Their services in pollination and soil management make insects vital for agriculture. But farming also poses grave threats to them. We need to better maintain and restore biodiversity in farmed landscapes.
Ecosystems depend on insects to function properly. Plant eaters, which chew on leaves or suck plant sap, are just as important as predators that feed on herbivores, or – like parasitic wasps – lay their eggs in a host insect, where their larvae hatch and consume their hosts from the inside. Carrion-scavengers and dung-eaters consume dead organisms. Litter-decomposers break down dead plants, making it easier for microbes to work.
Pollinators are an important part of many agricultural systems. By carrying pollen from one plant to another, insects enhance seed set and facilitate the mixing of genes in both crops and non-cultivated plants. Three-quarters of the world’s most important crops exhibit a yield beneﬁt from pollinators: they contribute directly to around one-third of global food production.
Promoting wild bees – which are usually more important pollinators than honeybees – can double the yields of strawberries and cherries. Insects can be harmful as well as helpful. If they eat crops, instead of weeds, they can cause huge amounts of damage. Worldwide, insects are responsible for between 17 and 30 percent of crop-yield losses, especially in countries already afﬂicted by hunger and poverty.
Insects also cause a lot of damage to crops after the harvest: postharvest losses may be as high as 40 percent in developing countries. Just as insects affect agriculture, so too does agriculture affect insect populations. Alongside climate change and light pollution, the spread and intensiﬁcation of farming is by far the most important cause of the global decline in insect numbers.
Intensiﬁed production makes agricultural landscapes structurally much simpler. Overfertilization leads to monotonous communities of plants that provide habitats for only a few species. Pesticides kill insects both directly and indirectly. The frequent use of herbicides to control weeds reduces the diversity of plants and impoverishes the food webs of the insects.
Insecticides usually kill insects directly. But even if they are not lethal at ﬁrst, they can still prove deadly – by reducing insects’ vitality and reproductive ability, by harming their ability to ﬁnd their resources, and by increasing their susceptibility to diseases. Plant protection using chemicals has increased steadily since the 1930s in many industrialized countries, as well as in Latin America, Asia and Oceania.
In the 1960s, the crop-protection industry was valued at less than 10 billion US dollars, and farmers could choose among products based on around 100 different active ingredients. Today the sector is worth over 50 billion US dollars, and customers worldwide have a choice of about 600 different active ingredients. What is more, the number of chemical products in use around the world continues to increase. And, their negative effects on the insect world are also becoming more and more evident.
This is not just because a growing number of chemicals are being applied; the formulations are also increasingly effective and can be used more selectively. The nature of agricultural production and the structure of the agriculture landscapes can be optimised to hinder harmful insects and promote those that are beneﬁcial. Pests beneﬁt from monocultures and from the fact that the same crop is planted season after season.
A diverse range of crop types, long rotations (planting different crops each season) and small ﬁelds all help to sustain a diverse insect population and make it easier for farmers to maintain a balance between pests and beneﬁcial insects. A comparison of eight regions in Europe and North America shows that smaller ﬁelds lead to a marked increase in species diversity. This is because insects, birds and plants can take advantage of the wider range of resources that are available.
The edges of the ﬁelds are especially important, as they enhance dispersal across landscapes. Reducing the average ﬁeld size from around 5 to 2.8 hectares in a landscape has the same positive effect on biodiversity as increasing the proportion of near-natural habitats from 0.5 percent to 11 percent. It is not just how individual ﬁelds are managed, but even more so, it is the makeup of the whole landscape that is important for maintaining insect diversity.
This is because most insect populations are not conﬁned to small locations, but range over a wide area. For example, chalk heathlands are home to one-third more species if they are surrounded by a high percentage of near-natural habitats instead of predominantly arable ﬁelds.
Efﬁciency of management is higher in monotonous, cleared landscapes, because introducing hedges and wildﬂower strips have a signiﬁcantly greater positive effect on insect diversity than in colourful, variegated landscapes where such structural elements are common. Further, conservation measures are necessary across all regions, because the composition of insect populations may be radically different from one region to another.