The Norwegian Environment Agency requested the Norwegian Scientific Committee for Food Safety (VKM) to assess the relationship between flowering Tilia and dead bumblebees. The Norwegian Environmental Agency also called for a summary of the hypotheses put forward to explain the phenomenon and whether it may have negative consequences for the populations of bumblebees in Norway.

Hypotheses

There has been dispute over the causes of bumblebee death. VKM have chosen to divide the hypotheses into our groups:

• The bumblebees die of natural causes
• The bumblebees starve to death because the flowers of Tilia trees do not contain sufficient nectar
• The flowers or other parts of Tilia are poisonous
• Bumblebee death are caused by human impact

"We have not found any scientific documentation for some of the hypotheses," says Anders Nielsen, scientific leader of the working group. "We do not know if Tilia have any negative effects on Norwegian bumblebee populations. Tilia trees may in fact be a valuable resource during a time of the season when few alternative nectar sources are available."

There is too little knowledge to be able to validate the death of natural causes hypotheses as well as the starvation hypotheses. The working group concluded that it is unlikely that Tilia trees are poisonous.

"It has been put forward that Tilia nectar contains mannose, a sugar that may be poisonous for bumblebees. Unpublished data from Dr Torgils Fossen, professor at the Institute of Chemistry, University of Bergen, revealed that mannose was not present in the nectar of the Tilia flowers. Studies from abroad have reached the same conclusion. We can therefore regard this hypothesis as very unlikely," says Nielsen.

The hypotheses of human impact are not relevant in a Norwegian context, since it is illegal to spray park trees in the cities with pesticides.

Suggest to establish a monitoring program

To get a better understanding of the spatio-temporal dynamics of the death of bumblebees in relation to Tilia trees VKM suggest to establish a monitoring program. The monitoring should assess the species, castes and age distribution of the bumblebees involved. It should also assess whether other insects are dying under Tilia trees and whether alternative nectar sources are available at the time the bumblebees are dying.

"We also suggest that the stomach content of bumblebees found dead under Tilia trees are analysed, to test the validity of the “starvation hypothesis”, says Nielsen.

Extensive assessment of the amount and constituents of Tilia nectar would also add valuable knowledge informing the assessments of the hypotheses related to the Tilia trees being the ultimate cause of the phenomenon.

"There is a need to establish whether dead bumblebees also occur in connection with other nectar sources," says Nielsen. "A monitoring program should therefore cover several regions and span over several years," concludes Nielsen.

The assessment was conducted by the Panel on Alien Organisms and Trade in Endangered Species (CITES).