Researchers also found that when used on hostas the organic pellets out-performed their rivals.
The research, which is supported by BASF, a manufacturer of nematodes, also found that mulch, which was one of the six control strategies being studied*, actually increased slug damage on beans.
The overall findings from year one of the research were that across the plants studied: daffodil; hosta; lettuce and beans; the synthetic, non-organic slug pellet treatment was the most consistently well performing, only slightly ahead of the organic version, and that mulch performed least well across the range of plants studied.
*The six different control strategies being tested are:
1. Control (no treatment) – this is important to compare with the other strategies
2. Cultural management (mulch) – a loose covering of material on the soil around the plants
3. Cultural + synthetic chemical (metaldehyde) – the most commonly used type of slug pellets
4. Cultural + organic chemical (ferric phosphate) – a slug pellet that is certified as organic
5. Cultural + nematode biological control applied reactively (once damage is seen)
6. Cultural + biological control applied preventatively (applied regularly from the early spring).
Other initial findings were that that when used preventatively nematodes provided good levels of control for daffodils and intermediate control for lettuces, however, when combined with mulch it was less effective on hostas and beans.
Scientist Dr Hayley Jones said: "The data gathered so far shows some interesting complexities, the most notable being the increase in damage caused by the mulch on some plants.
"The high levels of slug and snail damage found on the plants protected by mulch was unexpected, especially as many mulches are billed as being repellent to slugs and snails. It is possible that the negative impact of the mulch affected the effectiveness of the other control treatments when they were combined.
"In the second year of the study we will try to answer some of the questions generated by these results. We will look at the treatments with and without mulch, so we can better understand how the mulch interacts with the pesticides and nematodes."
Gavin Wood, from BASF, Europe’s largest producer of nematodes for use in pest control, and a partner in the research says: "These early results are useful, in year two we hope to be able to differentiate between damage caused by slugs and snails, as nematodes are not effective against the latter.
"It seems possible that using mulch may attract slugs and snails as they head for the continuous warm and moist conditions the treatment generates.
"We are happy to continue to sponsor the second and final year of the trial and remain confident about the slug control provided by nematodes. We will work closely with the RHS and look closely at the protocols for the 2017 trials."
Meanwhile, work begins on demolition of some buildings at the front of RHS Wisley this month. The plant centre and shop will move to new buildings that will replace the buildings next year. Trial grounds will then move to the former plant centre site.