As Climate Shifts, So Do Pests: a National Forum and Assessment

The climate is changing and the evidence is all around. Springs are arriving earlier, summers are longer and hotter, winter temperatures are warmer, and plant hardiness zones have shifted northward. Since 1895, temperature in the United States has increased by an average of 0.72–1.05°C, with most warming occurring in the last 45 years. The decade 2001–2010 was the hottest on record, and the trajectory is set for the average global temperature to continue to warm into the future from 1.8° to 4.0°C by 2100. This anticipated increase in temperature, along with changes in precipitation patterns and extreme events (e.g., heat waves, droughts), will have profound implications for pests (e.g., arthropod, disease, weed) that inhabit agriculture and forestry systems. To protect these systems and their benefits for the environment, economies, and society and culture, we need to understand how climate changes will affect pest distribution, ecology, and virulence, which are already taking a toll (Fig. 1). Only then can resource managers better prepare and adapt their practices. With funding from the USDA National Institute of Food and Agriculture, the National Forum on Climate and Pests (NFCP) was convened at the National Academies in Washington DC on October 4–6, 2016. The NFCP brought together 26 scientists from the climate and pest science disciplines in front of a live Internet audience to provide the latest information on how climate change affects many aspects of pest biology and to discuss ways to create more resilient and productive agriculture and forestry ecosystems. The long-term goal of the NFCP was to bring attention to and strategize ways to develop resilient agricultural and forest ecosystems in the face of climate-induced changes in pest distribution and severity. This goal aligns with the USDA Agriculture and Food Research Initiative's (AFRI) Agriculture and Natural Resources Science for Climate Variability and Change Challenge Area, which is focused on reducing the greenhouse gas footprint of U.S. agriculture and forestry as a whole, while maintaining resilient and productive ecosystems. (Photo credit: J. Hicke) The NFCP was organized by individuals representing various private and public organizations, including the American Society of Agronomy, an international organization with a membership base that includes researchers, extension personnel, non-governmental organizations, policy makers, and professional crop consultants who cover all sectors of agriculture and natural resources management; the Board on Agriculture and Natural Resources of the National Academies of Sciences, Engineering, and Medicine, which conducts studies and organizes conferences to address national science-related issues; the Cornell Institute for Climate Smart Solutions, which serves as a focal point to facilitate research, education, and outreach to help farmers become more resilient to extreme weather and climate variability and reduce their impact on climate change; the four Regional Integrated Pest Management (IPM) Centers, which fund and support programs focused on the adoption and development of IPM and include climate and the related impacts on pests; and the USDA's Northeast Region Climate Hub, one of seven regional centers in the United States that provides information and resources on climate change to stakeholders and, collectively, are a source for creating policy at the national level. These organizations developed the NFCP to gather a select group of scientists and thought-leaders in Washington DC along with an online audience to foster a scientific exchange of ideas and ways to plan for adaptable and resilient agriculture and forestry ecosystems threatened by pests due to climate change. The NFCP had the following objectives: (1) learn about critical research being conducted on the effects of climate on pests, microbes, and host–pest interactions; (2) understand the strategic consequences of potential shifts in agriculture and forestry-related land use due to climate-related pest outbreaks or reductions; and (3) develop regional frameworks to support pest management decisions that will lead to more sustainable U.S. agriculture and forestry ecosystems in changing climates. Two days were spent listening to speakers presenting in the room and having discussions that included the online audience and were insightful and often intense. Keith Dixon, U.S. National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Lab, started out with an introduction to climate models and their application for various purposes, one of which is pests. He pointed out that models, in general, have been used for detection, projection, and determining impacts. Models are used to study attribution in order to help explain the causes of behaviors and events, but Dixon showed how this can be a sticking point. The question is what will people do with models and how will the removal of factors cause different results? Scaling is one of the major issues with models along with extremes, which traditionally have been dealt with as outliers. Dixon ended by highlighting the challenges that he sees in modeling pests, which are (1) merging the focus between pure research and widespread application, (2) the strengths and limitations of downscaling, and (3) converting data into knowledge and the need for quality control. Nathan Mueller, Harvard University, spoke about model applications to agriculture, which connected to points made by Dixon. The “local” climate is regulated by ecosystems depending on the composition and amount of vegetation. Mueller suggested that the conversion of perennial grasslands to maize and soybeans in the Midwest is having a cooling effect and lowering summer extremes; thus, the