase of crop pathogens will depend on temperature, precipitation, humidity, radiation, wind direction and the occurrence of extreme events. Higher temperatures and precipitation are conducive to the spread of plant diseases since hot, humid conditions are ideal for spore germination and the proliferation of bacteria and fungi.

Gaps. Field-based estimates of production losses due to weeds, insects and diseases with increasing carbon dioxide and/or climate change are lacking. Little information is available on how changes in carbon dioxide and/or temperature could alter weed populations, seed bank dynamics and species diversity of weeds. Although changes in climate and carbon dioxide could alter the efficacy of chemical, mechanical and biological pest control, the nature of these changes is unknown. Basic research on the comparative ecophysiology of crops and pests to carbon dioxide and/or climate change to determine range and distribution of troublesome pests is unavailable. Potential pest migrants should be identified, as well as new combinations of crops and pests. For insects and pathogens, indirect effects of carbon dioxide on the host plant that could alter resistance or tolerance (e.g., changes in the production of secondary compounds, changes in water content, nitrogen status) have not been well characterized. For all pests, multiple interactions between carbon dioxide and other key climatic variables (e.g., high temperature, drought, tropospheric ozone and UV-B radiation) that directly affect productivity are unavailable.

• Quantify the growth, seed production, and viability of troublesome weed species and, in turn, potential crop losses due to weedy competition at the field level with increasing carbon dioxide and/or temperature;
• Provide multi-variate analysis of elevated carbon dioxide and other abiotic stresses (UV-B, tropospheric ozone, temperature, drought) on crop susceptibility to pests;
• Assess secondary effects of global change on host plants (e.g., plant water status, nitrogen concentration, secondary compounds) and determine if such changes alter susceptibility to pest infestation;
• Determine critical temperature thresholds for troublesome insects and pathogens to identify potential migrants and invaders;
• Provide long-term monitoring of pest populations as an early indicator of climate change effects on agricultural systems;
• Quantify potential changes in pest management strategies with multiple interactions of increasing carbon dioxide and abiotic stresses; and
• Utilize biological response data to strengthen the ability of simulation models to predict the distribution of pest infestations and projected changes in economic yield for a given abiotic perturbation.

An interdisciplinary approach among molecular biologists, plant physiologists, entomologists, pathologists and computer modelers will be used to measure, predict, and assess the impacts of carbon dioxide and climate change on the susceptibility of agriculture to weeds, insects, and diseases. Because of the varied and diverse nature of the goals to be addressed, it is recommended that a variety of facilities be utilized in the approach.

• Verifiable, physiologically based criteria will be determined to assess and predict losses in economic yield induced by weeds, arthropod pests, and diseases with climate change.
• Improved crop management systems will be developed that minimize economic and environmental uncertainties while maximizing the positive aspects of global change in agriculture.

A sustainable crop production management system that maximizes productivity while minimizing pest damage under changing climate conditions

Linkage with other ARS National Programs

• Air Quality
• Arthropod Pests of Animals and Humans
• Crop Protection and Quarantine
• Integrated Agricultural Systems
• Plant Diseases
• Plant, Microbial, and Insect Germplasm, Conservation, and Development
• Rangeland, Pasture, and Forages