Love ’em or hate ’em, we need insects, yet their numbers are dwindling. Of decline monarch butterflies in North America to disappear bumblebees in Europe there is growing evidence that insects are in rapid decline. This should worry us all, because insects are extremely important; they serve as food for countless larger creatures such as birds and bats, they control pests, recycle nutrients, help keep the soil healthy and pollinate three quarters of the crops we grow. Without insects, life as we know it would come to an end.
The causes of insect decline are many, with habitat loss and industrialization of world agriculture leading the way, aided by the increasing use of pesticides, depredations of invasive species, increased light pollution, and more. Populations of many insects are now much smaller than in the past, and most now exist in “islands” of habitat – fragments of their preferred habitat surrounded by inhospitable terrain.
Unfortunately, climate change will probably be the last straw for some. Of course, the climate has changed in the past, many times, and the insects have survived. Indeed, insects have gone through all five previous mass extinction events on our planet (although many individual species must have been lost). However, this time is different. In the past, climate change was generally much more gradual and species could gradually shift their ranges in response, moving poleward as the climate warmed and returning equatorward as it cooled. . Such movements were once easy, as there were large expanses of suitable habitat. Today, to move towards the poles, insects must somehow jump from one patch of habitat to another, crossing roads, landscapes filled with polluting factories, arable fields sprayed with insecticides, housing estates and other man-made alien landscapes.
Insects do not live in isolation, but depend on particular plants to provide food for their larvae, or nectar and pollen for adults. If these plants have not traveled before them – for example via blown seeds in the wild – then they cannot survive, so entire communities of organisms must make that journey. It seems that they fail to do so. Studies of bumblebees in North America and Europe have found that they disappear from the southern edges of their range as the climate becomes too warm for them, but the northern edges of their range have not moved north as one would expect. It has been described as a climatic “vice”, with bee ranges being gradually squashed from the south. Some mountain bumblebees in Colorado and Spain have responded to global warming by moving higher into the mountains, but eventually they will run out of mountains and have nowhere to go.
Studies of these mountain bees have also revealed that climate change can disrupt the seasons in subtle but important ways. Many insects plan their life cycles carefully to ensure that periods of activity coincide with the availability of a vital resource. As the climate warms, different organisms may use different signals to time their life cycles, leading to mismatches. For example, some mountain plants in Colorado now flower before the bumblebees feeding on them come out of hibernation, which means the plants are unpollinated and produce few seeds.
Bumblebees seem to be particularly sensitive to warming as they are creatures found primarily in cool, temperate climates. Their large size and fur coats are adaptations for keeping warm in cool conditions, and they literally overheat in hot weather. Above about 30 degrees Celsius (30oC), most bumblebee species seem unable to sustain prolonged activity. Recent heat waves that have plagued much of Europe (even the UK) temperatures have exceeded 40°C, preventing bumblebees from harvesting food for long periods of time.
Are bumblebees an exception? Many other insects are more “thermophilic” – heat loving – and thrive in hot climates. For example, many butterflies are at the northern edge of their range in the UK, so we can expect them to benefit from a warmer climate. To test this, the charity Butterfly Conservation analyzed changes in the populations of 46 species of butterflies which all reach the northern edge of their range in the UK – the species you would expect them to take advantage of the warming. Between 1970 and 2000, three-quarters of these species have declined significantly. The pattern differs between sedentary habitat specialists (difficult species with very specific and not very mobile needs, comprising 28 species) and generalist, highly mobile species (18 species). Of the habitat specialists, 89% had declined, while only half of the generalists had declined and a few, including nuisance species, thrived. This gives us a clue as to why global warming has so far not benefited even heat-loving butterflies. Mobile GPs can more easily move in response to warming and are more likely to find a place where they can survive when they get there.
Of course, climate change is not just a matter of slight increases in temperature. The increased frequency of extreme weather events, such as droughts, heat waves, wildfires, storms and floods, which are likely to become more frequent and more extreme in the future, may have more d impact on wildlife. We have no idea what impact this will have on insects, but of course very few will be positive. Fires will obviously kill insects, although the spurt of new flowers that follows fires in some ecosystems would benefit. Summer storms are likely to batter delicate adult insects such as butterflies, and flash floods are likely to destroy underground nests of creatures such as bumblebees. Drought causes water-stressed plants to stop producing nectar in their flowers, which will certainly harm pollinators. During prolonged droughts, plants wilt and become unpalatable to caterpillars – for example, in the hot British summer of 1976 many caterpillars of the Adonis blue butterfly died as their food plant, horseshoe vetch, shrivelled in the heat. As a result, adult numbers were much lower the following year and some populations disappeared. With the UK currently experiencing the worst drought since 1976, it’s a safe bet that 2023 will be a particularly bad year for butterflies.
While climate change is undoubtedly bad news for many insects, there is no doubt that some are thriving. Ironically, these tend to be undesirable, from a human point of view. Adaptable and mobile species that can thrive in urban landscapes, such as houseflies and mosquitoes, are on the rise. For example, the yellow fever mosquito (Aedes aegypti) appears to have adapted well to urbanization and thrives in cities, breeding in clogged gutters, discarded tires, barrels, buckets, and other human waste that traps puddles. It is one of the main vectors of several nasty diseases, including dengue fever, chikungunya, Zika fever and of course yellow fever, as the name suggests.
The Anopheles The mosquito, the main vector of malaria, also benefits from the spread of human activities. Cases of malaria tend to become more frequent in areas where forests are cleared for agriculture, as the mosquito likes to breed in sunny puddles and ditches, which it finds difficult to find in dense forests. Climate forecasts suggest that malaria is likely to spread to high altitude tropical regions, for example in Colombia, Kenya and Ethiopia. These regions are densely populated by humans in part because, until recently, they were largely free of malaria. The southern states of the United States, southeastern Europe, parts of China, and densely populated areas around São Paulo and Rio de Janeiro in Brazil are all likely to become malaria-prone by 2050. America, as far north as southern Canada.
There are no simple solutions to these problems. Clearly, we must make the fight against climate change humanity’s most urgent priority, both for the sake of our planet’s amazing biodiversity and for our own well-being. Preserving as much nature-rich habitat as possible and attempting to connect habitat patches can help insects and other wildlife cope in the meantime.