by Michael Wilson (Agresearch, Ruakura Research Centre, East Street, Private Bag 3123, Hamilton 3240, New Zealand),
The nematodes are sold as infective juveniles (dauer larvae) approximately 1 mm long, which can be just seen with the naked eye. The infective juveniles are formulated onto clays or gels that can be added to water and applied with conventional spray equipment.
The nematode is a soil dwelling animal and is generally found where slugs are abundant. It has been isolated from grassland and crops of wheat and oilseed rape (canola). Recent research has shown that the nematode is well adapted to life in leaf litter, compost, and organic soils.
Phasmarhabditis hermaphrodita is a facultative parasite and is capable of growing on dead invertebrates, slug feces, and decaying leaf litter. The nematode can form non-feeding, developmentally arrested juveniles (dauer larvae) which are a survival stage commonly formed by rhabditid nematodes. In the case of P. hermaphrodita, the dauer larvae also function as the infective stage in the parasitic life cycle. Dauer larvae are third stage larvae which retain a second stage cuticle, and in which the mouth and anus are closed. In the nematode’s preferred slug host, Deroceras reticulatum, the nematodes infect principally through a natural opening at the rear of the slug's mantle. The site of entry for other mollusc species has not been studied.
P. hermaphrodita has been shown to infect and kill a wide variety of pest species of both slugs and snails.
The most susceptible species is the grey field slug, Deroceras reticulatum, which is the most widespread pest species in the world and is responsible for most agricultural and horticultural losses. While all the above species have been shown to be killed by P. hermaphrodita in laboratory bioassays, many of the assays used high doses and unrealistic assay conditions, so it is not clear if P. hermaphrodita could be used to control all the above species under field conditions. Also, laboratory bioassays indicate that body size may be an important feature of susceptibility. For example the garden snail, Helix aspersa, is susceptible when its body weight is less than a gram, but larger individuals are not. Similar results have been found for the large slugs Arion ater agg. and A. lusitanicus. Thus, for some large species of slug, it would be advisable to apply nematodes at the time of year when only juvenile slugs are present.
Phasmarhabditis hermaphrodita has been used successfully in a number of field experiments in many European countries, including the UK, Croatia, France, the Netherlands, Spain and Switzerland. There have also been successful efficacy tests in Chile and the USA. Crops in which the nematode has been tested include high value horticultural crops such as lettuce, strawberries, and glasshouse orchids and low value field crops such as Brussels sprouts, wheat, potatoes, and oilseed rape. If applied at a dose rate of 3 x 109/ha, the nematode gives equivalent control to chemical-standard slug pellets. The rapid feeding inhibition caused in susceptible slugs by P. hermaphrodita means that the nematode can be applied with the same timing as chemical molluscicides.
There is no evidence that the nematode has any undesired non-target effects. It has been tested against several earthworm and insect species (Rae et al., 2007) and non-target mollusks (Wilson et al., 2000). The nematode is killed at temperatures above 25°C so it is unlikely to pose a threat to mammals. The nematode can kill some aquatic snails that are considered beneficial so it is recommended that the nematode not be sprayed adjacent to ponds and waterway.
Little is known about ways to conserve populations of P. hermaphrodita. Practices which typically conserve nematodes within the soil and favor build up of organic matter, e.g. reduced tillage and reduced pesticide input, are also likely to favor slugs.
Soil insecticides, nematicides, and fumigation should not be used if attempting to conserve P. hermaphrodita.
Phasmarhabditis hermaphrodita is available under the trade name "Nemaslug" in the UK, Ireland, Switzerland, Norway, Holland, Denmark, Finland, Belgium, Germany, Poland, France, Spain and Italy. It is manufactured in the UK by Becker Underwood UK Ltd and distributed nationally and internationally by a range of distributers. Key markets include domestic gardens and high value commercial salad crops such as lettuce and celery. More recently, sales have expanded to include vegetables such as brassicas and potatoes. The product is mostly used by organic growers, but many conventional growers use the nematode alongside chemical molluscicides. The nematode is not commercially available in North America at present, as the presence of P. hermaphrodita in this continent has yet to be confirmed.
Ester, A. & Wilson, M.J. (2005). Application of slug parasitic nematodes. In Nematodes as Biocontrol Agents (Eds. P.S. Grewal, R.U. Ehlers & D. Shapiro-Ilan). CABI Publishing, Wallingford. 431-444.