Written by Henrietta Appleton, Policy Officer (England)
Scorpion Fly
This week is insect week and on the back of the recent publication of a GWCT scientific paper on invertebrate abundance changes over a 50-year period at the Sussex study site1, it led me to explore the fascinating world of insects – all over again!
I don’t know about you but as a child I would try and catch butterflies or moths, earwigs, dragonflies and grasshoppers (probably to their detriment!), reflecting one’s innate inquisitiveness at that age, but also quite probably because they were in evidence in our garden or at the stream edge at the bottom of the neighbouring field. Would I see these insects in such abundance now?
Quite simply, you would assume not, given the headlines of so-called ‘Insectaggedon’, due to agricultural intensification (and concerns about the decline in pollinators in particular), climate change and the impact of light pollution. Whilst some scientists have contested that ‘Insectaggedon’ is not the reality, overall insect biomass has declined across all habitats.
But within this trend there are winners and losers, even within the pollinators. Recent biodiversity indicator figures for England show that hoverflies continue to decline, whilst wild bee species have actually increased in the last 10 years2.
These differences will reflect a number of factors including their behaviour and preferred habitats – whether they occupy urban spaces, woodland, the field edge/hedgerow or cereal ecosystems where the focus on increasing yields and the use of insecticides (amongst other inorganic inputs) has had a detrimental effect on several important functional insect groups. Climate change will also be influencing insect abundance and causing changes to species assemblages.
For example, moths would be expected to decline given concerns about light pollution and agricultural practices, yet research has shown that extreme weather events in particular cause population changes3. As a result, short term studies and infrequent sampling risks presenting a distorted picture and so the data from the Trust’s long-term Sussex study is particularly important in helping us understand the complexities surrounding the changes in insect populations by sub-class/order and by habitat over time.
But what are the insect sub-classes (with thanks to the Royal Entomological Society website!) and what are their roles in our natural world?
There are 5 sub-classes; Apterygota, Palaeoptera and Polyneoptera are all wingless or winged insects whilst the so-called higher insect sub-classes are Paraneoptera and Endopterygota (more detailed descriptions, with examples, are given below). Within these there are orders usually reflecting differences in construction/appearance, classified further by family, genus and species. And then there are the common names which I will use here. All these insect orders (and below) will have roles to play in our natural world from pollination and biological control to recycling and waste clearance and animal feed and insect protein.
Let’s look at them and consider their roles in the environment.
Apterygota are primitive wingless insects with incomplete metamorphosis. Probably the most common one that you will come across is the silverfish which is considered a pest as it can cause problems in the home – this reflects their role as scavengers.
Palaeoptera are primitive winged insects which hold their wings upright or outstretched. This sub-class contains the widely known mayflies, dragonflies and damselflies. These insects, in their larval stages that live in water, are vital for the cleaning of our freshwater and consequently are an indicator of water quality. Dragonflies and damselflies are also important predators of other ‘pest’ insects such as mosquitos. All three are a food source for birds, bats, fish and frogs.
Polyneoptera are winged insects with a fan-like extension to their hind wings. There are many that you will recognise – grasshoppers, crickets, stick insects, stoneflies, earwigs and cockroaches to name a few. Whilst they are an important link in the food chain for birds, fish, frogs and rodents, you may not know that these insects create a balance between populations of plants through facilitating plant decomposition and regrowth. They break down plants and leaf litter and through their droppings return nutrients to the soil. They can also act as pollinators. Increasingly crickets are being seen as a protein source for humans.
Paraneoptera are higher insects where the nymph generally resembles the adult. This is a rather disparate order covering true bugs, sucking and biting lice, booklice and thrips. Lice are obviously parasitic and generally host-specific and have been used to understand our evolutionary biology, so from the environmental perspective true bugs and thrips are the most interesting.
Aphids are one of the best-known groups of true bugs as they are an agricultural pest and perhaps not surprisingly in decline.. The Sussex study data suggests, in June, a 90% decline in aphid numbers in cereal ecosystems (1970-2019) which, given they are at the base of the food chain, (you could call them “terrestrial plankton”), is a serious concern.
