Plant Diseases and Pests Are Oft-Ignored Climate-Linked National Security Risks

When climate change is discussed as a national security issue, experts often cite food insecurity brought on by extreme weather events as an important contributor to the risk of political instability and other adverse outcomes. Indeed, the U.S. intelligence community’s 2021 Annual Threat Assessment, released last week, cites extreme weather, along with conflict and the Covid-19 pandemic, as key drivers of the current high levels of global food insecurity. Crop-damaging extreme weather events were a hallmark of 2020, with historic numbers of tropical storms, droughts, floods, and derechos across several continents.

Generally missing from such analyses, however, is discussion, or even a mention, of crop damages arising from agricultural pests or plant diseases, many of which are strongly linked to changing climate variables. This omission is even more surprising given that 2019–2020 witnessed the emergence of a ferocious transnational desert locust plague that devastated crops in East Africa and parts of the Middle East and Asia. Many scientists point to climate change as a major contributor to the severity of the locust outbreak, which caused billions of dollars of crop damage across dozens of nations. Swarms of tens of billions of locusts were common, with one swarm in northern Kenya reportedly almost 1,000 square miles, an area more than 14 times larger than the District of Columbia.

In the U.S. intelligence community, analyses of the security implications of climate change rarely stray from a predominantly meteorological framework. For example, analysts typically consider the effects of climate change operating through stressors such as heat, storms, droughts, sea-level rise, and Arctic melting—all examples of what ecologists call abiotic, or nonliving, systems—while largely ignoring security outcomes arising through stresses to biotic, or living, systems. Since increasing temperatures affect a vast number of physiological and behavioral dimensions across all kingdoms of life, from animals and plants to algae, bacteria, and fungi, it is reasonable to assume that a subset of biotic stresses will pose risks not only to food security but to ecological stability writ large. 

This subset is rather consequential; the security implications of climate change cannot be fully understood without robust consideration of these biotic stressors. In addition to triggering substantial economic losses, crop diseases and agricultural pests can decimate livelihoods, disrupt social order, and undermine health. In some cases, political instability or heightened migration is likely to occur. Of course, non-climate ecological stressors, such as habitat change, invasive species, and overexploitation of organisms are likely to compound these effects.

With respect to food insecurity, the most important climate-linked biotic stressors to agriculture are almost certainly pests and diseases (although stress to pollinators from climate change is also a significant and complex concern). According to a report released last month by the Food and Agricultural Organization of the United Nations, biological disasters from pests, diseases, and infestations accounted for about 9 percent of all crop and livestock production losses from 2008–2018, a period that did not even include the recent desert locust crisis. Such losses were greater than those incurred from extreme temperatures and wildfires. These food stresses also exclude the complex but likely deleterious effects of warming and acidifying waters on diseases in fish, especially in aquaculture, an industry responsible for 50 percent of global fish consumption by people.

In addition to triggering substantial economic losses, crop diseases and agricultural pests can decimate livelihoods, disrupt social order, and undermine health. In some cases, political instability or heightened migration is likely to occur.

Agricultural Pests in a Warming Climate

Spatial movements in geographical distributions of wild animals and plants, including pests, are among the clearest lines of evidence of anthropogenic global warming. For terrestrial systems, there is a general tendency of species moving poleward in latitude and upward in altitude. For example, the northern expansion of tree-munching pine and spruce beetles in North America, Europe, and Russia has caused enormous forest damage over the past two decades.

A 2013 scientific article demonstrated a measurable latitudinal shift north since 1960 in many species of Northern Hemisphere pests, which include insects, mites, and ticks. However, such studies are complicated by the influence of other anthropogenic stresses, such as land-use changes, habitat fragmentation, and the dispersal effects of transportation. It is likely that at least some agricultural pests will enter regions with few to no natural predators, which would augment their destructive potential.

Temperature increases also bring agriculturally relevant changes in pest metabolism and population sizes. Insects and other arthropods are ectotherms—animals that are dependent on external sources of body heat—whose metabolic rates increase exponentially with temperature. This means that food intake must grow accordingly, resulting in hungrier pests that increase crop damage per individual. At the same time, population growth rates are temperature dependent, and most regions (except those in the tropics) are expected to see increases in insect population sizes as a result of warming.

In a 2018 journal article, scientists examined the projected crop losses arising from changes in insect metabolism and population size due to increased temperatures, variables that are rarely considered in models assessing effects of climate change on agriculture. According to this study, these two factors alone associated with a projected 2°C of warming would produce estimated losses of 46, 19, and 31 percent per year in the major grain crops—wheat, rice, and maize, respectively. These projected losses are significantly higher than the already sizable 5 to 20 percent loss of major grain crops to insects observed currently.

Warmer temperatures are also expected to increase maturation rates and shorten life cycles of many insects, which could increase the number of generations per season. Some insect species may further benefit from faster onset of maturity by reducing exposure to predators by more rapidly exiting the more vulnerable larval stages.

For example, various species of the aphid—a major agricultural pest that causes direct crop damage through feeding while also transmitting an array of pathogens to plants—exhibit a strong correlation between warming temperatures and increases in development rate. One scientific study estimated that a projected warming of 2°C above pre-industrial temperatures could produce four to five extra generations of aphids and a warming of 3°C could produce seven extra generations.

