It has been well established that the use of toxic pesticides and disease outbreaks are the main driver of the declining bee population. However, a new study from North Carolina State University revealed that the increasing urban temperatures can also contribute to the decreasing bee population.

The study, published in the journal Biology Letters, showed that almost all of the 15 most common wild bee species in the southeastern cities can be negatively affected by the increasing temperatures in urban heat islands.

"This is certainly relevant for urban heat islands, but it may also help us understand potential effects of global climate change on bee species," said Elsa Youngsteadt, a research associate at NC State and co-lead author of the study, in a press release. "If species that have a lower CTmax are most sensitive to urban warming, they may also be most sensitive to warming in other environments."

For the study, the researchers first determined the so-called CTmax, or the critical thermal maximum of the different bee species. The bees were placed in a tube and the temperature was gradually increasing until each bee became incapacitated. The researcher noted that the CTmax is the temperature at which the bees were incapacitated in the laboratory settings. These insects could be negatively affected by lower temperatures, forcing them to leave habitat or reproduce less.

Among the bee species included in the study, carpenter bees Xylocopa virginica and Ceratina strenua appear to be the most heat-tolerant with CTmax value of 122 to 124 degree Fahrenheit. On the other hand, sweat bee (Agapostemon virescens) and a bumble bee (Bombus bimaculatus) are among the least heat-tolerant with a CTmax value below 113 degree Fahrenheit.

The researchers then sampled bee populations 11 times over two years at 18 urban sites in Wake County, North Carolina to determine how the bees' CTmax affect their behavior in real-world settings. The CTmax of the bees corresponded well to their abundance in urban yards. This means that the population of the bees decreases as the temperature in the urban yards increases.