But given that insecticide use has dropped and populations have not recovered other factors must be involved. Research (including the Sussex Study) suggests that changes in temperature and rainfall are also affecting abundance. Some true bugs are less common such as the water measurer which is one of the ‘pond skaters’ that we often see on the water surface. All these have hydrophobic hairs which allow them to walk on water and are sensitive to vibrations which allow them to locate their prey or food sources.
These are at risk of climate change with the lesser water-measurer found only in the Broadland fens in Norfolk and at risk of sea water contamination. Thrips are considered a pest as some species can damage leaves, flowers and fruit, reducing yields whilst also spreading disease between plants. However many species eat fungal spores whilst others called flower thrips act as pollinators.
The final sub-class is Endopterygota or the Holometabola (note there is some discussion about the naming and grouping of these) which are insects that have a clear metamorphosis from larva via pupa to adult. This group includes the well-known orders of beetles, true flies, ants, bees and wasps, butterflies and moths, lacewings, fleas and caddisflies or sedge flies.
These insects have important roles in pollination and the recycling of organic matter through breaking down both animal and plant debris. However, some may also be regarded as pests, i.e. some beetles such as cabbage stem flea beetle, whilst others such as lacewings play a valuable role in keeping pests such as aphids under control. But what of the lesser-known orders in this class?
There are scorpion flies of which there are 4 species in Britain which feed on dead animals and occupy woodland edges and hedgerows and twisted wing flies which are one of the strangest insect groups and little known. They are parasitic with unique adaptations and in Britain their hosts are mainly ants, bees and wasps.
The point about this detail is that there is still much we need to know about our insects in order to understand their role in the natural world, the causes of decline (indeed whether there is a long-term decline in some orders/families/species) and how we can address these to prevent a break in the complex web of natural world links.
There has been much emphasis on pollinators but these are not the only insect ‘ecosystem engineers’ we have to conserve, particularly if we are aiming for a more sustainable management of our land uses, especially agriculture. This is where the Sussex study data is important.
Whilst there are other insect datasets that have been monitoring species consistently over the long term, Rothamsted, for example, has data on macro moth and aphid populations, the Sussex study is unique in looking at the insect assemblage within the arable environment. The value of this is two-fold. Firstly, it enables us to understand the impact of farming systems; not just pesticide use but cultivations and changes in cropping patterns as well.
Ironically many beekeepers are mourning the loss of oil seed rape, a casualty of the removal of neonicotinoids as a seed treatment, as this crop was good for supporting bee populations in early spring. Secondly, it enables us to understand the link between insect numbers and farmland bird populations. We have used our insect data from Sussex, Loddington, Royston and two estates in France to create a Chick Food Index (CFI) which allows us to estimate the survival and population change in three farmland birds.
Many of the insect groups affected by intensive agriculture are the food sources for young farmland bird chicks such as the grey partridge, yellowhammer and corn bunting – all red-listed. Our Sussex data shows, for example, that the CFI for yellowhammer declined by 85% from 1970 to 2019; a picture reflected in the 62% decline in yellowhammers (BTO data) over a similar time frame (1967 and 2020).
The conclusions from our work suggest it is time to broaden agri-environment scheme options away from purely farmland bird habitats and pollen and nectar mixes and consider how we can support a much broader range of functional insect groups through integrated management techniques. For example, it is of concern that our Sussex study data shows a significant decline in predators - both aphid-specific and polyphagous such as some carabid beetles – which need to be supported if we are to move to a more integrated approach to pest management.
Happy insect hunting!
(Note: These monthly musings are intended as interest pieces and to provoke thought. I do not pretend to have deep knowledge about the subjects I am covering or indeed to be comprehensive in my coverage. I hope you enjoy reading them as much as I enjoy researching and writing them.)
[1] EWALD2024 - Game and Wildlife Conservation Trust (gwct.org.uk)
[2] 10 Status of pollinating insects - GOV.UK (www.gov.uk)
[3] Scientists find no evidence for ‘Insect Armageddon’ – but there’s still cause for concern - News and events, University of York