Tough decision making is not restricted to human societies. While male mosquitoes happily feed on the nectar in our gardens, female mosquitoes invade our bedrooms at night attracted by the proteins in our blood. They need this resource to produce eggs that are subsequently deposited in freshwater habitats where the larvae grow. However, not every pond or ditch is a good breeding ground and therefore gravid mosquitoes tend to be very picky. This is because mosquito larvae feature prominently on the menu of a whole range of predators such as fish and voracious larvae of water beetles and dragonflies. To ensure survival of their offspring, mosquito mums must therefore deposit their eggs in habitats where these killers are absent. It was already known that mosquitoes avoid the smell of fish when they are looking for a place to lay their eggs. However, in a new study in the journal Ecology Letters, we show that mosquitoes do not only avoid reproduction in local ponds that smell fishy, but also in neighboring fishless ponds in the surrounding landscape. Although several researchers have suggested that the smell of predators can be used as a chemical repellent to prevent mosquitoes from reproducing , we show that this may not work very well. If the smell of fish is everywhere, mosquitoes seem to be smart enough to realize that there are no better alternatives and happily lay their eggs, even if the environment smells fishy. Just like politicians, mosquitoes seem to be able to make compromises and, in the absence of better options, will settle for a bad deal.
It was the first time that this process, which in the specialized literature is known as habitat compromise, could be demonstrated in nature. Releasing fish is a classical strategy to control mosquitoes. However, this approach is controversial as many biting mosquitoes reproduce in habitats where fish cannot survive such as ephemeral pools, gutters, buckets and other containers where rainwater accumulates. Additionally, releasing fish in ponds has negative effects on the diversity of other organisms. For instance, in Western Europe, a number of rare dragonfly species and amphibians such as the tree frog and the crested newt, can only successfully reproduce when their larvae are not consumed by fish. Yet, even when fish cannot be released, there are perspectives for the application of predator smell for mosquito control. Artificial fish smell chemicals might help to concentrate mosquitoes in a few selected water bodies in a landscape where the eggs and larvae can be eradicated locally. From an environmental perspective it will certainly be preferable to spray large areas with fish chemicals rather than insecticides. However, further research will be needed to confirm whether such an approach is practically feasible. The exact volatile fish chemicals to which mosquitoes respond are also still unknown.
The paper is out now in Ecology Letters