Scottish winter oilseed rape cultivation 2015-2016: impact of the second year of neonicotinoid seed treatment restrictions

Discussion

This is the second of two consecutive surveys collecting information about winter oilseed rape cultivation from a large, representative sample of Scottish farmers. In the first survey in this series, the neonicotinoid restrictions made the control of insect pests challenging, both practically and economically, for some growers. The lack of an insecticidal seed treatment resulted in increased use of foliar insecticides and some CSFB related crop loss was encountered ⁽⁸⁾ . However, other growers experienced little or no impact, and the overall effect on Scottish cultivation was less severe than reported elsewhere in the UK, particularly in the South and East of England.

In the first year of the restrictions, climatic conditions were favourable for oilseed rape cultivation and autumn pest pressure was generally low to moderate. This second survey was conducted to help assess the effect of the restrictions, under different conditions, and upon growers who have gained some experience of WOSR cultivation without insecticidal seed treatments.

It was reported, by the majority of survey participants, that climatic conditions were generally unfavourable for the cultivation of Scottish WOSR crops harvested in 2016. The late harvest of 2015 crops, followed by a period of wet weather, led to problems with autumn drilling, and as a consequence some recruited participants were unable to grow WOSR in 2015/16. Those growers that did sow, drilled later than they had in the previous year. Poor climatic conditions were reported to have continued throughout the season. Growers reported a range of adverse weather affecting crop development, including: a wet autumn and winter, cold spring conditions, lack of sun in spring and summer and high winds prior to harvest causing seed shed in many areas.

The Scottish census area of WOSR declined by 14 per cent between 2015 and 2016 ⁽⁶⁾ and reductions in Scottish winter crop areas, both cereals and oilseeds, were partly attributed to the aforementioned late harvest and rainfall ⁽¹⁴⁾ . Although, it was also noted that changes to the EU Common Agricultural Policy ( CAP) support schemes, particularly the crop diversification requirement, may have influenced variation in the areas of crops sown ⁽¹⁴⁾ . A similar reduction in WOSR area, 11.2 per cent, was reported for the UK as a whole ⁽¹⁶⁾ . The AHDB 2016 planting survey stated that the key factor behind the lower 2016 oilseed rape area was economic, reflecting the prospect of historically poor market returns ⁽¹⁷⁾ .

In addition to reducing the area sown in Scotland, the adverse climatic conditions at crop sowing and establishment may also have negatively influenced pest populations. Reported aphid and flea beetle levels were significantly lower than in the previous survey. Reduced CSFB incidence was associated with significant reductions in autumn insect damage compared with the previous year. No CSFB-related crop loss was encountered in autumn 2015, in contrast to the one per cent of the sample lost in autumn 2014. Although the area re-drilled overall, to replace crop lost to a combination of slug, geese and weather related damage, was similar to the previous survey. Incidence of TuYV symptoms, transmitted by aphids, was also low but similar to the previous year.

The crop damage data collected in this survey was comparable to that reported by the ADAS cabbage stem flea beetle incidence and severity monitoring ⁽¹⁸⁾ . This monitoring was based on agronomist inspection of 62,000 ha of UK crops, 2,240 ha of which were in Scotland. Despite the differences in survey methodology, agronomist inspection of crops versus grower reports of pest damage, the results were similar. ADAS reported no CSFB flea beetle related crop loss in Scotland and the majority of crops were assigned to no or low damage categories (>90 per cent of crops) ⁽¹⁸⁾ .

At a UK level, the crop loss attributed to CSFB grazing by the ADAS survey also decreased between the two seasons; from five per cent in autumn 2014 ⁽¹⁸⁾ to one per cent in autumn 2015 ⁽¹⁹⁾ . In both years, UK CSFB damage varied with geographic area and the highest losses were in the English Eastern and South East regions, which reported 2 and 1.7 per cent crop loss respectively in 2016. As in this survey, crop loss unrelated to insect damage was also encountered, with an additional 3.1 per cent of the sample re-drilled, primarily in response to slug grazing ⁽¹⁸⁾ .

