Diesel Pollutants Disrupt Flower Odors and Honeybees

Pollution from diesel exhaust can decrease the honeybee’s ability to detect floral odors, according to researchers from the University of Southampton (1).

Pollinators use their sense of sight and smell to locate their food sources. They play a critical role in our global food production. But over the last few years, many pollinator populations, including that of the honeybee, have been declining. Many factors may be responsible for this decline.

Widespread air pollution, capable of disrupting the honeybee’s sense of smell, may be one reason for the decline in honeybees. Bees use smell to differentiate amongst flowers and locate them from a distance. In theory, pollutants could decrease the range at which bees can detect smell by reacting with the chemicals emitted by flowers.

One large source of air pollution is diesel exhaust. When burned, reactive chemicals such as CO and mono-nitrogen oxide (NO_x) are released. Their concentrations often exceed the limits placed by regulatory agencies.

A team of researchers at the University of Southampton in the UK first tested whether or not NO, NO₂, and CO at the concentrations of diesel exhaust could affect the concentrations of chemicals released by the oilseed rape flower. They chose eight chemicals that most strongly attracted bees. They then combined the chemicals to mimic the natural odor of the flower. The mixture was finally released into two sealed containers, one with clean air and the other polluted by NO, NO₂, and CO.

In the contaminated container, they found that after two hours, α-farnesene, a chemical that comprised over 70% of the mixture, and α-terpinene, a chemical that comprised about 1% of the mixture, were undetectable. In the clean air container, however, the concentrations remained stable.

The researchers then changed the ratios of NO:NO₂, ranging from that of federal regulations to those of diesel exhaust. They found that α-farnesene and α-terpinene either decreased or became undetectable at many, if not all, ratios. In all the tested ratios, NO₂ lowered the concentrations of the above chemicals to a greater extent than NO.

Finally, the researchers tested to see if the absence of α-farnesene or α-terpinene actually affected a honeybee’s ability to recognize the flower’s odor. Bees naturally extend their proboscis, a straw-like mouth part, when they detect sugar. By training the bees beforehand to associate the odor of the flower with sugar, the researchers knew a bee recognized the odor. When exposed to a mixture of odor without α-farnesene, the main component of the mixture, the bees’ recognition did not decrease significantly. However, when α-terpinene was removed, the recognition rate dropped sharply.

The results show that a flower’s odor is complex, with many chemicals involved in smell recognition. If one chemical, even a seemingly minor one, is missing, a honeybee may not be able to recognize the flower. This effect becomes even more severe when bees are farther away from flowers because the odor is less concentrated and more likely to be affected by pollutants. The bee, having lost recognition by smell, will have to use its sense of sight to find the flowers.

Pollution has dire ramifications for pollinators that need to find and pollinate flowers to feed themselves. More research is underway to find out what other factors are leading to the decline in pollination and pollinator populations. Any factor that disrupts a pollinator’s lifestyle will damage the pollinator’s population, its ecosystem, and even the human food supply.

Reference:

1. Girling, R.D., Lusebrink, I., Farthing, E., Newman, T.A. & Poppy, G.M. Diesel Exhaust Rapidly Degrades Floral Odours Used by Honeybees. Sci. Rep. 3, 2779; DOI:10.1038/srep02779 (2013).