By Prof John Holland and Dr Julie Ewald
The recent paper (Hallman et al 2017) on insect declines on nature reserves in Germany attracted a surprising amount of media attention last week. But then bad news always gets the limelight. Should we be worried? And is it good for farmers to have fewer pesky creatures around? Although there are a very small proportion of insect species that attack crops, overall, having a plentiful abundance and diversity of insects should be regarded as good thing. Many species provide vital services for agriculture – controlling crop pests, pollinating crops and recycling organic matter. They are also an essential component of the agricultural food web, being an essential dietary component for most farmland bird chicks; some mammals including mice, shrews, hedgehogs and bats; reptiles and amphibians.
The authors of this paper found a quite dramatic 76% decline. However, bear in mind that the findings are not definitive, and we should not assume that this has occurred everywhere, nor perhaps for all insect species. It is worth considering the results from our much more detailed studies in Sussex and Loddington. Unlike the samples taken in the German study, these studies are based upon sampling conducted in the same location every year, for a much longer time period (Sussex started in 1970 and Loddington in 1992). Additionally, our studies use a method that measures insect density rather than activity and includes identification of the insects, rather than just biomass as in the German study.
Change in the average number of invertebrates per sample from the Sussex Study. Invertebrates
collected from at least 100 cereal fields every year using a Dvac suction sampler.
In work published in 2015 (Ewald et al 2015), analyses of 26 insect groups showed that the abundance of 12 insect taxa declined, two increased and the remainder show variability, with some recovering then declining. Overall, though, there was a 35% decline in the total number of insects. This analysis also sought to determine whether changes in weather or pesticides were responsible for some of the observed changes in insects. Although hot/dry years and cold/wet years affected insect abundance, both positively and negatively, pesticide use alone or in combination with weather also had an impact.
Use of insecticides, fungicides and herbicides has increased over the duration of the Sussex Study, and all three types can have an impact on insects (Ewald et al 2016). Herbicides by removing the weeds upon which many plant feeding insects depend; fungicides by reducing fungi that some insects also feed on (e.g. Tachyporus species); and, of course, insecticides. Insecticides are designed to kill insects so we would expect them to have an impact, especially as there are very few selective products; even pirimicarb, which is “selective” for aphids, has an impact (Moreby et al 2001).
Thus, a combination of removing food and causing mortality or sub-lethal effects to insects will reduce their abundance.
Implications and the future
Importantly, in the Sussex Study two of the key groups of invertebrates that provide Integrated Pest Management (spiders and beetles) were most affected by pesticides. For some of these taxa, the sharpest declines in abundance occurred in the 1970s, and we know from previous analyses that the insecticides used in this period were particularly harmful (e.g. organophosphates, Ewald et al 2016). Since then many farmers are much more aware of the role of pest natural enemies and the impacts of insecticides. Alongside this greater awareness, there is also the introduction of agri-environment schemes and adoption of less intensive tillage, so things could be looking up for insects.
We shouldn’t be too complacent, though, as there are problems with modern agricultural systems. Insect populations can take time to recover, as many only have one generation per year. Some must remain unaffected if they are to rebuild their populations, so unsprayed areas in the landscape are needed. Block cropping doesn’t help in this respect, especially for the less mobile species, which includes many of the predatory pest natural enemies such as carabid beetles.
We also don’t fully understand the effects of spraying frequency, although evidence from the Sussex Study continues to show a carry-over effect into following years (Ewald & Aebischer 1999; Ewald et al 2016) and therefore we may expect that the more frequently insecticides are applied to a field, the less chance there is of recovery. Previous work that modelled the effect of insecticide treatment on sawfly numbers indicated that it would take seven years to recover numbers following treatment (Aebischer 1990).
As always in ecology, nothing is ever simple and insects are likely to be influenced by a multitude of factors. Of these, soil management and the loss of mixed farming are perhaps as important as changes in agricultural inputs. Ploughing is generally detrimental to insects, as some important groups overwinter in the soil. The switch to minimum tillage and increases in soil organic matter will foster a much richer insect community and provide benefits up the food chain (Holland 2004). Livestock and the associated pasture land and dung also play a role in supporting insects, so the loss of mixed farming in some regions will have had an impact. Finally, all of these changes are interrelated and have not happened in isolation, making it difficult to point to one smoking gun responsible for all of the changes in the flora and fauna of farmland that we have witnessed over the past half a century.
If we want to encourage insects then there are a number of actions that can be taken.
- Follow IPM guidelines and avoid using insecticides whenever possible.
- Don’t be too tidy. Try to tolerate a few more weeds in crops and gardens.
- Establish new habitats that are insect-rich, e.g. beetle banks, nectar-flower mixes, wild bird seed, herb rich grassland and ponds.
- Only cut hedges every two years to allow flowering and berry production.
- Avoid contaminating the hedgebase with fertilizer and pesticides.
- Adopt minimum tillage and return crop residues to the soil.
- Grow alternative livestock forage – legumes, especially sainfoin and whole crop silage.
For further guidelines and information on encouraging pest natural enemies, see the SAFE guidelines in the AHDB Encyclopaedia of Pests and Natural Enemies in Field Crops.