Investigating the potential for slug biocontrol

New research is making headway into battling one of the most persistent pests that cause endless heartache and significant financial frustration for horticultural and agricultural farmers and home gardeners alike.

AgResearch scientist Dr Michael Wilson is investigating biological control of slugs which could reduce reliance on largely ineffective pellets that contain environmentally toxic chemicals.

At present there are no biological control options available for slugs in New Zealand and our mild climate is perfect for them to thrive, because they’re rarely limited in activity by freezing temperatures in the winter or drought in the summer. Slugs cause a variety of damage and attack arable crops, vegetables, garden and horticultural crops.

Dr Wilson is working with the Foundation for Arable Research (FAR) on biocontrol options for slugs. Allister Holme, Research and Extension Team Leader from FAR says slugs are a pest of almost all arable crops, and can cause considerable damage, particularly at crop establishment.

“There is a competitive market for slug baits, but internationally the use of slug baits is facing restrictions due to poisoning of waterways and other environmental concerns. The possible development of a biocontrol agent in New Zealand is exciting work and will be a valuable tool in the ongoing management of slug pests”.

There is potential for a parasitic nematode to be utilised as a biocontrol. In Europe the nematode parasite Phasmarhabditis hermaphrodita is mass-produced by BASF and sold as a biological molluscicide. Recently, the nematode has been found parasitising slugs in New Zealand and has potential to be developed here as a biological molluscicide (Wilson et al., 2012).

Dr Wilson is now lead scientist in a Sustainable Farming Fund grant “Integrated management of slug pests” with co-funding from FAR and Ravensdown. Part of this grant aims to develop biocontrol of slugs in New Zealand.

While the nematode occurs in New Zealand, because it was not known to be present prior to the 1996 Hazardous Substances and New Organisms (HSNO) Act, P. hermaphrodita would be classified and regulated as a “new” organism which would preclude its commercial use, Dr Wilson says.

“To have this classification removed, we would need to show the nematode is widely distributed in New Zealand. We believe this is the case, and with the engagement of the community of interest, demonstrating this will be part of the SFF.

“We have found the nematode in several New Zealand sites and will be applying to have the ‘new organism’ status lifted early next year. If that happens, we’ll be able to begin field trials with the nematodes.”

The nematodes actively search for potential slug hosts which are then entered through the breathing pore. The nematodes feed and develop feeding on the slug tissues eventually killing the slug after 4-21 days, depending on the number of parasites and the temperature. The nematodes need several days to complete their life cycle and seem to be able to modify the host’s behaviour so that it remains below the ground surface before death and is thus not readily available to predators and scavengers. The nematodes eat the cadaver and produce another generation of infective juveniles which move off through the soil in search of new slug hosts.

The nematode appears to be present in low numbers and spread slowly so is currently ineffective as a biocontrol. However, Dr Wilson says future adaptation as a specific biocontrol would include the wider application of the nematodes as a biopesticide.

While Dr Wilson and his team were surveying sites they also found a new species of nematode and, in an interesting twist, found that the same nematode was simultaneously discovered by a group at the University of California, Riverside – Phasmarhabditis californica. The researchers from AgResearch and UoC have applied for a Catalyst grant to kick-start a collaboration to assess the biocontrol potential of this new species. Funding through the NGBP programme will enable biological activity and host range of the new nematode species to be tested.

Wilson, M.J., Burch, G.M., Tourna, M. Aalders, L. & Barker, G.M. (2012). The potential of a New Zealand strain of Phasmarhabditis hermaphrodita for biological control of slugs. New Zealand Plant Protection 65, 161-165.