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Crop pest will spread northward as soil temperatures warm

Corn earworm (Helicoverpa zea) is considered by some to be the most costly crop pest in North America. Also known as tomato fruitworm, sorghum headworm, vetchworm, and cotton bollworm, this pest feeds on over a hundred different plant species, favoring the buds, flowers and fruits for forage. This results in extensive damage to many important crops each year. The pest is not a worm, but the larva of a moth that flies strongly and is able to migrate up to 400km in its lifetime, depositing eggs along the way.

In the past, corn earworms have been found to occur in North America up to the latitude of around 40o. Further north than this was always too cold for the larvae to survive the winter. However, with climate changing it is possible that the species’ distribution on the continent will change and that farmers, whose crops were previously spared the ravages of this insect, may find themselves in the line of sight. Although the species feeds preferentially on corn ears, it also infests crops such as tomatoes, peas, cabbage, lettuce, okra, peppers, spinach, pumpkin and sweet potatoes. 

A new study from scientists at the North Carolina State University has proposed that soil temperature can help predict the success of corn earworm pests and therefore prepare farmers to protect their crops as global warming advances. The research, published in the Proceedings of the National Academy of Sciences, shows that soil temperature can be a good predictor of where the pest will survive successfully over the winter, and this will enable farmers in those areas to use pesticides more effectively, thus reducing the financial and environmental impacts of this practice. 

The researchers combined historical soil temperature data with long-term population data on corn earworms, along with information on how the pest survives cold conditions in a lab setting to better understand “overwintering success,” or how well the pest can survive underground during the colder winter months. 

Greater overwintering success can expand the areas where the pest can live and thrive, the researchers say, as the pest can migrate long distances. Generally, greater overwintering success in more northern latitudes increases the potential for crop damage from this pest further north. Climate change also affects overwintering success.

“There is a preconceived notion that pests have little overwintering success north of 40 degrees latitude,” said study co-author Douglas Lawton, a former NC State postdoctoral researcher. “That may have been true in the 1930s, but now we have more data-guided evidence to ask and answer the question, ‘Where can this species actually overwinter?’”

The researchers identified three overwintering suitability zones throughout North America, based on four decades of soil temperature data. These geographic zones included a “Southern Range” where pests predictably survive over the winter months, a “Northern Limits” area where pests are generally unable to survive during winter months, and a “Transitional Zone” in between the northern and southern areas, where pests may or may not survive the winter depending on the weather. 

“These areas are biologically relevant and supported through studies in the lab and the academic literature,” Lawton said.

The researchers used the three zones to show historical trends for corn earworm populations, and then used a model to make predictions about pest spread in future.  They show that the Southern Range has expanded by 3 percent since 1981; strikingly, the model predicts that this region, where corn earworm survival is a given, will double in size by the end of the century and will shift well to the north. The other two zones are predicted to shrink in size during this time. 

“As the climate changes, the overwintering zones are likely to shift northward,” said Anders Huseth, assistant professor of entomology at NC State and the paper’s other co-corresponding author.

These changes suggest larger populations of the pest may persist at higher latitudes in the future due to reduced low temperature lethal events during winter. In addition, because H. zea is a highly migratory pest, predicting when populations accumulate in one region can inform the likely population changes in other regions, explain the authors. 

“This is the canary in the coal mine for agricultural pests,” said Professor Huseth. “Making sense of what’s taking place with this pest is really important for agricultural producers. We’ve shown here the element of uncertainty that can have demonstrable effects on farmers and potentially new opportunities for pesticide resistance selection. Our models visualize that change and provide touchstones for pest management.”

“Now we’d like to come up with a better forecasting tool for this pest, along with a risk-prediction model, in order to give growers better information about pest spread. Success here could reduce both costs for farmers and pesticide into the environment.”

By Alison Bosman, Staff Writer

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