Honey bees transmit viruses to bumblebees in spring

According to a study by researchers at Pennsylvania State University, honey bees may play a significant role in increasing the virus load in wild bumblebees each spring.

By analyzing seasonal trends in the transmission of parasites and viruses among bees, the experts found that honey bees consistently carried higher concentrations of viruses than bumblebees.

Furthermore, virus prevalence was lower in both bee species in winter, while in spring only bumblebees showed negligible levels of virus.

Honey bees can re-infect bumblebees in spring

Heather Hines, associate professor of biology and entomology in the College of Agricultural Sciences at Pennsylvania State University and corresponding author of the study, suspects that these results indicate that honey bees may reinfect bumblebees in the spring, a time when the virus is otherwise very rare in bumblebees.

“Our data suggest that healthy wild bee populations require healthy honey bee colony management,” Hines said.

“Measures that help reduce the disease burden in honey bees include ensuring high-quality nutrition throughout the season, reducing pesticides and controlling Varroa mites, which are known to cause higher viral loads towards the end of the season.”

Importance of bee colony management

According to Hines, while the study highlights that honey bees are reservoirs of viruses in the spring, this does not mean that honey bees are harmful. Rather, it underscores the importance of good honey bee colony management for the overall health of pollinator communities.

Pollinators are crucial for the reproduction of many crops and wild plants around the world. Honey bee colonies make a significant contribution to agriculture by pollinating plants, while native bee species such as bumblebees and solitary bees provide natural pollination services that can improve fruit set and are essential for many plant species.

Dynamics of parasite and virus spread

Bumblebees, native to the United States, are especially important for pollinating crops such as blueberries and cranberries, while honey bees, originally from Asia, excel at pollinating most orchard crops due to their ability to recruit large numbers of foragers.

Hines argued that while honey bees and bumblebees share many characteristics, there are also differences that affect the dynamics of parasite and virus spread.

“Honey bees are perennial, farmed bees, while bumblebees are annuals – they overwinter as new queens and establish and expand their colonies the following spring,” she explained. These different colony dynamics likely have implications for how diseases are transmitted between farmed and native bees.

Bee viruses throughout the seasons

The study focused on the spread of viruses and parasites throughout the seasons, specifically examining deformed wing virus (DWV) and black queen cell virus (BQCV), which are harmful to both honey bees and bumblebees and can spread between species.

The researchers collected both bee species at different times over four years from six locations in Centre County. They then examined the bees for viruses using molecular biomarkers and for protozoan pathogens and parasites using a microscope, comparing the prevalence of viruses and pests in different seasons.

The results showed that both DWV and BQCV are common in honey bees and bumblebees, with DWV levels peaking in autumn and BQCV levels peaking in mid-season.

However, honey bees carry consistently higher concentrations of these viruses throughout the year, with the largest seasonal difference observed in spring.

Honey bees as virus reservoir

“In the spring, virus infection in bumblebees was negligible. This suggests that the queens are either resistant or, if infected, die during overwintering,” said Hines.

“Bee colonies also tend to have fewer viruses in the spring; however, they still have relatively high levels of viruses compared to bumblebees. This means that honey bees serve as a reservoir of viruses that can re-infect native bee colonies that would naturally shed these viruses in the spring.”

Further implications of the study

The study is a contribution to the growing body of research on the effects of honey bee keeping on the incidence of disease in bee colonies.

Specifically, the Hines Lab has focused on studying the impacts of landscapes in the Eastern United States on pathogen burden in bumblebees, with the goal of identifying controllable factors that can most effectively reduce disease burden in these bees.

In the future, the researchers plan to conduct further studies on queen bee immunity and susceptibility to pathogens. These could provide deeper insights into these patterns and contribute to the development of bee protection strategies.

The study was published in the journal Ecosphere.

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