Rising temps may alter moth outbreaks

PENN STATE (US) — As global temperatures rise, cycles of infestation from common plant pests may become more frequent and more violent, experts warn.

A new study may shed light on how temperature influences whether insects emerge as cohesive cohorts or continuously—and could have implications for both pest control and how climate change can alter infestations.

“While the influence of temperature on individual-level life-history traits is well understood, the impact on population-level dynamics, such as population cycles or outbreak frequency is less clear,” the study reports.

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Currently, researchers use temperature to predict the number of generations that appear each year and the timing of the various insect life stages, which is critical for scheduling pest control.

“While we had a really good record of temperature and the number of cohorts that appeared each season, we had no clear understanding of the difference between distinct and continuous reproduction,” says Ottar N. Bjørnstad, professor of entomology, biology, and statistics at Penn State.

“Understanding the timing of generations is important because typically insecticides work only during one or two of the life stages of these pests.”

For the study, published in Science Express, researchers looked at more than 50 years of data on the tea tortrix and also developed an independent mathematical population model that can predict population dynamics under both constant and seasonally driven temperature regimes.

While the tea tortrix is native to Japan, many similar moths exist in North America including the spruce bud moth, grape berry moth, light brown apple moth, and summer fruit tortrix.

Researchers used long-term data on the population dynamics of the tea tortrix that span 51 years and more than 200 outbreaks. The data were collected every five days at the Kagoshima tea plantation in Japan.

This type of insect remains dormant during the winter and emerges once the temperature reaches a certain level in the spring. Because the first generation is triggered by this temperature increase, the insects emerge all at once.

Population crash

“We found the tea tortrix data very interesting,” Bjørnstad says. “Often in North America we have one or two discrete early cohorts because winter synchronizes them and later, we find a background of multiple generations at all times.”

The tea tortrix starts out in this way, but the researchers found that desynchronization does not occur. Through the warm season, outbreaks become more and more synchronous and distinct from each other.

“When the temperatures are high, the reproduction rate is high and the developmental rate of the tea tortrix is high,” Bjørnstad says. “The population grows very fast and becomes unstable. Above a certain temperature, the population numbers overshoot the carrying capacity and the population crashes. After a bit, another generation comes in.”

To better understand how temperature influences tea tortrix and other insect populations, the researchers developed a mathematical population model that is based on the insect life cycle and the effects of temperature on individual stages, and used this to predict population dynamics.

“The model is developed to represent the biology of the insect,” says William A. Nelson, associate professor of biology, Queens University, Canada, who is currently on sabbatical at Penn State. “It is realistic, fully developed, and parameterized independently of the field data.”

The model is based on laboratory data and is fully independent of the Japanese data set.

“We speculated that temperature might do something to population dynamics,” says Bjørnstad. “We documented that temperature itself is destabilizing to the dynamics of this pest. This is the first clear demonstration that temperature has the ability to alter those dynamics, causing large cycles in the insect.”

The Natural Sciences and Engineering Research Council, Canada, and the US Department of Agricultures National Research Initiative supported this work.

Source: Penn State