Changing pest dynamics as a result of changing atmospheric conditions are of ecological and economic importance . While little is known about the direct effects of changing precipitation patterns upon invertebrates, it is known that increased rainfall can increase insect mortality . Information on direct effects of elevated atmospheric concentrations of CO2 on insects is limited , as are studies of the consequences of changing UVB levels on insect herbivores and other invertebrates. Existing studies suggest that direct effects of temperature are likely to be larger and more important than any other factor associated with climate change . Given the predicted increase in temperatures in California in the coming century, this is a key area upon which attention should be focused. Invertebrates require a certain number of degree days to develop from one point in their life cycle to another. The survival, range and abundance of many invertebrate pest species is mediated by temperature. Furthermore, temperature is the dominant abiotic factor that directly affects herbivory . Consequently, the diversity and intensity of insect herbivores increases with rising temperatures and constant latitude . In Multivoltine species , such as the Aphididae and some Lepidoptera, development time is expected to increase with climatic warming, allowing for increased generations within a year . A 2o C temperature rise, which is at the lower end of temperature increases predicted for California in the coming century , may result in 1-5 additional generations/ yr for a range of invertebrates such as insects, mites & nematodes . It is also likely that many pest species will expand their geographical range in a warmer climate, seen already in Britain in several butterfly species . The effect of higher temperatures on overall abundance of herbivorous insects remains unknown in the absence of equivalent data of their natural enemies . While warming speeds up the life cycles of many insects,growing tomatoes hydroponically suggesting that insect pest problems could increase , herbivorous insects may grow more slowly, as they feed on the typically protein poor leaves produced under conditions of elevated atmospheric concentrations of CO2 .
The increase in C:N ratio in plant tissue may cause insects to eat more herbaceous material, thereby causing more damage or change their feeding preferences to satisfy their dietary N requirements, slowing larval development and increasing mortality . Climate change may impacts host species in ways that make them more vulnerable to pests , for example, pine bark beetles would find pine trees easier attack . Adaptation to changing climate would be more rapid for insects than host plants, due to generation time , and the spread of insect pests may be accelerated if host ranges change rapidly due to environmental change or to socioeconomic incentives . . For example, the temporal synchrony of larval emergence of the Winter moth, Operophtera brumata, and bud burst of its host plant, sitka spruce Picea sitchensis are important. A temperature increase of 2o C is not expected to dramatically impact bud burst date; however, larval emergence is likely to advance dangerously ahead of bud burst . However, temperature does not act in isolation to influence pest status. Some insects are unable to cope in extreme drought, while others are disadvantaged by extreme wetness. However, the present forecasts of California’s future precipitation patterns are uncertain, making predictions of this nature difficult. Taken together, these examples highlight the complex climatic and trophic interactions that California agriculture will need to begin to consider in a changing climate .The global pesticide market was valued at $29 billion in 2000, with herbicides, insecticides and fungicides representing 48%, 27% and 19% of expenditure respectively . In addition to the high costs of chemical control, there are growing environmental and health concerns about the use of pesticides and their regulation , and applications must be timed precisely to maximize efficiency and minimize undesired impacts. Under increased temperature scenarios, the number of days that will be suitable for spraying is likely to increase where it is drier and decrease where it is wetter; however, as a result of increased pesticide application, invertebrate pests may build resistance to the chemical or its active ingredient .
Furthermore, the toxicity and/or stability/volatility of the chemical are likely to change under different climatic conditions . An important consequence of chemical spraying is that natural enemies present in the ecosystem are killed, further increasing the need for chemical applications to control pest populations. Health risks to workers and consumers, associated with increased pesticide usage in Californian agriculture, are also of importance. The efficacy of other control methods such as biological control and the use of genetically modified organisms are likely to be impacted by climate change. Factors that impact the abundance and activity of invertebrate pests will similarly impact beneficial invertebrates such as predators, parasitoids, and pollinators. Thus, biological control efforts will need to consider the impacts of climate change on complex pest/natural enemy dynamics. For example, high temperatures tend to decrease the efficacy of the entomopathogenic fungus Beauvaria bassiana in controlling wax moth in soil treated with certain pesticides . In Australia, the effectiveness of Ingard cotton which has been genetically modified to produce a Bt toxin precursor, appears to be greater at a given node when that node is produced at a higher temperature . This adds an additional layer of complexity that needs to be considered as GM crops are grown in some instances to not only reduce pest pressure but to also decrease insect vectored plant pathogens . Taken together, these examples highlight the need for multi-trophic studies of pest, biological control agent and host plant dynamics in a changing climate.Invertebrates not only cause direct damage to crops, but can also act as vectors of disease causing organisms. Environmental conditions play a significant role in vector borne diseases, and the impact of climate change has the potential to shift geographical ranges . Some examples of vectored diseases include Curly Top virus, which affects several hundred varieties of ornamental and commercial crops in California and is vectored by the Sugar Beet leaf hopper, Tomato Spotted Wilt Virus, vectored by Western Flower Thrips and Pierces Disease vectored by the Glassy Winged Sharpshooter. These will be considered in more detail in the following section.
