Decision

Defra’s Chief Scientific Adviser’s advice on the use of Cruiser SB for sugar beet in 2024

Updated 18 January 2024

Applies to England

Gideon Henderson, Defra Chief Scientific Adviser, 12 December 2023

This is the fourth year in which I have given advice regarding emergency authorisation of Cruiser SB. My advice this year is largely similar to that offered in late 2022, with the addition of consideration of new field data collected in 2022 and 2023.

1. Background on active substances in question

Cruiser SB is a coating for sugar-beet seeds that contains the active substance thiamethoxam. This chemical, and its breakdown product clothianidin, are neonicotinoid pesticides. There is clear and abundant evidence that these neonicotinoids are harmful to species other than those they are intended to control, and particularly to pollinators such as bees. The general ban on use of these chemicals for pest control is well justified scientifically and environmentally. In that context, it is ecologically encouraging that water monitoring data indicate reducing concentrations of both these neonicotinoids since their widespread use was banned in 2016. Following the lowest values yet recorded in 2022, the Health and Safety Executive (HSE) states that in “data available so far for 2023, concentrations are low and reflect a trend for decreasing levels over time”.

The emergency authorisation (EA) application for use of Cruiser SB is in the context of this general ban. It is seeking use only for the limited and specific application of thiamethoxam on sugar beet in England for the 2024 crop. This is the fourth consecutive year that such an EA has been sought. The EA was granted in the three previous years, but Cruiser SB was only used in 2022 and 2023 (the proscribed threshold for use was not exceeded in 2021).

2. Nature of the application and existing environmental risk assessment

The emergency authorisation (EA) proposes use of thiamethoxam only in a prescribed manner, intended to minimize environmental impact. These limitations are identical to those applied for use of Cruiser SB in 2023:

• the use of a seed coating rather than a spray, and the drilling of coated seed directly into soil
• limitations on application rate per hectare
• use of herbicides to prevent flowering weeds during growth of the sugar beet, and the fact that sugar beet is harvested without flowering
• a 32-month ban on subsequent planting of flowering crops on the same field
• a 48-month ban on subsequent use of Cruiser SB on the same field
• use only if the predicted incidence of virus yellows during the growing season is sufficient to cause at least as much economic loss as the additional costs associated with use of Cruiser SB

Given these limitations on use, HSE judges many environmental risks to be acceptable, but consider there are still risks to bees.

3. Risk to bees

Thiamethoxam and clothianidin are toxic to bees, leading to acute mortality, chronic mortality (i.e. death due to exposure over a number of days), or damaging sub-lethal effects, depending on the exposure level. For this risk to be realised, there must be a pathway by which bees are exposed to one or both of these chemicals, at levels which have been assessed to be harmful by previous experiments.

HSE considers the risk to bees of exposure from the sugar beet crop itself as acceptable. They also consider that risk from the breakdown product, clothianidin, does not need to be assessed separately from that of thiamethoxam. Past publications indicate that clothianidin production is low when using seed coatings rather than sprays (Hilton et al. 2019), and low levels of clothianidin were found in pollen and nectar from flowering crops grown after use of thiamethoxam coatings on drilled seeds (Peterek 2020).

The remaining risks to bees come from three pathways of possible exposure to thiamethoxam:

3.1 Chemical transport to field margins and incorporation into flowering plants

Recent research assessed the chronic toxicity of thiamethoxam to bees (Wilkin – cited in HSE report 2022) and led HSE to indicate, last year, that the risk to bees from this pathway is acceptable. That conclusion was supported by the absence of detection of high thiamethoxam in measurements made on field margins following use of Cruiser SB 2022, though the limits of detection for those measurements were too high to add significant useful information. New measurements made on field-margin soils, vegetation and pollen around fields sown with Cruiser SB in 2023 were made with a technique with a lower limit of detection. These continue to support the absence of risk from field-margins.

3.2 Flowering crops planted following use of Cruiser SB

This has been the primary pathway of concern to HSE and has prevented them recommending approval of EAs for Cruiser in all four years they have been sought. This year’s HSE assessment follows that of last year, citing a submitted study into chronic toxicity by Wilkin (2022). HSE concludes that likely exposure levels of bees to thiamethoxam are 7.5 times lower than that required for chronic toxicity, and 2.6 times lower than that required for sub-lethal effects. Although exposures are lower than toxicity levels, HSE looks for a factor of 10 times lower to give a margin of safety and, as this factor is not reached, have concern about risk from this pathway.

