I'll state first up that I am not anti-pesticides. Without them modern agriculture couldn't exist, we'd have much higher food prices and no doubt people's diets would be worse for their absence. But they will have an environmental impact that is far less easy to detect than any 'endocrine disrupting' effect that groups like the Pesticide Action Network rave and shout about. What is needed is an informed discussion about the risks that these chemicals have, not only to bees and humans but to wildlife further afield. Using long-term demographics and detailed studies of birds and other animals we can hope to determine these impacts. Unfortunately, many of the staunch anti-pesticide campaigners are also very anti GM: a technology that has the potential to actually reduce our impact on the environment and increase sustainability.
Pesticides kill insects, that is what they are designed to do. Almost all non-ratpor or seabird species (I can't think of an easier way of writing this!), whether they as adults each insects or seeds, feed their chicks insects. As a food source, as many an alternative website will state, insects are very high in protein - the ideal thing for a growing chick! With the majority of the UK being either urban or under agriculture that leaves environments that are either a) probably fairly low in insects or b) get sprayed with pesticides to reduce the number of insects. Many species of native British birds are on the decline, house sparrows declined by 71% between 1977 and 2008, common cuckoo number are hugely down, there is obviously a problem but is it a singular one or is each species decline due to their own specific factors? However unique the behaviour and ecology of each declining species, It seems that more and more studies and ornithologists are at least in part blaming pesticides for the reduction in bird numbers. If birds are unable to feed their offspring then recruiting individuals into the population is reduced, and as birds age or die from disease or predation then the population will decline. And if the adults themselves rely on insects for their own food then surviving or getting up to reproductive weight becomes even harder.
It is very difficult to show causation in situations like this, as all that most researchers have is historical records of bird populations and pesticide use. To complicate things further pesticides are not the only thing that can lead to a reduction in insect abundance. Changing land use practices, such as cutting down hedgerows or increasing stocking density are also likely to have impacts on insect abundance. Controlled trials where specific areas are allowed to use pesticides and others are not would be great, but they are expensive and, for an issue like this, long-term. And with our governments current track record for environmental experiments (i.e. the badger cull) few would likely trust the results.
However, the decline of British birds cannot be put solely at the feet of pesticides. Many of our birds have long and complex migrations which we are only now starting to understand. Changes in land use, hunting or a number of other things may be impacting them at various stages along their migrations. For example, in Malta every year thousands of migrating birds are shot - which I find repellent. The decline in insect numbers in the UK may also be due to climate change, insect populations are likely to decline as flowering times and insect emergence times become increasingly out of step.
So conclusions? I don't have any. I'm just hoping that this issue gets wider attention and enters the debate into both the use of pesticides and the potential benefits of GM.