The risk of agricultural yield losses due to disease, weeds and insects, is likely to increase with climate change, but is rarely considered in climate assessments . Disease onset requires a susceptible host,hydroponic growing supplies a virulent or infective pathogen, and a favorable environment. Disease-causing microbes are dependent on temperature and moisture optima for establishment and reproduction, with most diseases occurring in warm and wet conditions . Pathogenicity, or the degree to which the host is harmed by its parasite, depends on this three-part interplay. Disease often occurs outside of the temperature optima of the pathogen and the host, and often results from the host organism being more susceptible than the pathogen to being outside of these optima . Climate change in California, especially in the context on increased temperatures, and its impact upon plant disease development is likely to be of great consequence to California agriculture.An increase in average temperatures of just a few degrees can hypothetically lengthen the growing season as well as the growth rate of a pathogen dramatically . While increased CO2 may increase plant growth, it may also increase pathogen fecundity, thereby negating or reversing positive effects on plant growth, should conditions conducive to disease development, such as increased temperatures, manifest . Similarly, increased O3 and UV-B levels, while harmful to plant tissues, may also harm obligate host pathogens, decreasing plant disease . The global impacts of pathogen outbreaks in agriculture have been profound . One example is the Irish potato famine in the 1840’s, caused by potato late blight . Since the 1960’s millions of livestock and poultry have been destroyed in response to combined outbreaks of Influenza A Virus, Foot and Mouth disease, and Mad Cow Disease alone , with anomalous climate patterns often flagged as alleged triggers to such natural economic disasters . The introduction of new agricultural pathogens through species range shifts will undoubtedly be a major effect of changing climates . Climate-driven pathogen range extensions in terms of both latitude and elevation have been widely reported in mosquito-borne human diseases such as malaria and dengue and yellow fevers ; however, debate exists on whether such range expansions are better attributed to anthropogenic causes . Similar climate-range interactions have been anticipated in aphids by influencing winter survival and spring flight timing .
Evolutionary responses of pathogens are an additional source of uncertainty in changing agricultural systems. It is well known that microbial agents can quickly evolve resistance to antibiotics and herbicides, often within time scales less than a decade . However, adaptation potentials are not unlimited and interactions between pathogen evolution and their environment, having been rarely studied. For example, increased atmospheric CO2 concentrations have been shown to increase fungal disease severity in crop plants in short-term experiments , while in a long-term experiment in the same system, Chakraborty and Datta showed a decreased ability of the fungal pathogen to evolve aggressiveness in elevated CO2 environments, purportedly due to enhanced host resistance. Furthermore, climate change will enable plant pathogens to survive outside their historical geographic range; consequently, climate change may lead to an increases in the significance of pre-existing pathogens as disease agents, or provide the climatic conditions required for introduced pathogens to emerge .In the multi-billion dollar grape industry of California , Pierce’s Disease has caused Riverside County alone $13 million in damage as of 2002, and the state has aided the industry with more than $65 million in control efforts since 1998 Pierce’s Disease is a prominent bacterial disease of California grapes that is caused by Xyllela fastidiosa and vectored by the Glassy-Winged Sharpshooter, a native to the southeastern U.S. that is more mobile than existing leaf hoppers, is limited to climates with mild winters such as southern California . The optimum temperature for growth of the Pierce’s bacterial pathogen is 28°C . Consequently, northern and coastal California grape-growing regions are currently suboptimally cool for Pierce’s Disease. However, under climate change, these regions may face increased risk of establishment of Pierce’s disease. The threat of the glassy winged sharpshooter is not limited to grapevines; its host range includes more than 100 species of plants, including almonds, citrus, peaches, plums, alfalfa and ornamental plants produced by the state’s commercial nursery industry, and therefore has the potential to disrupt the state’s agricultural economy, especially if it will increase under future climate scenarios. In 2004 West Nile virus was reported in horses in more than half of California counties, resulting in a 42% mortality rate of infected animals . Assuming that warming climates lower developmental thresholds for mosquito vectors , WNV incidence could potentially increase in California in areas historically less prone to mosquito outbreaks. Similarly, changes in amounts and timing of precipitation, snow melt and stream flow dynamics , may lead to an increase in the abundance of mosquitoes in California, and hence, WNV. Disease forecasting models are essential in order to be able to quickly respond to high risk trends. In California several crop disease models have been developed and are in use. Downy mildew in lettuce is an example of a disease whose incidence can be predicted by a very simple model; morning leaf wetness after 10 am, influenced by low midday temperature and high relative humidity, directly affect disease incidence . In this system, warming alone may actually reduce disease risk for this pathogen in certain areas; however, with future precipitation patterns uncertain at best, there is need for further information. Interactive risk assessment and forecast models are currently available through the University of California Integrated Pest Management Program for powdery mildew on grapes and tomatoes . The fungal mildews in these systems, as well as others, such as the devastating late blight in potato and tomato , are tightly linked to temperature and precipitation, with severe disease outbreaks occurring in relatively wet winters with mild temperatures such as in El Niño years . Esca, a fungal disease in California table and wine grapes, appears to respond to above-normal rainfall and summer temperatures .