How does climate change affect agricultural pests and disease?
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.
Insects
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.
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How rising temperatures affect insects
Climate change resulting in increased temperature could impact crop pest insect populations in several complex ways. Although some climate change temperature effects might tend to depress insect populations, most researchers seem to agree that warmer temperatures in temperate climates will result in more types and higher populations of insects.
