Climate change poses a threat to the control of pest and disease invasions. These "pests and diseases" include insects, plant diseases, and invasive weeds. As climate variables continue to change in the Central Valley, new pests and diseases may become able to invade previously uninhabitable areas like Yolo County. Climate factors that aid in pest and disease invasions are mostly temperature related and include increasing average temperatures, warmer winter minimum temperatures, changes in precipitation patterns, and water shortages.
Where do pests and diseases come from?
Some crop diseases and pests are native to California, while others originate from other regions of the continent or world. In their native regions, potential pest species are kept under control by predators and other ecosystem processes. However, once introduced to a different region such as the California Central Valley, these pests and diseases may no longer have natural predators or other environmental variables to control their population size. Climate change is aiding these invasions by widening the "invasion niche," or the set of environmental conditions under which pests can successfully invade.
What are some examples of pest and disease invasion threats due to climate change?
Increasing temperatures contribute to a poleward migration or expansion of the ranges of many organisms . In the case of California, this means ranges will move northward. Average annual temperatures have been modeled to increase by up to 4.5 degrees Celsius 2100 . Additionally, winter minimum temperatures may increase by up to 2.4 degrees Celsius in that same window . This will likely result in an increased amount of new pest and disease species, as the ranges of many pests and diseases have historically been limited by cold winter minimums. This general hypothesis has been used to study specific pest invasions in further detail.
Milder winters have been shown to increase the survival of many frost-sensitive insects . Increasing temperatures also allow for higher rates of growth and reproduction in insect herbivores . Studies on aphids and moths have shown that increasing temperatures can allow insects to reach their minimum flight temperature sooner, aiding in increased dispersal capabilities . Multiple studies have shown the northward expansion or shift of insect ranges, such as Edith's checkerspot butterfly or the mountain pine beetle, to be correlated with increasing temperatures .
While not nearly comprehensive, the factors discussed above provide a sense of the types of responses we can expect from insect pests in light of climate change. Positive physiological responses to increasing temperatures will allow for faster insect growth and movement. Additionally, milder winters will allow for earlier insect growth and a reduction in overwinter deaths. The expansion or shift in ranges coupled with an increase in growth and numbers will likely result in an increase in insect invasions.
Insect pests already present in Yolo County may also benefit from many of the same factors. Historically, cold winter temperatures have helped to keep pest and disease life cycles at a minimum and otherwise delay the growth and dispersal of pest organisms. Just as we expect an increase in growing degree days and a reduction in chilling hours for crops in Yolo County, we can expect the same for insect pests. Therefore, the challenges presented by currently established pests in Yolo County will become greater, on top of those challenges presented by invasion threats.
Crop diseases can be animal, fungal, bacterial, or viral in origin. Crop diseases are often spread through an insect vector (MPB). Fungal diseases are also common, and can spread via spores carried by wind. Dispersal plays a key role in the spread of crop disease. Anthropogenic activities have been found to contribute to the spread of sudden oak death (phytopthera) . An increase in severe weather events such as hurricanes may also catalyze the spread of crop diseases such as soybean rust across continents or oceans . Climate change can also aid in the dispersal of plant and crop disease. A local increase in summer precipitation due to climate change has been found to be responsible for the increase and spread of needle blight in British Columbia . An increased fungal pathogen load in grassland communities was found in response to climate change events such as increased CO2, decreased plant diversity, and increased nitrogen deposition . While these studies are based on non-crop plants, we can expect similar responses from crop pathogens as a result of climate change factors.
A local example of new disease in Yolo County due to climate change is the alfalfa stem nematode. The stem nematode parasitizes alfalfa crops and can cause severe crop losses. The nematode disperses through several vectors, including waterways and irrigation runoff, contaminated farm equipment, and other anthropogenic means similar to other plant diseases. As a new and increasing threat in Yolo County, stem nematode populations are thought to have appeared due to warmer minimum temperatures moving closer to the reproductive threshold for the nematode.
Climate change is expected to increase the range, or "damage niche" (also called "invasion niche") of many weed species. Research suggests that the composition of invasive weed communities will be fundamentally altered by the end of the century under increasing temperature scenarios, with new weed species entering communities as a result of geographic range shifts . For example, the range of the yellow star thistle, a California weed, is expected to increase to more northern parts of California and Nevada due to climate change .
The effects of climate change on weed/plant interactions are likely to vary by region and crop type. Understanding of the underlying physiological mechanisms responses to such factors is needed in order to address these effects. Because the interactions between crops and weeds are "balanced" by various environmental factors, local changes in these factors may tip the scale towards either crop or weed. Furthermore, as the geographic distribution of weed species changes, so will the community composition, posing both challenges and opportunities for invasion control. If the invasion of new weed species can be detected, efforts can be made in advance to prevent and control their establishment.
Figure 1. Climate change is likely to expand invasion risk of Centaurea solstitialis, creating minimal retreat potential by 2100. (a) C. solstitialis dominated lands in the western United States and climatically suitable habitat based on Mahalanobis distance. (b) Change in future invasion risk based on the number of atmosphere-ocean general circulation models (AOGCMs) that project maintained climatic suitability. Colors represent risk of invasion based on the number of AOGCMs that project climatic suitability; black lines denote regions of expanded risk. (c) Retreat potential of currently invaded lands. Note that most areas currently suitable for C. solstitialis maintain their climatic suitability in five or more of the 10 AOGCMs tested .
Figure 2. Historical and projected distribution of the damage niche for S. halepense in U.S. maize cropping systems. Projections are for climatology centered on 2030 and 2084 under a "business-as-usual" GHG emission scenario. Towards the end of the century, the damage niche for S. halepense may experience a pole-ward advance of approximately 200-600 km north of present-day boundaries .
What solutions exist to address climate change and pests?
Integrated pest management lies at the center of insect, disease, and weed control. The combination of farming strategies, biological control agents, and necessary pesticide and herbicide use has helped California farmers address pest problems using a variety of methods. Climate change may make IPM solutions less effective and harder to implement. Additionally, the spread of pests and disease through human vectors will continue to become a problem, especially as they become more tolerant to environmental conditions.
An increasing resistance to pesticides among invasive insects is a problem to farmers and the effectiveness of current IPM and other pest management strategies. For example, Liu (2007) showed that invasive populations of the potato psyllid have a higher resistance to certain pesticides than the native populations . Plant growth responses to increasing CO2 will aid in herbicide resistance by increasing leaf thickness and decreasing stomatal openings and soil nutrient uptake, which could make plants less likely to absorb applied herbicides.
As a result, IPM solutions will have to adapt to these problems. Biological control agents can be effective against insect or weed populations that are resistant to pesticides or herbicides. The establishment of hedgerows containing native vegetation can aid in the establishment of beneficial insect and plant communities near crops to aid in biological control. Farming practices such as cleaning equipment between crop use can help to minimize the spread of diseases or pests, and planting strategies such as changing planting times can counter weed growth.