Gideon Henderson, Chief Scientific Adviser, 5 December 2024

This is the fifth year in which I have given advice regarding Emergency Authorisation for use of Cruiser SB. My advice this year has many similarities to that offered in late 2023, but is informed by:

• the increasing timeseries of measurements from fields treated with Cruiser in 2022
• knowledge about uptake of Cruiser SB by farmers in the past 3 years, and the implications of their use for virus incidence

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. Data collected since last year continue this broad trend, though decreases in clothianidin concentration are slower and may now be less systematic.

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 2025 crop. This is the fifth consecutive year that such an EA has been sought. The EA was granted in the 4 previous years, with Cruiser SB being used in 2022, 2023, and 2024 (the 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 46-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

Given these limitations on use, HSE judge many environmental risks from use of Cruiser SB to be acceptable, including risk in aquatic environments and to birds, but consider there is still risks to bees.

3. Risk to bees

Thiamethoxam and clothianidin are toxic to bees, leading to acute mortality, chronic mortality (that is, 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 consider 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 indicates 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 3 pathways of possible exposure to thiamethoxam:

i. Chemical transport to field margins and subsequent incorporation into flowering plants

Research published in 2022 assessed the chronic toxicity of thiamethoxam to bees (Wilkin 2022) and led HSE to indicate that the risk to bees from this pathway is acceptable.  That conclusion continues to be supported by the absence of detection of high thiamethoxam in measurements made on field margins following use of Cruiser SB in 2022. Measurements on field margins in 2023 and 2024 using sensitive techniques show no statistical evidence for an increase of either thiamethoxam or clothianidin in vegetation or pollen in field-margin plants. This data continues to indicate that there is no significant movement of either pesticide from treated fields to field-margins.

ii. 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 5 years they have been sought. This year’s HSE assessment follows that of the last 2 years, using the chronic toxicity study of Wilkin (2022). HSE conclude 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 look for a factor of 10-times lower to give a margin of safety and, as this factor is not reached, have concern about possible 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 calculate 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 at a rate varying by soil type and conditions. I consider the implications of this break down on expected bee exposure in Section 4 below. 

iii. 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 agree with 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 non-flowering crops that succeed sugar beet on fields drilled with Cruiser SB; risk that is not mitigated by the 32-month exclusion 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 (that is, 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 for most fields because:

i. 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. 
ii. The HSE calculation assumes that all water uptake for bees comes from guttation fluids similar to that of maize which is likely to be unrealistic.

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

4. The break-down of thiamethoxam with time following use of Cruiser SB

The rate of chemical break-down of thiamethoxam sets the rate of decrease of soil concentrations that justifies planting of flowering crops 32 months or more after Cruiser is used.  If the break-down is too slow, concentrations in soils planted with Cruiser SB will still be high at 32 months, and risk to bees from flowering crops planted on the field will not be 10 times lower than chronic and non-lethal risk levels to bees. 

Reported values in the literature for breakdown of thiamethoxam include DT50 values (that is, the time for the concentration to half) averaging 16 days for European soils (Hilton et al. 2019), 75-109 days for UK soils (Jones et al. 2014) and a longest reported DT50 value of 172 days. Last year, my advice used this longest DT50 to assess breakdown of thiamethoxam over a 32-month period, which indicated a 11.2-times decrease of concentration between 12 months and 32 months and expected exposure levels of bees to thiamethoxam 84 times lower than that required for chronic toxicity, and 29 times lower than that required for sub-lethal toxicity.

The use of Cruiser SB on some sugar-beet fields in 2022 has allowed assessment of the breakdown of thiamethoxam on fields in the specific region of sugar-beet cropping in the UK.  This research was initiated and funded by Defra at the time of Cruiser planting in 2022 to enable improved assessment of risk from use of Cruiser SB.  Soils from 5 typical fields on which Cruiser SB was planted were measured for thiamethoxam concentrations 9, 17, 22, and 27 months after planting and show decreasing concentrations (Fig 1). The measurements made at 22 and 27 months (that is, since my advice last year) indicate that the rate of decrease is slower than expected even using the longest published DT50 value (172 days). A best fit to the field data suggest a DT50 of 362 days. 

