Slug Biological Control

Slugs cause damage to arable crops, vegetables, garden and horticultural crops and are a particular problem when crops are being established. They thrive in New Zealand’s temperate climate, making them a significant pest for Kiwi farmers and horticulturalists, year round. Existing slug control options are toxic baits. But they pose threats to mammals and cause problems in waterways. As well, their use in commercial food crops is becoming increasingly unacceptable to consumers. 

Dr Michael Wilson is an AgResearch scientist who is leading research into finding more environmentally friendly and effective ways to control slugs. He is involved in an “Integrated Management of Slug Pests” project, and an MBIE funded project ‘Next Generation Pesticides’. 

There is no biocontrol currently available for slugs in New Zealand so toxic baits are often used. Less toxic methods like copper rings and eggshell barriers are really only viable for home gardeners, and even then gardeners need to be careful to ensure no slug-sized holes remain in those barriers when applied. Toxic sprays are not an option as slugs simply produce more slime in order to reduce the toxin’s impact. 

Baits are problematic as they are also toxic to mammals (domestic pets are often accidentally poisoned as a result of ingesting bait). Further, there is evidence to suggest that some bait ingredients are also toxic to beneficial invertebrates.   An ingredient used in some baits (metaldehyde) has been detected in waterways in Europe. Present methods available to clean up contaminated drinking water do not remove metaldehyde. 

Michael says, “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”. 

The nematode that is currently being studied has already proved an efficient control option in Europe. It is called Phasmarhabditis hermaphrodita. The nematodes seek out potential slug hosts using chemotaxis, essentially searching by smell. (Mike says slugs do have a unique smell.) The nematodes enter the slugs via a natural opening under the mantle located on their back. The nematodes then develop and reproduce, feeding on the slug’s tissue and body fluids, eventually killing their host after 4-21 days, depending on the number of parasites and the ambient temperature.

The nematodes appear to be able to modify their host’s behaviour so that it remains below the ground surface before dying, and so are not readily available to predators and scavengers. The nematodes then eat the cadaver and produce another generation of juveniles, which move off through the soil in search of new slug hosts. Even when fully grown, the nematode is barely visible to the naked eye. 

The nematodes are grown in Europe. They are reared in vats and fed on bacteria in a nutrient-rich medium. They are supplied partially dehydrated on an organic polymer. The farmer then mixes the polymer with water and sprays the solution onto the crop. 

In 2013 the patent on the nematode as a biocontrol ended, opening up the possibility of commercializing and growing them within New Zealand. The first hurdle to overcome was to change the existing classification of the nematode as a ‘new organism’. Because it was not known to be present prior to the introduction of the 1996 Hazardous Substances and New Organisms (HSNO) Act, this meant it could not be introduced or commercialized.

Initially it had to be established that the nematode was already in New Zealand. This involved collecting slugs and testing them for the nematode – done by anesthetizing the slug and cutting off its head. If, a week later, the slug cadaver is covered in the nematodes, the result is positive. After extensive research, the project managed to have the nematode’s ‘new organism’ status overturned in 2016. Mike believes it was likely to have been established along with introduced slugs at the end of the 19th century. 

The team is also looking at related nematodes to assess their potential as a biocontrol and, while it’s early days, the other species are not proving to be as effective as Phasmarhabditis hermaphrodita. 

Small populations of Phasmarhabditis hermaphrodita are spread throughout the country but are currently ineffective as a biocontrol due to their lack of numbers. With present research coming to an end, Mike is now looking for further funding in order to collect data on how the nematode will work for New Zealand crops and conditions. He also needs to establish how many nematodes are required per hectare for good control. 

Mike points out that biocontrol will never completely remove slugs from the ecosystem as populations will follow the well known predator-prey cycle. To compund matters, slugs are remarkably efficient breeders. All slugs are hermaphrodites and mate to reproduce – but all slugs produce eggs, usually around 250 in a life cycle (up to one year). Slugs are also known to go deep underground during drought conditions. As a result, it is likely that farmers will need to apply the control on a yearly basis. 

Mike believes it will be a couple of years yet before the bait will be commercially available.