HSE are explicit that they do not take into account the 32-month delay on planting of flowering crops imposed by the EA. HSE calculates exposure levels from pollen and nectar for crops planting at 12 months (Peterek 2020), because no study has directly measured thiamethoxam levels in flowering crops planted at 32 months. It is known, however, that thiamethoxam breaks down in field conditions with a DT50 (i.e. time for the concentration to half) varying by soil type and conditions. Reported values for DT50 average 16 days for European soils (Hilton et al. 2019), and 75 to 109 days for UK soils (Jones et al. 2014). Taking a precautionary approach, and using the longest DT50 value reported (172 days), thiamethoxam concentrations are expected to decline by a further 11.2 times between 12 months and 32 months. For flowering crops planted at 32 months, exposure levels of bees to thiamethoxam are therefore expected to be 84 (i.e. 7.5 x 11.2) times lower than that required for chronic toxicity, and 29 times lower than that required for sub-lethal toxicity. The additional breakdown of thiamethoxam is expected to lead to an acceptable level of risk to bees from flowering crops planted at least 32 months after use of Cruiser SB.

3.3 Guttation fluids

The fluids some plants excrete on their leaf edges are a possible pathway for bees to be exposed to pesticides, even for non-flowering plants. HSE agrees with the European Food Safety Authority (EFSA 2018) that the risk to bees from thiamethoxam residue in guttation fluids of the sugar beet itself are low. This is supported by other literature which has indicated that exposure of bees to neonicotinoids dissolved in guttation fluids of coated sugar beet seeds is very unlikely (for example, Wirtz et al. 2018)

There is also possible risk from guttation fluids on crops that succeed sugar beet on fields drilled with Cruiser SB. This risk is not mitigated by the 32-month exclusion on planting of flowering crops. HSE assess this risk using information for maize as a succeeding crop planted at 12 months (Peterek 2020). Their calculation suggests exposure 7.8 times lower than the chronic toxicity level, and 2.8 times lower than the level for sub-lethal effects (i.e. factors that are lower than the value of 10 used by HSE to provide a safety margin). These exposure estimates are likely to be too high because of two factors:

• maize is only grown in 5% of cases as a succeeding crop, with wheat and barley the most commonly planted follow-on crops after sugar beet. Wheat and barley produce lower levels of guttation fluid than maize so exposure levels from these crops are likely to be lower than those from maize
• the calculation assumes that all water uptake for bees comes from guttation fluids similar to that of maize

These factors make expected exposure of bees from guttation fluids on succeeding crops lower than those calculated by HSE for maize (i.e. further below the levels required for chronic or sub-lethal effect).

4. New field and field-margin measurements during use of Cruiser SB

The use of Cruiser SB on some sugar-beet fields during 2022 and 2023 has allowed new measurements of concentration of thiamethoxam and clothianidin in soils which may lead to an improved ability to assess risk from use of Cruiser SB. The data sets are small (based on five fields in each year), and detection limits of measurements made in 2022 were unhelpfully high.

Drawing new understanding about risk from Cruiser is not yet possible, but some observations about the measurements are:

1. Clothianidin is above detection limits in baseline (i.e. pre-drilling) measurements at most measured sites in both years. Clothianidin has a longer DT50 than thiamethoxam, but its presence is still surprising and may suggest clothianidin breakdown from historical use in these fields is even slower than measured in previous studies (particularly in finer grained silty and clay soils).
2. Clothianidin concentrations show no statistically significant changes in either fields or field-margins following application of Cruiser SB. This observation is expected because of the low production of this byproduct during drilling. The new data is not yet convincing, however, because of the high limit of quantification in 2022 data and because of the lack of time for breakdown of thiamethoxam in 2023 data. Further measurements on soils from fields measured in 2022 and 2023 will be important to test the expectation that clothianidin production is low following use of Cruiser. This is particularly important given the slow breakdown of this chemical.
3. Several measured fields had detectable levels of thiamethoxam before drilling. This is puzzling and, assuming the measurements are accurate, suggests either a previously unrecognized source of thiamethoxam on these fields in the recent past, and/or that thiamethoxam breaks down on some fields very much more slowly than reported in previous studies.
4. Thiamethoxam concentrations increase post-drilling in 2023 data, as expected given the use of this chemical. Planned post-harvest measurements (and measurements further in the future) will provide information about the rate of breakdown of thiamethoxam in these specific soils. If breakdown is found to have a DT50 > 308 days (i.e., significantly longer than 172 days – the longest DT50 reported so far) then the levels found in soils may be less than 10 times below that at which sub-lethal effects on bees are expected. This would suggest re-evaluation of the period for exclusion of flowering crops.

