Members' experiments

Testing a novel method of pest control

Each year we organise several citizen science experiments that our members can participate in to help inform the way we grow organically today. This experiment is Testing a novel method of pest control and took place in 2021.


Biostimulants are an alternative approach to controlling pests that build up the plant’s natural defence mechanisms rather than trying to kill them, so they have a much less negative impact on wildlife than using a pesticide.

Garden Organic trialled a new biostimulant made from frass. It contains chitin which is thought to provide protection against attack from insect attacks such as flea beetle. It is made by feeding food waste to soldier fly larvae then collecting the discarded insect cases and droppings, so it is a natural product with good environmental credentials.

In conjunction with NIAB and Wasware, we tested the effects of frass on pest damage. It was applied either directly to the soil or as a seed coating at 14 of our members’ gardens. ‘Green in Snow’ mustard was chosen as a test crop. It is an oriental green that is rapidly growing, and often suffers from problems with flea beetle, like many oriental greens.


At each of the participant sites, three plots, each 1m2 were prepared for direct sowing in mid-May. Four rows with 10 seeds in each row were sown into each plot. Each plot had a different treatment:

Plot 1 – Control treatment
Plot 2 – Biostimulant applied as a soil amendment
Plot 3 – Biostimulant applied as a seed coating

Monitoring pest damage

Pest damage was assessed on a weekly basis, where possible, observing the central two rows in each plot to minimise edge effects. The following scales were used for assessment:

Slugs and flea beetle

1 = no damage,
2 = low (0 – 24% leaf area lost)
3 = moderate (25 – 49% leaf area lost)
4 = high (50 – 75% leaf area lost)
5 = severe (>75% leaf area lost)


1 = no damage,
2 = low (0 – 24% leaf area covered)
3 = moderate (25 – 49% leaf area covered)
4 = high (50 – 75% leaf area covered)
5 = severe (>75% leaf area covered)


The seed treatment may have caused a small reduction in the incidence of flea beetle, but it was difficult to tell whether this effect was real. Flea beetle was only present in low numbers, making it difficult to assess whether the treatments were having an impact. To see the results please download the report below.

We would like to continue testing for an additional growing season to build on our evidence base, so if you are interested in this experiment and want to help us further please sign up for our 2022 Testing a novel biostimulant - Help us assess if a new biostimulant could revolutionise organic pest control here.

Biostimulant seed treatment report 2021.pdf