‘It’s a form of biological investment’

Scientists discovered that azuki bean beetles — an agricultural pest — lay larger male eggs when under stress from climate change. 

What’s happening?

According to Kyushu University, researchers published a study in Scientific Reports showing how elevated temperatures and carbon dioxide levels influence the beetle’s reproduction. When scientists raised the temperature and carbon dioxide, they found that beetles infected with two strains of Wolbachia bacteria (wBruCon and wBruOri) laid larger eggs that hatched into male larvae.

This showcases how Wolbachia microbes can benefit the species, and how a species with chromosomal sex determination can change egg size and sex based on environmental factors. Before this, no one had observed sex-dependent changes in egg size in this kind of species.

“One characteristic we’ve observed in some insects, including these beetles, is that they increase egg size when exposed to environmental stress,” professor Midori Tuda, the study’s lead author, explained. “Offspring hatched from larger eggs tend to survive better and develop more quickly during the early stages. It’s a form of biological investment under stressful conditions.”

Why are the beetle eggs important?

Azuki bean beetles are a pest that lay their eggs on stored legume seeds. Hatched larvae bore into the seeds, reducing germination rates. This threatens the human food supply. Discovering that they grow larger and faster in the changing climate is cause for alarm.

According to the news release, insight into the beetles’ biology is crucial for stopping this seed damage. Rising temperatures endanger crops in many ways, from dry climates to extreme heat. Knowing how pests such as the azuki bean beetle will behave in the future can help farmers protect their crops better.

“With human activities raising atmospheric CO2 and global temperatures, it is critical to predict the future population dynamics of agricultural pests like the azuki bean beetle,” Tuda said.

How can legume seeds be protected from these beetles?

These fast-developing, large male beetles could spell trouble for crops. The combination of Wolbachia bacteria, elevated temperatures, and increased carbon dioxide levels triggers that “biological investment.” But with this information, researchers and farmers can develop strategies to safeguard legume seeds.

“From a pest control perspective, targeting Wolbachia may be a strategy worth exploring,” Tuda said. “However, because nearly half of all insect species are infected with Wolbachia, non-selective use of bactericides could threaten non-pest insects as well.

“A nature-positive approach is essential as we adapt to a changing climate.”

Further research is needed, but this discovery is a step forward in mitigating the harmful pests. And the goal is to find a solution without using harmful pesticides or bactericides.

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