Investigation of the Impact of Changes in Pesticide Use on Invertebrate Populations

Abstract

This report summarises recent changes in pesticide use (2005-2012) in the GWCT's Sussex Study, considers the use of seed treatments for the first time, and examines the effect of this pesticide use on invertebrate food resources for farmland birds. This work covers a period that has seen both the expansion of agri-environmental management directed towards reversing the trend of declining farmland flora and fauna as well as changes in pesticide availability due to legislation and agrichemical development. The analysis on which this report is based draws on a unique dataset, the GWCT's Sussex Study, which has monitored both the farming decisions and the cereal ecosystem on 62 km² of the Sussex Downs since 1970. This study is the longest running cereal ecosystem monitoring exercise in the world and collates information on cropping, pesticide use, cereal weeds and invertebrates. Results from the analysis of this dataset allow long-term changes in crop management and the effects of these changes on cereal ecosystem biodiversity to be assessed. Two earlier reports have examined changes in pesticide use and the effect of this use on the food resources of farmland birds (Ewald & Aebischer, 1999 and GCT, 2007).

When the entire time span of the Sussex Study (1970 to 2012) is considered, there have been long-term increases in every measure of foliar and residual pesticide use (herbicides, fungicides and insecticides), including the intensity of use. Considering the recent time period, however, we found no significant changes from 2005 to 2012 compared to the period from 1970 to 2004 in the overall use or intensity of use of pesticides in Sussex (Ewald & Aebischer, 1999 and GCT, 2007). This stabilisation reflects changes in cropping on the study area, with recent declines in the area sown to winter wheat and an increase in spring cereals and break crops. On average, over half of all winter cereals and break crops planted since 2005 on the study area were treated with seed treatments containing neonicotinoids. We did not find that the use of neonicotinoid seed treatments in winter cereals reduced the number of subsequent foliar insecticide applications; in fact we found that, conversely, winter cereals treated with neonicotinoids were more likely to be treated with foliar insecticides. This may reflect either farmer risk-aversion or timing of the sowing of crops.

We examined the trends in the average annual abundance of six invertebrate groups and three chick-food indices over the 43 years of the Sussex Study considered here. Of these, all six invertebrate groups and three chick-food indices declined in the early part of the Sussex Study, in concert with the advent of foliar insecticide use across the area. Two invertebrate groups, Carabidae & Elateridae and Aphididae, have declined over the whole of the Sussex Study, with no evidence of a recent recovery. Four of the six invertebrate groups and all three chick-food indices have shown some signs of recovery in abundance, with the abundance of three groups in particular, Araneae & Opiliones, Chrysomelidae & Curculionidae and Non-aphid Hemiptera, increasing over the past ten years.

The main finding of this work reinforced that of previous assessments of the Sussex Study dataset: foliar insecticide use, adjusted for the use of other types of pesticides, is associated with significantly lower abundances of all groups of chick-food invertebrates. Additionally, the use of foliar insecticide is associated with a carry-over effect in the year following an application, with the abundance of seven of the nine chick-food invertebrate groups examined significantly lower. With regard to seed treatment, aphid abundance was negatively affected by neonicotinoid seed treatments, with seed treatments as a group negatively impacting the abundance of four other chick-food invertebrate groups. The overarching negative effect of foliar insecticide applications remained when controlling for seed treatments. Our results indicate that foliar insecticide applications are more of a threat to the abundance of chick-food invertebrates examined here than the use of neonicotinoid seed treatments, in a cereal ecosystem. The role and use of neonicotinoids should be considered in light of the wider suite of evidence, including their potential impact on the main groups of pollinators not monitored in this study.