Pesticide Usage in Scotland: Outdoor Vegetable Crops 2023

Data and methodology

Sampling and data collection

Using the June 2023 Agricultural Census [9], a sample was drawn representing vegetable cultivation in Scotland. The first sample was selected from holdings growing any vegetable crops excluding vining peas, and the second from holdings known to have grown vining peas. Two samples were taken to achieve a better representation of all vegetable crops, as most vining pea crops are grown on farms growing arable crops rather than vegetable crops.

The country was divided into 11 land-use regions (Figure 25). Each sample was stratified by these land-use regions and according to holding size. The holding size groups were based on the total area of either vegetable or vining peas crops grown. The sampling fractions used within both regions and size groups were based on the areas of relevant crops grown rather than number of holdings, so that smaller holdings would not dominate the sample.

The survey covered pesticide applications to vegetable crops where all or the majority of the growing season was in 2023. As well as recording treatments applied directly to the crop, data was also collected on land preparation treatments prior to sowing or planting the crop.

Following an introductory letter and phone call, data was collected during a phone interview or by email. Where necessary, information was also collected from agronomists and contractors. In total, information was collected from 58 holdings growing vegetable crops and 30 holdings growing only peas (Tables 12 and 13), (with a further five pea holdings also growing veg, eight veg holdings also growing peas). These 88 holdings represent 10% of the total crop area grown.

Raising factors

National pesticide use was estimated by ratio raising. This is a standard statistical technique for producing estimates from a sample. It is the same methodology used by the other UK survey teams and has been used for all historical datasets produced by the Pesticide Survey Unit, allowing comparability over time. The sample data were multiplied by raising factors (Tables 18 and 19). These factors were calculated by comparing the sampled area to the areas recorded in the Agricultural Census within each region and size group. An adjustment (Tables 20 and 21) was made for each crop within each region by applying the raising factors to the sample area of each crop grown and comparing this with the census area. This adjustment modifies the estimate to take into account differences in composition of crops encountered in the sample and those present in the population. A second adjustment was necessary for some crops which were present in the population but were not encountered in the sample in some strata.

Detailed data tables and data used to create the charts in this report are available in Excel format in the supporting document published seperately.

Changes from previous years

There are changes which should be noted when comparing the 2023 data with the previous survey.

Different crops are included in the 'other brassicas' and 'other vegetable crop' categories in different survey years due to sample sizes. For example, broad beans were included in the 'other vegetable' category in 2021 due to too few crops being encountered in the holdings sampled in 2021 to adequately represent usage. Broad beans are reported as individual categories in the 2019 and 2023 reports. Therefore differences must be taken into account when making comparisons between surveys.

In previous reports, the active substance garlic was always classified as a physical control. However, garlic may be used as a physical control (repellant) or as a nematicide. In this report, all garlic use was as a nematicide and is therefore reported under the insecticide/nematicide pesticide category. Where comparisons are made between the 2023, 2021 and 2019 surveys, adjustments to reporting garlic under different pesticide categories have been taken into account. As a result, the insecticide/nematicide totals for 2019 and 2021 included in this report will be different to those published in previous reports.

This report contains information about grower adoption of Integrated Pest Management (IPM) and makes comparisons to data collected in 2015 and 2019. IPM data was not collected during the 2021 survey. It is anticipated that IPM data will be collected and published every 4 years. This allows IPM uptake to be monitored over time but reduces the burden on growers and surveyors.

The 2023 report follows a different structure to earlier survey reports. Previously full data tables were included in survey reports. This report only includes short summary data tables for each of the crops but references full data tables published in Excel format in a supporting document. This is a similar format used in other Scottish Government accredited publications. Figures have also been improved to meet accessibility criteria.

Data quality assurance

The dataset underwent several validation processes as follows; (i) checking for any obvious errors upon data receipt (ii) checking and identifying inconsistencies with use and pesticide approval conditions once entered into the database (iii) checking of data held in the database against the raw data. Where inconsistencies are found these are checked against the records and with the grower if necessary. Additional quality assurance is provided by sending reports for review to members of the Working Party on Pesticide Usage Surveys and other agricultural experts. In addition, the Scottish Pesticide Survey Unit is accredited to ISO 9001:2015. All survey related processes are documented in Standard Operating Procedures (SOPs) and our output is audited against these SOPs by internal auditors annually and by external auditors every three years.

Main sources of bias

The use of a random stratified sample is an appropriate survey methodology. A stratified random sample, grouped by farm size and region, is used to select holdings used in this survey. Sampling within size groups is based on area rather than numbers of holdings, so that smaller size groups are not over-represented in the sample. The pesticide survey may be subject to measurement biasas it is reliant on farmers/growers recording data accurately. As this survey is not compulsory it may also subject to non-response bias, as growers on certain farm/holding types may be more likely to respond to the survey than others. Reserve lists of holdings are held for each stratum to allow non-responding holdings to be replaced with similar holdings.

Experience indicates that stratified random sampling, including reserves, coupled with personal interview technique, delivers the highest quality data and minimises non-response bias.

Standard errors

The figures presented in this report are produced from surveying a sample of holdings rather than a census of all the holdings in Scotland. Therefore, the figures are estimates of the total pesticide use for Scotland and should not be interpreted as exact. To give an idea of the precision of estimates, relative standard errors (RSE) for total pesticide use on vegetable crops for both area and weight have been calculated. Standard errors are produced using the raising factors. An overall variance is calculated by summing the variance estimates for individual strata (region and size group) multiplied by the square of their raising factors. These variance estimates include a finite population correction. The overall standard error is calculated from the overall variance by taking its square root. This method of standard estimation was implemented as it is both relatively straightforward and has advantages over ratio estimator methods when within-strata sample sizes are small.

Standard errors are expressed as percentage relative standard errors for both total pesticide use by area treated and for weight applied. Larger relative standard errors mean that the estimates are less precise. A relative standard error of 0% would be achieved by a census. A relative standard error of 100% indicates that the error in the survey is of the same order as the measurement. Relative standard errors may be reduced with larger sample sizes. However, larger relative standard errors can also result from greater variability in pesticide use among holdings.

The RSE for estimates of total pesticide use on vegetable crops in 2023 was 12% for area and 17% for weight, compared with 9% for both area and weight in 2021. In contrast to previous years, RSE are not reported for constituent crop groups due to the small sample size of some crops encountered in some regions which presented issues with RSE calculation. Alternative methods for calculating RSE for minor crops with low sample numbers are being investigated.

Response rates

See supporting document (Table 22) which summarises the number of holdings contacted during the survey.

Financial burden to farmers

In order to minimise the burden on farmers, the survey team used non-visit methods of collection such as email, post or telephone call.

To determine the total burden that the 2023 outdoor vegetable crop survey placed on those providing the information, the surveyors recorded the time that 101 respondents spent providing the data during the surveys. This sample represents 100% of growers surveyed. The median time taken to provide the information was seven minutes.

The following formula was used to estimate the total cost of participating:

Burden (£) = No. surveyed x median time taken (hours) x typical hourly rate* (* using median "Full Time Gross" hourly pay for Scotland of £18.17) [12]

The total financial burden to all growers resulting from participation in the 2023 outdoor vegetable crop survey was calculated to be £153.