AGRICULTURE AND DROUGHT OUTLOOK PATRICK FRILET/HEMIS/CORBIS PATRICK

Swarms of grasshoppers and locusts have troubled scientists for hundreds of years, and the problem may be about to get even worse.

CROP PESTS Under attack The threat of to agriculture is set to increase as the planet warms. What action can we take to safeguard our crops?

BY AMY MAXMEN know if it was the heat, water loss or mites that breadth and hard numbers. This information killed the wheat,” he says, “but the crop could is increasingly important because climate ohn Stulp shakes his head as he surveys not survive the combined stress.” models predict that by the end of this century, the fields of his farm in Lamar, Colo- Dry fields infested with pests are the stuff of the hottest seasons on record today will be Jrado, which have been ravaged by the nightmares for farmers, but they also trouble the norm in many areas1 and droughts may worst drought the United States has seen for the minds of scientists. In 1665, the first issue be routine. Farmers urgently need to know over 80 years. He says he’ll be lucky to har- of one of the world’s oldest scientific journals, whether dry, -ridden crops reflect unu- vest a third of the 2,000 hectares of wheat he Philosophical Transactions of the Royal Soci- sual situations or expected outcomes. planted last year, and he has either sold or ety, contained a report on swarms of locusts What’s more, in order to feed the projected killed all but 3 of his 200 cattle because the devouring fields of corn. The anonymous 9 billion people who will inhabit the Earth grass they feed on never greened. “I’ve been author wrote that once the swarm’s eggs by 2050, farmers must increase their cereal through dry weather,” Stulp says, “but noth- hatch, they “produce again such a number of yields by at least 40%. An average of 15% of ing this severe.” locusts that then they do far more mischief crops worldwide are currently lost to insects, This year’s wheat crop was planted in mid- than afore, unless rains do fall, which kill both so controlling pests is crucial to achieving that 2012, but by spring 2013, Stulp’s water-stressed eggs and the insects goal. “When we think about what the major plants were spindly and covered with brown themselves.” NATURE.COM impediments are to increasing crop yield, I wheat mites. These tiny arachnids use their Despite centuries of Find some of the don’t think we spend enough time consider- syringe-like mouthparts to suck the sap from enquiry, the science latest research on ing pests,” says Joshua Tewksbury, director of the stems and leaves, leaving gaping wounds behind drought-driven crop pests here : the WWF’s Luc Hoffmann Institute in Gland, through which more fluids evaporate. “I don’t infestations lacks both go.nature.com/wwfbtc Switzerland. We need to know which insects

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eggs eat the ’s non-vital tissues and

