In Oregon there are more than 600 native bee species. A recent release by Oregon State University gives us a fascinating insight into how bees respond to the earth crisis caused by wildfires.

The blue orchard bee, Osmia lignaria, is a well-known and important native pollinator that has been found to produce more female offspring at a higher rate following forest wildfires. In fact, the more severe the fire was, the higher percentage of females. In the worst burned areas, the figure was more than 10% greater than in the areas that were less damaged and burned by the fires.

Jim Rivers, an animal ecologist at the OSU College of Forestry, says this is one of the first studies that has looked at how forest fire severity influences bee demography. The number of young produced did not vary, but the sex ratio did vary based on fire conditions. This indicates bees intentionally altered the sex of their offspring depending on the severity of the fire.

This 1:09-minute video by KOIN 6 discusses this very finding: 



Female bees control the sex of their offspring. They lay eggs that are fertilized with sperm that become female bees, or non-fertilized eggs that become male bees.

Fire is one of the greatest threats to bees in the short term, but brings great bounty for them in its wake. Fire destroys their food sources but post-fire landscapes often have an abundance of flowers. Bees pollinate many flowering plants in the native ecosystems in the area where they live, and this way they are responsible for managing their own food chains as well as that of humans.

Maybe bees know somehow that fire increases in frequency and severity, expands and destroys. If we understand how the concept of fire influences their reproductive output, we will have an important key to how our post-fire management can help or harm bees.

Sara Galbraith, a postdoctoral researcher at College of Forestry, said that they positioned bees at different sites in a mixed-conifer forest that had recently been burned in southwestern Oregon in order to use them as a measuring stick to see how good the bee habitat was. When bees adjust offspring production towards the more expensive offspring sex it shows a functional response to changes in habitat quality via an increased density of flowering plants.

Pollinators benefit from canopy-reducing fires in dense conifer forest ecosystems. After a fire, the abundance of flowering plants usually increases for several years. This results in food resources that enhance wild bee diversity and abundance.  

Of all Earth’s pollinators, bees are the most important with an annual global economic impact of $100 billion. Animal pollinators enhance the reproduction of almost 90% of Earth’s flowering plants, including many food crops, and are an essential component of insect and plant biodiversity.

Bees are the standard bearer since they are usually present in the greatest numbers and they are the only pollinator group that feeds exclusively on pollen and nectar their entire lives.

For this study, Galbraith, Rivers and James Cane of the USDA set up blue orchard bee nest blocks with a standardized number and sex ratio of pre-emergent adult bees. They evaluated the relationship between fire severity and reproductive output, sex ratio and offspring mass at the local (within 100 meters of the blocks) and landscape (750 meters) scales. Female bees forage across both scales when caring for offspring.

According to Rivers, there is a variation in species-level response to wildfire in fire-prone landscapes that serves to maintain ecosystem structure and function. Foraging blue orchard female bees (and similar species) invest in larger progeny and more females when more resources are available.

These findings show that burned mixed-conifer forest provides forage to the blue orchard bee along a gradient of severity. The rise in floral resources following high-severity fires causes females to reallocate resources to the larger and more costly sex—females—when nesting.

In closing, Galbraith said that the study revealed more female progeny than is typically observed with blue orchard bees. The greater proportion of females in areas surrounded by a more severely burned landscape indicates an investment in more female offspring due to greater resource availability.

These findings were published in Oecologia. The Bureau of Land Management and the OSU College of Forestry supported this research.