expansion of crops may be having a positive effect on climate. However, he was quick to qualify that expansion alone is less important than intensification and cropland type (e.g., tropical, annual, temperate, high lands) and that the effects are confined to location. The recent increase in the adoption of cover crops is affecting the climate models that Mueller is working on. During the beginning of the talk by Matt Ayres, Dartmouth College, he noted that there is nothing more practical than good theory. This is clearly true with respect to understanding and predicting the effects of climate change on forest pests. Ayres further drilled down on the points from Mueller and Dixon by focusing on southern pine beetle, which has recently expanded its range from the southeast into the Pinelands of New Jersey and Long Island, NY. This has been permitted by warming of the coldest nights of winter. The expansion of this highly aggressive tree-killing insect is a challenge for forest management in the newly occupied range partly because land owners and forest managers have no previous experience with this insect. Suppression of the current epidemic is difficult because of positive feedbacks in the population dynamics of the beetle: more beetles lead to greater success in attacking and killing trees, which leads to high reproductive success, which leads to even more beetles. At the same time, as southern pine beetles have become a new pest in the northeastern United States, they have become less abundant and less of a pest in their traditional range in the southeast. Evidence indicates that decline in beetle damage in the south is not due to temperatures getting too warm for the beetles but rather because of effective management, which includes monitoring of abundance, active suppression when beetle abundance begins to rise, and reduction of risk by thinning overstocked pine stands. Ken Linthicum, U.S. Department of Agriculture, Agricultural Research Service Center for Medical, Agricultural and Veterinary Entomology, focused on mosquitoes and their interaction with humans and their domestic animals through viral disease transmission. In Africa, Rift Valley fever (RVF) occurs in humans and their domestic animals who are bitten by virus-infected mosquitoes that emerge from eggs in soil after exceptionally heavy rainfall events. The unusual rainfall events can be forecast using specific global sea surface temperatures, rainfall, and vegetation development. Forecast information dissemination networks that use climate data provide early warning to at risk areas. In 2015–2016, predictions for RVF outbreaks were highly accurate and the early warning drove massive vaccine, mosquito control, and public education campaigns in various countries in the Horn of Africa and for the first time stopped a regional outbreak of this devastating disease. The warning and disease control activities likely prevented thousands of human deaths and the loss of millions of domestic cattle, sheep, goats, and camels. This disease prevention also sharply reduced starvation and the occurrence of significant negative national economic and security incidents in impacted countries and regions. Other mosquito-transmitted diseases can be forecast and an example is shown in Fig. 2, which depicts projected vector-borne diseases (including mosquito-transmitted RVF, dengue, and chikungunya) in 2014–2015 (Chretien et al. 2015). The current challenge is to enhance the spatial and temporal accuracy of forecasts and increase lead time to permit disease mitigations in impacted countries and regions of the world to be most effective. Enhanced forecasts will be developed by closely monitoring and analyzing both temperature and rainfall departures from the norm, the most critical climate parameters impacting mosquito-transmitted disease outbreaks. Chonggang Xu, Los Alamos National Laboratory, talked about another important pest, mountain pine beetle, and the interaction with pine forests in the western U.S. The use of earth system models for measuring carbon fluxes from vegetation does not include tree mortality due to insect populations. Xu showed that as mountain pine beetle moves north and potentially east across Canada with warming climate, earth system models will need to be more robust to include insect population dynamics. A number of solutions were suggested, including demographic vegetation models, stochastic representation, and benchmarking data. Another solution is forest management practices, where small trees are thinned to prevent fire and large trees are removed to keep mountain pine beetle populations low. Could ecosystem demography models also be part of the answer? Jim Stack, Kansas State University, provided an assessment of climate and pests along with implications for policy. It was one of the most comprehensive presentations and a very compelling argument for action. The growth and development of plants, including crops, is occurring differently because of climate change. The changes are causing disruptions to the way food is produced and how it is distributed, yet the problem has bigger implications than a few fields going fallow; food security is equal to national security. Stack suggested that tipping points are on the horizon if developed nations continue on the same course. There are many unknowns about the trajectory of nations and historically, some have become obsolete due to lack of proactive steps to avoid disaster. In Illinois, trees are replaced with pine, ash, and others that are susceptible to the next invasive pest, which begs the question of how long will it take for us to learn from the past to better prepare for the future? The current solutions are to increase intensification (high yielding, genetically uniform) and efficiency of