Winter temperatures in the soil can be used as a way to predict the following growing season’s insect pest populations
Corn earworm is a significant pest that ravages not only sweet corn but cotton, soybeans, peppers, tomatoes and other vegetable crops. Monitoring it in a way that would help farmers predict where it could appear in the next growing season could be significant in controlling it more effectively.
An adult corn earworm is a buff or tan-coloured moth, but it is larvae that do crop damage. In Ontario, female moths migrate from southern areas in the U.S. where they have overwintered. They arrive in late July or sometime in August.
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Earworms lay their eggs mostly on fresh silks. The female moth can lay more than 100 eggs in a single night and more than 1,000 in its lifetime. Eggs hatch within two to 10 days depending on temperature and, on hatching, larva first feed on the silks inside the husk and then consume kernels at the ear tip.
Recent research from North Carolina State University has shown that soil temperature can be used to effectively predict the pest’s spread.
“Many temperature insect species undergo a dormant period during winter to survive harsh cold conditions,” said Douglas Lawton, former postdoctoral researcher with NCSU and now a data scientist with AgBiome, a biotechnology company.
“One very common strategy is to dig underground and wait out the cold temperatures in a larval form. Therefore, soil temperature is a critical factor in whether insects are able to survive or not. Since many growers are worried about insects during the peak of the growing season, we can use the prior winter soil temperatures as a predictor of the following season’s insect pest populations.”
The researchers looked at historical soil temperature data along with long-term corn earworm monitoring data. They also looked at how the pest survives cold conditions in a controlled lab to better understand its overwintering success and cold tolerance levels.
Traditionally, it has been thought the pest cannot survive with much success north of 40 degrees latitude. But the influence of climate change is altering that, and the data-guided evidence used by researchers shows new variables.
“Using remotely sensed data, we show that there is a lot of variation around this 40th parallel split and going into the future we can expect a northward shift of where this species can overwinter,” said Lawton.
Researchers made specific data-guided maps to show three geographic zones. The southern range set parameters for where the pest survived the winter months. The northern range defined where they were unable to overwinter. The transitional zone defined where it was possible for them to survive winter.
“These areas are biologically relevant and supported through studies in the lab and the academic literature,” Lawton said.
The models have shown that the southern range has grown by three percent since 1981, indicating it will overtake the transitional zone.
“This is the canary in the coalmine for agricultural pests,” said Anders Huseth, assistant professor of entomology at NCSU and a co-author of the paper. “Making sense of what is taking place with this pest is really important for agricultural producers.”
Climate change and warmer temperatures appear to be driving changes in the pest and its survival ability.
“We expect the transitional zone and the northern range to shift northward,” said Lawton. “Our future soil temperature predictions are very coarse due to the fact that there are no future soil temperature data products available at this time. With that said, we (still) expect to see significant increases by 2050.”
He said that other environmental factors may also limit or regulate the species spread and survival success such as the influence of climate change on precipitation or extreme heat waves.
“Another critical factor to understand is the complicated interactive effect of climate change and land use change. Where farmers currently grow crops will likely change due to certain areas becoming too dry for corn or soybean. Earworms are heavily influenced by crop composition and if this changes in the future, we can expect where and when earworm populations build to reflect that change.”
Given the pest’s ability to migrate long distances, expanded success in overwintering can also expand options for where they can live and thrive. For instance, Minnesota, with its harsh winters, saw no corn earworm overwintering from 1950 through 2021, according to the data. But, by the end of the century, predictive models indicate the entire state will be in the transitional zone.
“Farmers have a lot on their plate early in the season,” said Lawton. “They have a lot to think about when to plant their crop. Too early or too late and you risk losing a lot. For farmers in more northern areas, they are not thinking about insect pests until much later into the season.
“In terms of the earworm, typically these pests don’t arrive until August. Going into the future, farmers will likely need to consider increasing early season pest pressure that just simply wasn’t a thing, historically speaking.”
Corn earworm moths have been seen in Ontario earlier than usual in the past few years, with some sightings as early as June. They have become more pesticide resistant.
“We are experiencing BT/pesticide resistance in corn earworm populations now and that won’t be going away,” said Lawton.
Researchers are now focused on understanding and building a continent-wide forecasting system for the pest.
“Traditionally speaking, earworm control has been done region by region with little interaction or co-operation between regions. However, given the migratory nature of this species, we should establish a continental wide monitoring system. As we have shown in our paper, southern populations influence the timing and intensity of more northern populations. As such, we can likely predict when corn earworm migrants will arrive in the north.”
Huseth said they want to devise a better forecasting tool for the pest, along with a risk-prediction model to give growers better information about pest spread.
“We’ve shown here the element of uncertainty that can have demonstrable effects on farmers and potentially new opportunities for pesticide resistance selection,” said Huseth. “Our models visualize that change and provide touchstones for pest management. Success here could reduce both costs for farmers and (the effects of) pesticides in the environment.”
The research paper was published in the Proceedings of the National Academy of Sciences.
