A possible link between neonicotinoid pesticide use and a decline in bird numbers is worrying.
Does anything eat wasps? Despite the popularity of the question, which spawned a series of bestselling books, the answer is less interesting than one might think. Yes, lots of things eat wasps — bigger wasps, for a start. Then there is the bee-eater (don’t blame it, we chose the name) and hundreds of other bird species. Perhaps one might instead ask, does anything not eat wasps?
Plenty of these birds eat bees, and other insects too. So it is reasonable to ask a different question: if the numbers of these insects fall, will the birds that feed on them have enough to eat?
This is a complicated question, more difficult to answer than it might seem. So the provocative findings published in a paper on Nature’s website this week must be taken as a question themselves, rather than a definitive answer. In the paper, researchers suggest that common agricultural pesticides that have long been linked to the decline in some bee species could also be affecting birds (C.A.Hallmannet al.Naturehttp://dx.doi.org/10.1038/nature13531;2014).
The suggestion is based on an observed correlation between declines in some farmland bird populations and the use of neonicotinoid pesticides in the Netherlands. The analysis indicates that pesticide use may reduce the amount of prey insects available to birds, causing the observed association, and suggests that neonicotinoids pose an even greater risk to wildlife than previously thought.
Correlation, as every science blogger who reports this story will point out, is not the same as causation. The evidence that the agricultural chemicals can be blamed for the loss of the birds is circumstantial, being based on the finding that the steepest declines are in regions of the country that also show the highest water concentrations of imidacloprid, the most commonly used neonicotinoid. But the authors also rule out confounding effects from other land-use changes or pre-existing trends in bird declines.
Insects form a large part of the diet of many bird species during the breeding season and are important for raising offspring. Nine of the 15 species investigated exclusively eat insects, and all feed insects to their young, which supports the theory that pesticide use is negatively affecting bird populations by depleting their food sources. Still, although many birds do eat bees, none of those studied would ordinarily eat them in any quantity. So a new question must be addressed: if the pesticides are killing the birds, then how are they doing it?
In an accompanying News & Views article, ecologist Dave Goulson suggests one possibility (D. GoulsonNaturehttp://dx.doi.org/10.1038/nature13642;2014). Almost all of the neonicotinoid used ends up in the soil and water as a pollutant. Some could be taken up by non-crop plants, such as hedgerows. Surrounded by tainted food and water, aquatic insects and grasshoppers, beetles and caterpillars could be poisoned — just as bees are — and so not be around to feed the birds.
The picture is incomplete, and thus Goulson’s invocation of Rachel Carson’s Silent Spring, a tale of ecological doom, may seem premature. But his is not a lone voice. An international assessment on the likely effect of neonicotinoids, released by scientists (including Goulson) late last month, warned that the consequences for biodiversity and human food supplies could be severe (see go.nature.com/gzhg94). And Europe has already imposed a two-year moratorium on the use of three common neonicotinoids on flowering crops.
So, does anything eat wasps? Yes, and we must do more to ensure that that continues.