finally kill it while emerging from its body. ICARDA pupae normally cover themselves in feces, and Eigenbrode suspects that this fae- cal shield acts as a barrier to keep the wasp at bay. But producing faeces requires water, and Eigenbrode thinks that in extremely dry conditions, beetle larvae are unable to deploy their shields quickly enough to prevent pre- dation. The US Department of Agriculture has funded Eigenbrode’s project to test this hypothesis — part of a US$20 million grant to address the effects of climate change on US wheat production. A GROWING THREAT Farmers in the tropics lose up to 50% of their crops to pests — including insects and plant pathogens — compared with just 25–30% in Europe and the United States4. Part of the problem is that pests are a year-round problem in the tropics, and farmers are often poorer and rarely have access to safe and effective pes- ticides, robust varieties of plants and adequate irrigation. ICARDA entomologist Mustapha El-Bouhssini looks for pests on wheat crops in Morocco. Recent droughts in tropical areas are caus- ing concern that an already dire situation will be the most damaging under drought mountain pine beetle, for example, killed could worsen. In 2011 and 2012, the Horn of conditions, and where droughts will occur about 750,000 hectares of trees in 2010–11 Africa experienced its worst drought for 60 more often. in the western United States in an infestation years. Crop failures contributed to the deaths thought to be fuelled in part by droughts2. of nearly 260,000 Somalis, and millions of PEST PERFORMANCE Pines normally produce a sticky resin, which people across the region were malnourished. Droughts tend to be hot and insects generally suffocates that burrow beneath the Climatologists expect droughts to become thrive in warmer weather. As the temperature bark, but water-stressed trees are unable to more frequent in other arid regions too. The rises they eat more, mate more and produce secrete enough sap. Beetles finding such trees International Panel on Climate Change pre- more young. Freezing winter temperatures emit chemical signals called pheromones that dicts that over the next 90 years, rainfall in can kill insect eggs laid the previous autumn, attract other beetles. The resulting mass pre- some parts of North Africa and western Asia and can act as a latitudinal barrier to geo- dation weakens the tree further and the beetle will decline by 30% and the average tempera- graphical spread. population multiplies. ture will increase by 4 °C. Although anecdotes such as Stulp’s are However, the relationship between insect The effects of climate change are already powerful, they mask the nuances present in populations and drought is not consistent. being seen. Mustapha El-Bouhssini, an ento- every agricultural ecosystem, including the “Sometimes there are more insects and some- mologist at the International Center for Agri- biology of different herbivorous insects, the times less,” says Claudio Gratton, an entomol- cultural Research in the Dry Areas (ICARDA), natural resilience of certain crops, and the ogist at the University of Wisconsin-Madison. says that some pests once found at low levels community of predators and pathogens that One explanation for this variation, Gratton have recently become more frequent and have kill crop pests. There says, is that although insects can respond extended their ranges. In the unusually hot may be several rea- “Our goal is to positively to dry conditions for a while, even- and dry year of 2010, for example, a previously sons why the brown inform farmers tually they suffer as the plants they feed on rare pest, the barley stem gall midge, besieged wheat mites on Stulp’s deteriorate. For example, aphids can flour- Syrian farms. “Ten thousand farmers aban- farm flourished, for so they can make ish during short droughts as plant nutrients doned their fields because of these pests,” says example. Maybe the decisions based become more concentrated, but these benefits El-Bouhssini. The cereal leaf miner, which was warm winter allowed on what is right cease during prolonged droughts because of once restricted to Syria, can now be found more mite eggs to for them.” a drop in fluid pressure within the phloem of throughout neighbouring Jordan and Iraq, he hatch. Maybe the water-stressed plants3. says. And the hessian fly, a pest of wheat, has dried-out plants left the wheat more vulner- Predators of pests are also affected by crossed the Mediterranean from North Africa able than usual to predation. Or perhaps one drought. Some, such as fungi, require into Spain, Portugal and southern France. “In of the mite’s predators had not survived the humidity to survive and so are less effective my opinion,” El-Bouhssini says, “these migra- drought, leaving its prey free to roam. By fig- during droughts, whereas others fare bet- tions are the effects of climate change.” uring out which factors contribute most to a ter in dry conditions. Sanford Eigenbrode, Jürgen Kroschel, an agroecologist at the crop’s demise, agroecologists hope to identify an entomologist at the University of Idaho International Potato Center in Lima, Peru, strategies to mitigate pest damage. in Moscow, is studying parasitic wasps that says that outbreaks of potato tuber moth Droughts also make plants more nutritious attack the cereal , a pest of wheat are more severe in years that are hotter and to pests, as the lack of water concentrates throughout the northwestern United States. drier than usual. There is less rain to wash the the amino acids. Studies dating back to the The wasp lays its eggs in the beetle’s pupa, moths off the leaves and stems of the potato 1970s suggest that some herbivorous insects a life-cycle stage between the and the plant, and the dry conditions mean the larvae specifically target water-stressed plants. The adult. The wasp larvae that hatch from these can crawl down through cracks in the soil to

