Drivers of change in cereal insects: Climate and pesticide use

Dr Julie Ewald, Head of Geographical Information Science, with Chris Wheatley, Steve Moreby, Dick Potts and Nicholas Aebischer

Sussex studyThe GWCT’s Sussex Study is the longest-running cereal ecosystem monitoring exercise in the world. It started in 1968 to investigate the decline of grey partridges (Perdix perdix) on the South Downs. It encompasses a study area in West Sussex of approximately 62km2 of arable/grass farmland. Since 1970, information on cereal invertebrate abundance, occurrence of arable flora in cereal crops, cereal growth stages and cereal inputs and management has been collected, in addition to information on changes in grey partridge numbers.

The habitat data are all collated within a Geographical Information System (GIS), where changes in non-cropped habitats (boundaries, etc) have also been recorded in detail. It therefore offers a unique opportunity to examine changes in invertebrate abundance in relation to climate, landscape attributes and land management practices, including the application of both foliar pesticides and seed dressings. Over the past two years, Natural England has provided funds to allow the analysis of this dataset, focusing on two topical drivers of change in cereal invertebrates, namely the effect of climate change and pesticide use. In both cases we considered local/short-term effects and widespread/long-term effects.

Short-term drivers of change in cereal invertebrates

Extreme weather events

One proposed effect of climate change is an increase in the frequency and intensity of extreme weather events and the effect that these extreme events may have on wildlife. Examining the response of cereal invertebrates to extreme weather events in the past will give some indication to what might be possible in the future. We used weather variables (average temperature and overall rainfall) in spring/summer (April, May, June) to identify hot/dry and cold/wet years across the available time series from the Sussex Study (1970 to 2011). From this we identified six years that fell into the hot/dry category (1976, 1984, 1989, 1995, 2007 and 2008) and four that fell into the cold/wet category (1972, 1986, 1991 and 1996) – note that we did not have access to weather variable data from 2012 or 2013 at the time, both of which would likely have been classified as cold/wet.

We compared changes in abundance of 28 invertebrate taxa (common taxa that were found, on average, in over half the insect samples in a year). Indices in abundance change were calculated for each invertebrate time series, together with 95% confidence intervals to give a measure of variability year-to-year. Change in indices were compared for cold/wet, average and hot/dry years to determine if an extreme weather year was associated with higher than expected change in abundance. We found that the abundance of eleven of 28 taxa were affected by these extreme events, with the general pattern being that hot/dry years were associated with increases in abundance and cold/wet years with decreases in abundance. Cereal invertebrate abundances very quickly returned to the long-term trends, usually within the year following an extreme weather event, with thrips taking the longest time (1.5 years) following a cold/wet event.

We looked to see if any habitat variables such as the connectivity of field edges, amount of natural habitat around a field, size of field, or aspect and elevation were related to either the amount of change in invertebrate abundance in response to an extreme weather event or the ability of invertebrates to return to pre-event levels. The only variable that showed up across several taxa was aspect, with invertebrate abundance on sites with a west-facing aspect less susceptible to cold/wet extremes.

Foliar pesticide use at a field level

Another short-term driver of change in invertebrate abundance is the use of foliar pesticide applications. We have collected data on pesticide applications from 1970. Herbicide use on arable crops was widespread on the Sussex Study area before 1970. Fungicide use became ubiquitous starting in the late 1970s, and by the 1980s more than 90% of all arable crops on the area were treated with these chemicals. Insecticide use began in the 1970s and increased steadily, although over half of all arable crops are still not treated with insecticides. We examined changes in the abundance of invertebrate groups important in the diet of farmland birds (spiders and harvestman; ground and click beetles; sawflies, butterflies and moths; leaf beetles and weevils; plant bugs/hoppers, excluding aphids; aphids) and three indices of chick food abundance (indices for grey partridge, corn bunting and yellowhammer chicks). As in previous analysis, insecticide use was particularly damaging and this extended into the year following treatment, i.e. there was a negative carry-over effect with the use of organophosphate and pyrethroid insecticides particularly damaging to invertebrate abundances.

Long-term drivers of change in cereal invertebrates

We used long-term trends in yearly weather (average spring/summer temperature and total spring/summer rainfall as defined above) and intensity of pesticide use (average number of times treated with herbicides, fungicides and insecticides) for the Sussex Study from 1970 to 2011 to compare the effect of changes in weather and agricultural intensification on the long-term trends in invertebrates. Long-term trends in some invertebrate taxa, for instance plant/bugs and hoppers, were driven chiefly by changes in weather, increasing with rising average temperatures in spring/summer.

Others, particularly beetles, were driven by increases in agricultural intensification, measured as increasing use of pesticides. The long-term trends in other invertebrate taxa were driven by both weather trends and agricultural intensification. Aphids decreased with higher rainfall events while spiders increased with temperature, but both of these groups declined with increasing use of pesticides. The long-term trend in the abundance of flies was unaffected by either weather or pesticide use.

Further work

In addition to collecting information on foliar pesticide applications on a field-by-field basis, we have also collected information on seed dressings used throughout the Sussex Study area and are currently writing up analysis on the abundance of chick-food invertebrate groups in fields in relation to both foliar pesticide applications and seed dressings, exploring the effect of neonicotinoid seed dressings. Results for this are expected shortly.

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