The new field-based rate of thiamethoxam break-down suggests only a 3.1-times decrease of concentration between 12 and 32 months, and suggests exposure levels of thiamethoxam 24 times lower than required for chronic toxicity (that is 7.8x3.1), and 8.7 times lower than that required for sub-lethal toxicity (that is 2.8x3.1). While these values are lower than required for toxicity, the sub-lethal value does not meet the factor of 10 margin of safety used by HSE to assess lack of risk to bees. To reach that margin of safety, using the best-fit field-measured rates of thiamethoxam break-down, would require the exclusion period on planting of flowering crops to be extended from 32 to 35 months.

Figure 1 shows soil thiamethoxam concentrations in fields planted with Cruiser SB in 2022 measured at 4 times after planting. Uncertainties are 1 standard error of measurements from the five fields measured.  The blue curve shows the rate of decrease expected if thiamethoxam had a DT50 of 172 days (the longest DT50 reported in the literature).  Field data indicate that the rate of breakdown is slower than this literature value, with a best fit to the measurements (shown by the dashed line) equivalent to a DT50 of 362 days. 

5. Risk from the break-down product, clothianidin

Data collected on fields planted with Cruiser SB indicate greater persistence of clothianidin in the environment than anticipated from literature-based breakdown rates. Clothianidin was measurable in a number of fields prior to seed drilling, reflecting persistence presumably from use of neonicotinoids many years previously. Soil samples collected pre-drilling and post-2022 drilling of Cruiser in fields show constant concentrations over the 27 months of measurements. It is notable that the values differ consistently between fields and field margins, again indicating lack of any significant movement from field to margin with time. The constant concentrations observed in field also suggests that there is no significant production of clothianidin from breakdown of thiamethoxam on Cruiser SB (as expected from literature assessment that clothianidin is not a significant breakdown product when seeds are drilled).  

The new field data for clothianidin, and the increasing river timeseries, indicates a longer term general risk to wildlife from this neonicotinoid from past use.  They do not, however, indicate that drilling of Cruiser SB, in the manner prescribed by this EA, is leading to an increase in this risk. 

6. Alternatives to use of Cruiser SB

Attempts are being made to breed new varieties of sugar beet resistant to virus yellows, but these are not yet sufficiently mature to offer a viable alternative. A traditionally bred variety with partial resistance, Maruscha KWS, is available but has a low yield and is not favoured by growers. There is active research to use precision breeding to develop further resistant varieties, including research funded by Defra, but this is at an early stage and new varieties will not be available for some years.

At present, the only alternative to using Cruiser SB is simply to use an untreated seed, rely on existing and approved foliar sprays to reduce aphid populations, and accept some loss of yield due to virus yellows. 

It is notable that, in the 3 years that Cruiser SB has been available, a significant proportion of farmers growing sugar beet have chosen not to use it, and also that this proportion has increased (from 30% in 2022, to 41% in 2023, to 42% in 2024). In these 3 years, the average incidence of virus yellows in fields not using Cruiser SB was 4.1%, relative to 2.9% in fields using Cruiser. These statistics suggest that the yield implications of using Cruiser SB have been relatively slight in the last 3 years. The last three years do not include any year akin to 2020, however, when a very high virus incidence (38%) caused significant yield loss. Better prediction of risk from virus yellows, early enough to set plans for planting of sugar beet, is a desirable goal and would be a useful focus of future R&D. 

7. 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 there will be significant damage to crop yields and therefore to the economic viability of farms and the UK industry. 

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 (from the Rothamsted Institute) has been used to set the threshold for planting of Cruiser SB in the last 3 years. This model systematically overpredicts incidence at low values, likely because use of foliar sprays are 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. 

Identifying a threshold for use needs to assess the point at which foliar sprays cease to be effective and where there is a significant risk of high crop loss, such as caused by the 38% incidence in 2020. Data from this year’s sugar-beet crop complicate setting of such a threshold because, when added to data from previous years, they indicate considerable uncertainty in the relationship between the predicted and realised incidence of virus yellows. 

Given this uncertainty, the choice of a suitable threshold cannot be made simply as the point at which use of Cruiser is expected to cost less than the damage caused to farm income without use of Cruiser. Instead, some assessment is required of the expected risk to farms and the sugar industry, and the ability of the sector to manage loss of income.  The detailed economic analysis conducted by Defra officials provide full details of these factors and will allow ministers to form a view about a suitable threshold for predicted incidence if required.