Despite some problems with the measurement approach in 2022, the data collected in 2022 and 2023 indicate the utility of this research. These measurements, and others of field-margins and surface waters, should be continued to provide more robust assessment of risks from use of Cruiser SB.

5. Alternatives to use of Cruiser SB

This is the fourth successive year that an EA has been sought for use of Cruiser SB.

Last year a new partially resistant variety of sugar beet (Maruscha KWS) was available for the first time, but has a low yield and is not favoured by growers. There is active assessment of use of alternative foliar sprays, but this appears unlikely to have a substantial impact in the near term. A longer-term solution is likely to be available by use of gene-editing to produce a precision-bred sugar beet with resistance to virus yellows but it is estimated this will take at last five years to develop.

6. Setting the threshold for use of Cruiser SB

If the EA is granted, it is important that Cruiser SB is only used when there is evidence that growers might otherwise face economic losses. Setting a stringent threshold for use of Cruiser SB will enable its use only in years when clearly beneficial.

In years when winters are cold, aphid populations and the spread of virus yellows are low and use of Cruiser SB is not beneficial (for example, it may cost more than use of alternative pest control approaches). A model to predict incidence of virus yellows (Rothamsted) has been used to set the threshold for planting of Cruiser SB in the last three years, and is suggested again this year. It is now possible to assess the behaviour of this Rothamsted model; the relationship between the incidence of virus yellows it predicts, and the incidence subsequently observed. The model dramatically overestimates observed incidence at low values of incidence. This is likely to be because use of foliar sprays is able to control aphids in years where incidence is low. As incidence increases, foliar sprays become less effective, and the predicted incidence is closer to observed incidence.

Better understanding of the accuracy of the Rothamsted predictions now enable a more informed choice of the predicted level at which use of Cruiser SB is financially beneficial. I have assessed the approach used by Defra’s pesticide team to calculate the breakeven point above which use of Cruiser SB is expected to be economically beneficial (which is similar to that used last year). I support the calculated value of 64% predicted incidence as the breakeven point based on all available data. The threshold for use should be set at or above this breakeven point.

7. References

(Hilton et al. 2019). Hilton, M.J., Emburey, S.N., Edwards, P.A., Dougan, C. and Ricketts, D.C., ‘The route and rate of thiamethoxam soil degradation in laboratory and outdoor incubated tests, and field studies following seed treatments or spray application’. Pest management science, 75(1), 2019, pp. 63–78.

(Peterek 2020). Peterek, S., Collison, E., Ortoli, V. and Faure, A., ‘Practical and regulatory experience in the conduct of bee residue trials’. Julius-Kühn-Archiv, (465), 2020.

(Jones et al. 2014). Jones, A., Harrington, P. and Turnbull, G., ‘Neonicotinoid concentrations in arable soils after seed treatment applications in preceding years’. Pest management science, 70(12), 2014, pp. 1780–1784.

(EFSA 2018). ‘Conclusions on the peer review of the pesticide risk assessment for bees for the active substance thiamethoxam considering the uses as seed treatments and granules’. EFSA Journal 16(2):5179, 2018, pp. 59: https://doi.org/10.2903/j.efsa.2018.5179.

(Wirtz et al. 2018). Wirtz, I.P., Hauer-Jákli, M., Schenke, D., Ladewig, E., Märländer, B., Heimbach, U. and Pistorius, J.,‘Investigations on neonicotinoids in guttation fluid of seed treated sugar beet: Frequency, residue levels and discussion of the potential risk to honey bees’. Crop Protection, 105, 2018, pp. 28–34.