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Crop pests and climate change: the real story

Climate change could affect the spread of certain pests, and bean leaf beetles are one species that has spread, possibly as a result of warmer temperatures.                  Photo courtesy of Horst Bohner, OMAFRA.

Climate modelling,” “drought tolerance” and “range expansion” are some of the buzz words that dot recent articles predicting the impact of climate change on agriculture. Such articles have been showing up in the media on a regular basis. While some studies predict Canada’s growers will benefit from future climatic conditions, most offer dire warnings.

How are producers to make sense of it all?

Start with the basics, says Dr. Jonathan Newman, chair of the department of environmental biology at University of Guelph. Newman studies how climate change predictions could affect cereal aphids, swede midge in canola, blossom beetle, Colorado potato beetle and other insect pests. “Crop growers should first and foremost realize there is a good deal of unknown about climate change predictions,” he says. “The impact of climate change is a reasonable concern, but we need to keep in mind it’s a complicated story.”

Newman says that while the common broad generalizations about warmer future conditions being good for crop pests do hold merit, “Climate change is not just about warmer temperatures. It’s about humidity, and precipitation and changes in weather patterns. These all have direct impacts on pest life cycles in themselves, and also through their interactions with each other.”

Increased carbon dioxide levels, for example, interacting with temperature and precipitation, can cause changes to plant growth and quality. This, says Newman, could mean that the part of a crop that a particular pest is interested in eating may become less nutritious, so that the pest stops eating it. Conversely, it could result in the pest having to increase its intake to get the same nutritional benefit.

In terms of the pests themselves, Newman stresses that some are more sensitive to temperature, and some to moisture. Some are found only in certain regions and some are widespread. “It’s therefore the case that giving a continental-scale answer is very difficult for all pests of corn, for example,” he says. “Some will probably increase in number and range due to predicted climate change and some will probably decrease.”

Making predictions
Part of what makes climate change impact prediction so complex and difficult, says Newman, is the assumptions and resulting models on which the predictions are based. Assumptions about future greenhouse gas levels and other factors are used to create “scenario” models. These are then used to make climate models, which are spatial representations of all aspects of the atmosphere.

Newman says the United Nations Intergovernmental Panel on Climate Change (IPCC) accepts seven commonly used scenarios and 23 climate models. “They don’t all agree with each other,” he says. “They feature different assumptions and simplifications. The projected temperature increase tends to be similar among models, but precipitation, for example, is quite different among them.”

Recent analysis by entomologist Christian Krupke and his colleagues at Purdue University in Indiana used an IPCC emissions scenario called A2, which assumes greenhouse gas emissions continue to increase exponentially. In all of the study’s projections, chosen insect corn pest species (corn earworm, European corn borer, northern corn rootworm, and western corn rootworm) were predicted to spread into agricultural areas where they are not currently endemic, with the greatest potential range expansion predicted to be in corn earworm.
The analysis was based on the predicted number of days warm enough for pests to grow and the number of days cold enough to kill them. More warm days provides them with additional time to feed, mate and reproduce.   “However, the model cannot take the dynamic nature of any ecological system into account,” Krupke says. “Additional important factors could come into play such as pest interaction with the environment, where corn is grown and advances in pest management.”

Newman agrees that warmer growing seasons and milder winters predicted to occur with climate change are generally expected to benefit crop pests. It is generally predicted that more generations of an insect will occur during a growing season than has been the norm, and that the northern part of a pest’s range might expand. “However,” he says, “southern populations may be negatively affected. It may be too hot or too dry for them to survive in these areas.”

However, Newman stresses that these are broad generalizations that do not say much that is concrete about a specific pest. “An increase in annual average global temperatures of three degrees that some climate models predict is not going to affect all pests or all regions of the world similarly,” he says.

Preparing for the future
While it is difficult to make definitive predictions about which pests will do well due to projected climate change and how well they’ll do, Newman does think it is important to think about preparation. “If, for example, your breeding programs focus on pest resistance,” he says, “you have to know what you’re breeding for. You need a systematic approach for each crop.”

Newman observes that if any breeding to mitigate the impact of predicted climate change is being conducted right now, it is for drought tolerance. “Again, however, not everywhere will suffer drought. Some areas will get more precipitation,” he says. “And if you’re predicting temperature increases of 10 degrees C on summer days, drought is the least of your worries. Some crops won’t survive that, even if they get good rain.”

The way to best manage risk mitigation, says Newman, is to study a given region and crop in some detail. “Future scenarios are only that, scenarios. Farmers need secure predictions, not opinions on what might happen.”

Tracey Baute, field crop entomologist program lead for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), says there are specific pests the Ministry would like to work on to try and predict how successful they will be at overwintering here or spreading to Eastern Canada from other regions in terms of predicted climate change conditions. “Examples would be bean leaf beetle and western bean cutworm,” she says.

Crop pathogens and climate change
Most plant pathogens are fungi, says Tom Fetch, a crop pathologist with Agriculture and Agri-Food Canada in Winnipeg and Canada’s leading stem rust expert. “Fungi usually do best under humid conditions and not so well if it is very dry.”

How climate change impacts fungal disease prevalence depends mainly on two factors, says Fetch. If warmer and longer growing seasons occur, fungal diseases could do well, but only if there is enough moisture to support them. “That is, usually when the weather is good for growing a crop, you see the crop’s diseases flourishing as well. We could even see some new fungal disease here, especially if crops that currently are marginally grown (such as corn or soybeans) increase in acreage because of the increased heat units that would occur if warmer and longer seasons occur.” He adds “But if that warmth is accompanied by dry conditions, it is unlikely to see increased fungal disease.”

There are no data at this point on how Ug99 stem rust would be affected in Canada by climate change. Fetch can, however, report that “in recent months, there has been no appreciable spread of this disease in Africa, where it originates.”