fertilizers, pest management, and irrigation, yet are these sustainable? Both global trade, which has eliminated geographic barriers to pests, and an increasing standard of living (and rising energy and food demands) are causing the environmental and social systems of the earth to be strained, thus two major near-term tipping points that may be passed without any action. Ben Cook, NASA Goddard Institute for Space Studies/Lamont-Doherty Earth Observatory, Columbia University, focused on drought, a phenomenon that affects meteorological and hydrological cycles, to which many plants (both agricultural and natural) are susceptible. Importantly, drought is not just precipitation; as temperatures increase, transpiration and evaporation also increase, while snow and soil moisture decrease, all processes that can have significant ecological and socioeconomic impacts. Under most current warming scenarios, models indicate a trend toward increased drought risk and severity for much of the world, including many agriculturally important regions. This means inefficiencies and the sustainability of our management systems must be addressed. “Whew. I am glad I don't live there”, is the response most people give after looking at a world map showing locations of projected increases in drought with warming. Cook wondered if this is the right response. In sports, the champions crave the times when the game is on the line. How much do we need that mindset now in order to develop solutions to increase resilience when faced with extreme drought? Lew Ziska, U.S. Department of Agriculture, Agricultural Research Service, Beltsville Agricultural Research Center, addressed the topic of climate change and weeds, which are one of the biggest pests in crop production. Rising CO2 affects all plant growth, including weeds. Scientists have thought that C4 weeds would do poorly with C3 crops because of the latter having a more efficient carbon assimilation pathway. Whether C4 or C3, weeds and crops compete for resources. Lists of the “worst” weeds are mostly based on peoples' perspectives, but with climate change the ideal situation for weeds to be successful competitors are those wild relatives of the crop. Rice has many great examples. In an experiment conducted by Ziska, he found that wild oat seed from samples collected in 1965 were inferior competitors compared to 2013 wild oat seed. Weeds are adapting to climate change, but crops are not because they are bred for economic success with a very narrow genome. Weeds are just the opposite, which explains their success. Breeding corn is too slow to keep pace with changes in climate and competition from weeds. Ziska ended by stating that the more we try and control nature, the less we can and instead we should adapt and mimic nature, which in most cases means diversify. Jeff Dukes, Purdue University, also focused on invasive weeds, but diverged from Ziska by emphasizing the invasion pathway and systems and predictive modeling. Well known is that for invasive species to be successful in establishing, they must first be transported to their non-native range, survive abiotic and biotic “pinch points,” increase in population, and finally spread across the landscape. Climate change is affecting the process by which invasives become established with some arising and others disappearing, but in general, non-natives respond to climate change better than natives. Less diverse systems have lower biotic resistance to invasion; thus, increasing climate variation increases successful establishment of non-natives. Dukes suggested that current niche models could be improved by empirical studies. Future invasion threats will continue to occur at seaports, airports, and animal and plant import sites and as biome shifts and wild fires increase. Most countries are ill prepared to deal with invasive species whether in a proactive or reactive capacity. Jeff Hicke, University of Idaho, used a case study on pine trees and mountain pine beetle (MPB) to frame the climate and pest interaction in the western United States. Mountain pine beetles have killed trees in approximately 7% of forested areas, and the factors of host tree, density, stand age, drought stress, MPB populations, and temperatures all played roles (Fig. 3). Hicke and colleagues have looked specifically at whitebark pine mortality using a combination of empirical studies and modeling to estimate potential outbreaks (Fig. 4). Future conditions are expected to be continually favorable for whitebark pine as the climate warms. However, when comparing fire and MPB, forests can recover faster from the latter. (Photo credit: J. Hicke) (Photo credit: J. Hicke) Karen Garrett, University of Florida, brought up the social aspect of research and began by stating that a healthy plant and healthy soil is known, but less is known about what a healthy group of scientists might look like. In addressing simple questions, such as what are the causes of yield losses, to more complex ones, such as what do we need to know about climate change, there is a need for groups of researchers to work together. For climate change, the effects on plant disease are not well known, and information for within field management could include a variety of tools, such as host resistance and cultural and biological controls. Garrett gave the example of potato mixtures being used to overcome climate change. How do farmers make decisions? An impact network analysis can help in understanding the complexities of farmer decisions, and how much information is needed before the knowledge gained results in a change in behavior. Steve Frank, North Carolina State University, the last speaker, talked on a topic not extensively addressed: urbanization and pest outbreaks. 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