The decreases in Scottish pest pressure and damage levels reported in this survey were associated with significantly reduced insecticide use, with sprays primarily focussed on CSFB control. Spray regimes in autumn 2015 were very similar to pre-restriction levels. However, there are no data on comparative pest pressure between these two seasons and 38 per cent of growers still stated that they incurred more damage during autumn 2015 than when neonicotinoid seed treatments were available. The insecticides encountered were almost exclusively pyrethroids. Despite the presence of pyrethroid resistance in UK populations of CSFB, only one grower reported concerns about pesticide efficacy.

UK use of insecticides was also reported to primarily consist of pyrethroid compounds, although around 9 per cent of agronomists reported using Insyst (acetamiprid), which had an emergency use authorisation for CSFB in autumn 2015 ⁽¹⁸⁾ . However, in line with lower pest pressure and damage, Scottish pesticide use was lower than reported for the UK crop, where 75 per cent of the crop was reported to be treated with an insecticide, with an average of two sprays applied overall ⁽¹⁸⁾ . In contrast, 44 per cent of the Scottish crop surveyed was treated and received, on average, 0.5 sprays.

There was a significant decrease in WOSR yield in 2016, both in this survey and at a Scottish and UK level (19, 20 ⁽¹⁵⁾ and 21 ⁽¹⁶⁾ per cent respectively). Amongst our survey participants, this decrease was primarily attributed to climatic factors, with other factors such as late drilling, clubroot, competition from weeds and pest damage (including CSFB) playing a minor role. The role of weather conditions was echoed at a Scottish census level ⁽¹⁵⁾ . In relation to the UK yield, ADAS reported that there were a number of contributory factors, including a range of climatic factors, high weed pressure, high pest pressure (slugs and, in the South East, CSFB), high disease pressure and, in some cases, the influence of short rotations ⁽²⁰⁾ .

Overall, despite 2016 being sub-optimal year for Scottish WOSR production, this survey corroborates the findings of the first survey. Under current conditions, the neonicotinoid restrictions have introduced additional challenges for some WOSR growers which will have an economic impact on their operation. In addition, in both surveys, a small proportion of growers stated that these restrictions reduce the likelihood of their growing the crop in future. However, other growers appear to be relatively unaffected and it is clear that the impact of the restrictions are far less severe in Scotland than in other regions of the UK which have greater pest pressure and levels of resistance to approved foliar insecticides.

Whilst both pest pressure, and in particular resistance occurrence, may change with time, the current situation gives Scotland an advantage in this period of uncertainty in relation to pesticide approvals. It appears that Scottish growers can, on the whole, continue to successfully cultivate WOSR whilst research and guidance about alternative control strategies is formulated. ADAS, funded by AHDB, is investigating the factors influencing UK CSFB population dynamics and damage potential, including the relationship between adult and larval infestation and resultant yield impact ⁽²¹⁾ . This project is also investigating the development of non-chemical control strategies for CSFB, such as; varietal susceptibility, use of oilseed rape volunteers as a trap crop and targeted defoliation to stop larvae moving from leaves to stems ⁽²¹⁾ . This research will help to create an integrated pest management ( IPM) strategy for CSFB control which can be transferred to agronomists and growers.

There also appears, in this second survey, to be an element of adaptation to the restrictions amongst Scottish growers. Growers reviewed the impact of operational changes to crop cultivation, and made fewer changes in this second season. Growers also made greater use of action thresholds and published data sources when making pest management decisions and displayed a better knowledge of the pest species present on their crops. In this survey we also encountered a greater number of growers implementing more targeted pest control regimes, leaving some areas untreated rather than spraying the whole crop area. The relative success of all of these inputs, alongside the results of future research, can be used by growers to determine which actions should be adopted in future IPM strategies for their crops.

Currently, no follow up surveys on Scottish WOSR crops are scheduled. However, SASA's pesticide survey unit will continue to monitor the situation and reassess this position if the evidence suggests future monitoring is necessary. It should also be noted that the 2016 pesticide use on Scottish arable crop survey, which will be reported in autumn 2017, will provide comparative national estimates of insecticide use on oilseed rape between the crops harvested in 2014 (sown prior to the restrictions) and 2016. These arable pesticide use surveys are routinely conducted every two years.