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Blue orchard bees respond to fires by producing more females

Researchers have discovered a new strategy that bees use to cope with wildfires. In the aftermath of a fire, blue orchard bees produce more female offspring, according to a new study from Oregon State University.

Female bees control the sex of their offspring. The eggs that they fertilize with sperm become females, while the non-fertilized eggs become males.

The study revealed that the percentage of female offspring was even higher in the most severely burned areas compared to areas that were moderately burned. 

“This is one of the first studies that has looked at how forest fire severity influences bee demography,” said co-author Jim Rivers, who is an animal ecologist with the OSU College of Forestry. “Sex ratio varied under different fire conditions but the number of young produced did not, which indicates bees altered the sex of their offspring depending on the degree of fire severity.”

Many of the flowering plants that make up native ecosystems and food chains depend on bees for pollination. If experts can gain a better understanding of how bees respond to wildfires, which are expected to increase in frequency in severity, they can develop more effective post-fire recovery strategies.

“We placed bees on different sites within recently burned mixed-conifer forest in southwestern Oregon and used them as a measuring stick to tell us how good the bee habitat was,” said study lead author Sara Galbraith. “Adjusting offspring production toward the more expensive offspring sex shows a functional response to changes in habitat quality via an increased density of flowering plants.”

Pollinators can benefit from canopy-reducing fires in dense conifer forest ecosystems. For example, flowering plant abundance usually increases for several years following a fire, which leads to more food resources. This ultimately boosts wild bee diversity and abundance.

Pollinators are an essential component of insect and plant biodiversity. Animal pollinators enhance the reproduction of nearly 90 percent of the Earth’s flowering plants, including many food crops. 

The blue orchard bee, Osmia lignaria, is a solitary mason bee that is native to the United States and Canada. 

To investigate the relationship between fire severity and reproductive output of blue orchard bees, the researchers set up nest blocks containing a standardized number and sex ratio of adult bees, and then monitored the offspring across local and landscape scales

“In fire-prone landscapes, there is variation in species-level response to wildfire that serves to maintain ecosystem structure and function,” said Rivers. “With the blue orchard bee and similar species, foraging females invest in larger progeny and more females when more resources are available.”

The analysis showed that burned mixed-conifer forest provides floral resources for the blue orchard bee along a gradient of severity. An increase in these resources after high-severity fire prompts blue orchard bees to invest their energy in producing larger female offspring. 

“Our study revealed more female progeny than is typically observed with blue orchard bees,” said Galbraith. “The greater proportion of females in areas surrounded by a more severely burned landscape indicates an investment in more female offspring because of greater resource availability.”

The study is published in the journal Oecologia.

By Chrissy Sexton, Staff Writer

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