The number of insect species known is about a million, and the number of individual insects alive at any one time is a mind boggling 10 billion billion (1019), with about 300 times the mass of the human population; estimates of the total number of insect species waiting to be discovered go up to 30 million. (1,2)
It was therefore concerning when recently it was reported that populations of flying insects had declined by between 76 and 82% in Germany over just 27 years (3). The study was carried out in 63 sites in nature reserves between 1989 and 2016. The technique was a simple one: tent traps were set up and the insects caught by these in a certain time were weighed. The decline affected all kinds of insect.
A long-running study in another German nature reserve showed a decline of 40% in moths and butterflies over 150 years. More recently, the European Environment Agency reported that 50% of grassland butterflies had been lost in 20 years in 19 European countries. It suggests loss of managed grasslands, either to scrub or to crop growing, and pesticides on neighbouring farmland as potential causes (4). And a worldwide study of invertebrate species (of which about 80% are insects) showed a 45% decline over the past 40 years (5).
Anecdotally, the British media have commented on the disappearance of the “moth snowstorm” due to which night-time drivers (in the countryside) would have to clean their windscreens of the corpses of splattered moths which had mistaken their headlights for the Moon (2).
Serious
This decline is serious for two main reasons.
The wealth of insect species supports a large number of food chains with most obviously birds, but also fish, amphibians, reptiles and mammals (especially bats), at or near the top. Birds affected in Britain include the grey partridge and spotted flycatcher, both having declined by 95%, and the red-backed shrike, extinct since the 1990s, while the house sparrow has also shown a 50% decline since the 1970s (2). Furthermore, a great many plants, including many food ones, rely on insects to pollinate their flowers. These insects include not only bees but also moths, butterflies, beetles and hoverflies. Another may be that if predatory insects decline, populations of prey species that eat food crops could explode, leading to economic losses either from reduced yields or increased use of pesticides.
What is causing the decline and what should be done?
Agricultural practices, such as monocultures, great swathes of just one crop, reduce biodiversity. The removal of hedges, ponds and other refuges for wild life also reduce niches for insects and their food web members.
Pesticides are also a factor, especially when they affect other insects as well as crop pests. Many of the most harmful, such as DDT, have been banned but modern less harmful ones seem not to be entirely harmless. This may be the case with neonicotinoids (see below). The evidence about these is contradictory but seems to be coming down on the harmful side (6)
Climate change does not seem to be a factor in the decline at present but as warming accelerates it may become one. If anything, increased temperatures should increase insect biomass. For example, the warmer winters of recent years may have allowed pest species to overwinter more successfully, leading to more crop damage. However, species that rely on particular plants for food may suffer if those plants cannot cope with climate change and become more scarce. Also, increased extreme weather events such as droughts would negatively affect insects.
Changing agricultural incentives to favour greater crop diversity, to keep or restore hedges and so on, to reduce pesticide use, for example by applying it directly rather than spraying it into the atmosphere, are all initiatives that could help. In a rare example of evidence-based policy-making, Environment Secretary Michael Gove says the UK will support Europe-wide ban on neonicotinoids after the German study (7).
What are neonicotinoid insecticides?
These insecticides, developed from the 1970s onwards, have rapidly become popular because, unlike the organophosphate and carbamate insecticides, they have low toxicity to mammals, including humans, while being very toxic to insects. They now amount to about a quarter of the global insecticide market, with one of them, imidacloprid (patented in 1985 by Bayer), being the most widely used insecticide in the world.
Neonicotinoids (“new nicotinoids”) are similar to nicotine, an alkaloid produced by the tobacco plant (Nicotiana tabacum) and other members of the Solanaceae family (which includes deadly nightshade, potato and aubergine). Presumably it is produced as a defence against insects that would otherwise eat the leaves of the tobacco plant.
In humans, nicotine stimulates the brain’s nicotinic acetylcholine receptors (NAChRs), a class of receptors that promotes the release of dopamine and endorphin, stimulating the brain’s reward system. Nicotine is said to cause feelings of calmness and relaxation, while also making the user more alert. It can reach the brain some 15 seconds after inhalation of tobacco smoke. These effects are often desirable and even useful so it is unfortunate that nicotine intake is usually accompanied by a cocktail of carcinogens. It is even more unfortunate that it produces tolerance, where the user requires more and more to achieve the desired effect, and that it is highly addictive. Nicotine was previously widely used as an insecticide as it overstimulates insects’ central nervous systems, rather than making them feel relaxed, and kills them. It was phased out over its harmfulness to mammals, including people using it or their children. However, it should be noted that it is impossible to get a fatal dose of nicotine from smoking.
Neonicotinoids are chemically different to nicotine: they cannot cross the blood-brain barrier in mammals (and so do not mimic the effects of nicotine), and bind much more strongly to insect NAChRs than to mammalian ones. They are also thought to be less harmful to fish, an important consideration when rain can run off fields into rivers. While nerve gases such as sarin prevent the breakdown of the nerve transmitter acetylcholine (ACh) in humans, neonicotinoids mimic the action of ACh in insects and also cannot be broken down. The result is the same: nerves are stimulated to fire continuously, causing paralysis and death.
Resistance
Neonicotinoids were introduced after many insects had developed resistance to organophosphate, carbamate, and pyrethroid insecticides.
Predictably, resistance has started to develop to them as well (travellers may wish to note that bed bugs in New Jersey are now resistant).
Neonicotinoids are absorbed by plant roots and leaves and travel to all parts of the plant, where they are taken in by herbivorous insects. Also, they are more persistent, that is they break down more slowly, than nicotine, offering more long-term protection to crops. They are active against a wide range of pests, such as aphids, whitefly, wireworms and leafhoppers. However, their wide range includes many non-target insects, some beneficial, such as bees (8). In theory, it should be possible to minimise exposure of other insects by applying the insecticide more carefully directly to the roots, rather than spraying. It is common to treat seeds before sowing which is a less dangerous process.
It was reported last year that the use of neonicotinoids on oilseed rape in England from 2002 is linked to an average decline in all bee species of 7%, with the worst affected being those that collected nectar from rape flowers. This is serious news not only for the natural world but specifically for that substantial section of agriculture that relies heavily on pollination by bees and other insects. This is especially so since bee numbers have already suffered a lot from the parasitic mite Varroa and the mysterious Colony Collapse Disorder. It seems that neonicotinoids can get into pollen and nectar and thence into the bees. The amounts involved are not lethal but it is suggested that they may cause behavioural changes that make bee colonies less viable. One study shows that bumblebee colonies affected put on less weight before winter and are less able to survive.
In any case, neonicotinoids are found in bees and at least some bees seem to be adversely affected so, on the precautionary principle, neonicotinoid use should be restricted.
References
Some 40% of insects are beetles. The great socialist scientist JBS Haldane, when asked what he deduced about God from contemplating the living world, replied “God has an inordinate fondness for beetles.”
1 https://www.si.edu/spotlight/buginfo/bugnos
2 https://www.theguardian.com/environment/2017/oct/21/insects-giant-ecosy…- collapsing-human-activity-catastrophe (author Michael McCarthy, originator of the term “moth snowstorm”).
3 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185809
4 https://www.eea.europa.eu/highlights/populations-of-grassland-butterfli…
5 https://e360.yale.edu/features/
insect_numbers_declining_why_it_matters
6 https://www.nature.com/news/the-bitter-battle-over-the-world-s-most-pop…- insecticides-1.22972?WT.mc_id=TWT_NatureNews&sf159501690=1
7 https://www.theguardian.com/environment/2017/nov/09/the-evidence-points…- direction-we-must-ban-neonicotinoids
8 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4284386/