Fungal Pathogens

Fungi and fungus-like organisms called oomycetes are the most important emerging pathogens affecting plants. Long before our era of climate change arrived, these microorganisms posed threats to crops and social stability worldwide. For example, the oomycete Phytophoria infestans causes the potato disease called potato or late blight, which triggered mass starvation in Ireland and other European countries in the nineteenth century and shaped the trajectory of Western civilization.

Fungal and oomycete infections in major crops are a growing threat to global food security. Epidemic outbreaks include rice blast, wheat rust, maize smut, potato blight, soybean rust, and coffee leaf rust. One study estimates that losses to these pests, if mitigated, would be sufficient to feed almost 600 million people, or about 8 percent of the global population.

As plants and animals shift geographically due to climate change, so do the pathogens they harbor. Although the general trend is poleward in latitude and upward in altitude, the actual speed and direction depend on many variables, such as characteristics specific to the organism, its interactions with other organisms and its environment, and geographic properties. New assemblages of species or populations will be produced over time and this differential biotic mixing will provide many pathogens more opportunities to jump into new hosts. An uptick in emerging infectious diseases across the animal and plant kingdoms is almost certain to follow.

As is the case for most climate hazards, climate change alone is often not the dominant factor in the emergence of fungal pathogens. Human activities, such as globalized trade and intercontinental transport, are widely accepted as the most important drivers of new fungal transmission chains. For example, the importation and cultivation of Japanese chestnut trees infected by Cryphonectria parasitica (chestnut blight) around 1904 led to the virtual elimination of chestnut trees in North America 40 years later.

As plants and animals shift geographically due to climate change, so do the pathogens they harbor . . . New assemblages of species or populations will be produced over time . . . An uptick in emerging infectious diseases across the animal and plant kingdoms is almost certain to follow.

Many fungi and oomycetes are evolutionarily predisposed to take advantage of rapidly changing environmental and ecological conditions, including new patterns of biotic mixing. These organisms can reproduce both sexually and asexually, exhibit flexibility in habitat, and survive independently outside their hosts as environmentally durable spores. The many available avenues of genetic recombination can result in rapid adaptive changes, such as increased virulence or ability to jump hosts. Among pathogens, fungi and oomycetes already infect the widest variety of hosts. For example, Phytophthora ramorum, the oomycete responsible for sudden oak death (SOD), can infect over a hundred different host species.

Climate variables affect several other aspects of fungal behavior. The first appearance of fruits—the visible fungal structures that produce spores—is generally coming much earlier in the year and the last appearance coming later. This extended fruiting period provides more opportunities for adaption to changing conditions. Some studies suggest that temperature and elevated carbon dioxide levels make pathogens more aggressive while increasing the susceptibility of the host plant. An important research question is whether environmental and climate conditions induce more drug resistance in fungal pathogens, already a growing concern worldwide.

Climate and Plant Viruses

Not many scientific studies exist that examine the effects of climate change on plant viruses, although they are a substantial burden on crop yields. Climate change effects on only a handful of the over 700 known plant viruses have been studied, but they may provide some insights of what may transpire.

Barley yellow dwarf (BYD) disease, caused by the barley yellow dwarf virus, is considered the most destructive disease of major cereal crops. Transmitted by aphids as they feed, the virus can infect wheat, maize, oats, barley, and rice. In wheat alone, the disease is estimated to cause between roughly 10 to 30 percent losses in production. Cereal crop losses of over 70 percent from BYD are not uncommon when viral infections occur early in the plant growth cycle.

Scientists generally agree on some of the effects of climate change on BYD. For example, the transmission efficiency of viruses from aphid to plant is expected to increase with warmer temperatures. Elevated carbon dioxide levels, such as those that cause warming through the greenhouse effect, increased the viral load in some cereal crops. Both effects would dampen yields in these crops.

Some effects of climate change on BYD would be telegraphed through changes in aphid populations and behavior. As with agricultural pests, one should expect changes in geographic distribution, metabolism, and reproductive cycles. A recent study indicates that the occurrence and migration periods of some aphids have lengthened from climate warming, which would presumably have a further depressing effect on yields.

In some countries, such as Australia, Italy, and the United Kingdom, management of BYD has been successful enough that growers there consider it only a minor problem. It would not be surprising, however, to see outbreaks of BYD return or intensify if climate change effects were to overpower the heretofore successful management techniques of insecticides and precision planting between viral lifecycles. These problems may be compounded as insecticides with adverse effects on pollinators become more unpopular to deploy.

A Blind Spot for the Security Community?

It would be an understatement to say that analysis of agricultural pests and plant diseases in a national security context is not common in government circles. Indeed, one would be hard pressed to find an analyst in the 17 agencies of the intelligence community, or an officer in the defense enterprise, that understands plant pathology or disease ecology, or their effects on food security. That expertise exists in spades in the federal government, at agencies like the Department of Agriculture, but is typically siloed away from the security community.

Climate change—and its interconnected sister ecological disruption—is poised to bring a pattern of increasing uncertainty, perturbation, and surprise. Experience in both the scientific and national security communities suggests that much of this surprise will be conveyed through ecological agents.

If true, the national security community has much to gain through deeper engagement with ecological experts and principles.

This commentary is made possible by support from the Hewlett Foundation.

Rod Schoonover is a non-resident senior associate with the Global Food Security Program at the Center for Strategic and International Studies in Washington, D.C. He is also founder and CEO of the Ecological Futures Group, which examines the natural security and societal dimensions of global ecological change.

Commentary is produced by the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s).

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Rod Schoonover
Senior Associate (Non-resident), Global Food and Water Security Program