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burrow into the tuber. Potatoes grown traditional practices. For example, in the Peruvian mountains, about Scott Merrill, an agroecologist at the 3,800 metres above sea level, have University of Vermont in Burling- escaped these outbreaks because of ton, encourages farmers to change the cold evening temperatures. But as the way they monitor pests. Typi- the planet warms, the moth may move cally, he says, US farmers check for to higher elevations. Kroschel’s team insects on a specific day of the year predicts that over the next 40 years, and decide whether to use pesticides. a further 90,000 hectares of potato A smarter approach, Merrill says, is crops will be at risk of infestation as to use a ‘degree day’ model and check the moth spreads to mountainous their fields when the temperature regions in Bolivia, Ecuador and Peru5. reaches a predetermined level. That As global temperatures rise, pests way, farmers will be prepared for the that are now confined to the trop- early hatching of insects that gener- ics may spread to cooler parts of the ally follows a spell of higher-than- NIGEL CATTLIN/VISUALS UNLIMITED/CORBIS / NATHANIEL FOOTE UNLIMITED/CORBIS / NATHANIEL NIGEL CATTLIN/VISUALS world. Farms at mid-latitudes may normal temperatures. face a doubling of crop loss due to If such strategies fail to reach farms, pests by the end of this century, crop loss will not be the only dire according to a preliminary model consequence. Another danger is that developed by Tewksbury, David Bat- farmers may become so desperate tisti, an atmospheric scientist at the to defeat crop-eating bugs that they University of Washington in Seattle, increase their use of chemical pesti- and Curtis Deutsch, an atmospheric cides. Pesticide companies predict that scientist at the University of Cali- markets in countries with emerging fornia, Los Angeles. The model pro- economies, such as India and China, jects global pest damage to wheat, will grow rapidly as farmers are able maize and rice, and is based on the to spend more money spraying their relationship between insect popula- crops. The pesticide market in India, tion growth and temperature, and China and Japan was worth US$11.7 incorporates projections about cli- billion in 2011 and is expected to reach mate change over the century. If the An adult cereal leaf beetle and a larva deploying its faecal shield (top). $16.2 billion by 2016. Many of the pes- predictions are accurate, solutions are A turnip cutworm (Agrotis segetum) in a potato tuber (bottom). ticides used today in the United States desperately needed for agriculture in and Europe — so-called third-genera- temperate zones. “Today, insect pests reduce loss. In other cases, farmers will benefit more tion compounds — are much less poisonous the yield of maize in the United States by about from education on sustainable agricultural than the ones used 50 years ago. But most 8% annually, which amounts to about US$800 practices, such as intercropping (introducing farmers in the developing world can’t afford million,” says Battisti. “A doubling of pest dam- plants among the main crop that are either the expensive new pesticides and are more age would roughly double that amount.” more attractive to pests to act as bait, or that likely to turn to older, more toxic versions that Battisti says that models will be refined as are unpleasant enough to pests to repel them). are still quite dangerous to health and the envi- more data become available. For example, “Our goal is to inform farmers about their ronment. If regulations regarding the use of the current model (which has not yet been options so they can make decisions based on these harsh chemicals are not enforced, their peer-reviewed) does not consider how insect what is right for them,” Davis says. use will escalate, putting the environment and predators, such as parasitic wasps, will limit An arsenal of tools and techniques to protect human and health at risk. pest populations. “When I think about the crops from droughts and ravenous insects is We are faced with a hungry planet and ris- response of pests to climate change,” he says, now being assembled. Plant varieties that are ing temperatures, and pests will be an increas- “the detailed ecology is so complicated it will selectively bred or genetically engineered to ingly important part of the story. Once the require a lot of work to understand it.” resist insect pests or withstand water stress will scientific details are understood we can adopt be one line of defence (see ‘Discovery in a dry policies that will enhance sustainable agricul- AFFORDABLE FIXES spell’, page S7). This year, the agriculture com- ture and hopefully mitigate problems in the Once the models incorporate enough infor- pany Monsanto, based in St Louis, Missouri, long run. But in the garden of possibilities, the mation to make detailed predictions, scien- sold the first genetically modified drought- one certainty is that there is no time to waste. tists can recommend strategies to control tolerant plant, a variety of maize, to farmers As El-Bouhssini warns: “Climate change is insects that are specific to each farmer’s par- across the western United States. The idea is with us.” ■ ticular crop, region and finances. For exam- that these more robust plants will be able to ple, subsistence farmers in Africa may not withstand pest attack even under drought con- Amy Maxmen is a freelance science writer be able to afford genetically modified plant ditions. Ideally, these varieties would be com- based in New York City. varieties that are resistant to pests, or highly bined with other means of defeating insects, specific pesticides that reliably kill crop- such as pesticides and biological controls — 1. Battisti, D. S. & Naylor, R. L. Science 323, eating insects while leaving the rest of the for example, deploying ladybirds to eat sap- 240–244 (2009). 2. Chapman, T. B., Veblen, T. T. & Schoennagel, T. ecosystem intact. sucking aphids. “There’s a misconception Ecology 93, 2175–2185 (2012). The Food and Agriculture Organization of that people will just rely on biotechnology, 3. Huberty, A. F. & Denno, R. F. Ecology 85,1383– the United Nations (FAO) is aware of these and that’s not the case,” says Troy Coziahr, an 1398 (2004). limitations to farmers’ abilities to respond. In agroecologist at Monsanto. “We cannot rely on 4. Yudelman, M., Ratta, A. & Nygaard, D. Pest Management and Food Production (International some cases, says Mark Davis, an agroecolo- one or two tools to make land more produc- Food Policy Research Institute, 1998). gist at the FAO, providing access to technol- tive. It’s a huge undertaking.” 5. Kroschel, J. et al. Agric. Forest Meteorol. 170, ogy might be the best plan to combat crop Other strategies involve changing 228–241 (2012).

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