When people talk about the decline in insect populations, the focus is generally on pesticide use, and habitat destruction. There’s no question that these are major factors, but there’s another that has apparently been under-estimated: air pollution. I talk about air pollution a lot on this blog, and while that’s mostly focused on how it affects humans, I did post last November about how air pollution made it harder for fig wasps to find their aphid prey. The researchers speculated that the presence of diesel fumes and ozone masked the scent of their prey, but that prey feeding on cabbages and other brassicas were smelly enough to cut through the pollution. Now a new study has come out, which demonstrates that air pollution particles can collect on an insect’s sensory organs, affecting their sense of smell in general:

The research team conducted several related experiments:

• Using a scanning electron microscope, they found that as air pollution increases, more particulate material collects on the sensitive antennae of houseflies. This material comprises solid particles or liquid droplets suspended in air and can include toxic heavy metals and organic substances from coal, oil, petrol, or woodfires.
• They exposed houseflies for just 12 hours to varying levels of air pollution in Beijing and then placed the flies in a Y-shaped tube ‘maze’. Uncontaminated flies typically chose the arm of the Y-maze leading to a smell of food or sex pheromones, while contaminated flies selected an arm at random, with 50:50 probability.
• Neural tests confirmed that antenna contamination significantly reduced the strength of odour-related electrical signals sent to the flies’ brains – it compromised their capacity to detect odours.

In addition, continuing research in bushfire-affected areas in rural Victoria has shown that the antennae of diverse insects, including bees, wasps, moths, and species of flies, are contaminated by smoke particles, even at considerable distances from the fire front.

Insect antennae have olfactory receptors that detect odour molecules emanating from a food source, a potential mate, or a good place to lay eggs. If an insect’s antennae are covered in particulate matter, a physical barrier is created that prevents contact between the smell receptors and air-borne odour molecules.

“When their antennae become clogged with pollution particles, insects struggle to smell food, a mate, or a place to lay their eggs, and it follows that their populations will decline,” Professor Elgar said.

“About 40 per cent of Earth’s landmass is exposed to particle air pollution concentrations above the World Health Organisation’s recommended annual average.

“Surprisingly, this includes many remote and comparatively pristine habitats and areas of ecological significance – because particulate material can be carried thousands of kilometres by air currents,” Professor Elgar said.

I’ll be honest: If you had asked me how air pollution was contributing to the decline in insect populations, I would have guessed ill health through inhaling, drinking, or eating air pollution, but I wouldn’t have gone with “it messes with their sense of smell”. Given the fig wasp thing I mentioned above, I guess it should have been higher on my list, but I apparently didn’t give it enough thought. I think it’s partly that being a visual creature that gets food from stores, I sometimes forget the importance of smell to other animals. Repetition aids memory, though, so now I’m more likely to remember it. I suppose the next question here will be how big this olfactory problem is, but while we wait for a number, we can add this to the already-huge pile of reasons why it’s good to reduce air pollution.