Left: Deroceras reticulatum infected with Phasmarhabditis hermaphrodita.
Right: Healthy Deroceras reticulatum. Note the swollen mantle of the infected slug. R.Harvey

Phasmarhabditis hermaphrodita
(Rhabditida: Rhabditidae)

by Michael Wilson and Randy Gaugler, Department of Entomology, Rutgers University, PO Box 231, New Brunswick, NJ 08903

Phasmarhabditis hermaphrodita is a nematode parasite of slugs which has recently been commercialized in the UK as a biological molluscicide. The methods of production and formulation for this nematode are similar to those used for entomopathogenic nematodes.

Appearance

The nematodes are sold as infective juveniles (dauer larvae) approximately 1 mm long, which can be just seen with the naked eye.

Habitat

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.

Life Cycle

Phasmarhabditis hermaphrodita forms non-feeding developmentally arrested juveniles (dauer larvae). These are third stage larvae which retain a second stage cuticle, and in which the mouth and anus are closed. The dauer larvae move through the soil looking for slugs. Once they have located slugs they infect. For 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.

Once inside the slug, the dauer larvae develop into self-fertilizing hermaphrodites which reproduce, and go through another generation. During this time, the slug usually develops a characteristic swelling of the mantle region. The slug typically takes between 7 and 21 days to die, but its feeding is markedly reduced from the onset of infection. In laboratory experiments, a dose of nematodes roughly equivalent to half the recommended field dose inhibited slug feeding by 90% within four days of exposure. Once the slug dies, nematodes spread out over the cadaver and feed. When the food-source is depleted the nematodes again form dauer larvae which leave in search of other slugs.

Phasmarhabditis hermaphrodita is a bacterial-feeding nematode. Unlike entomopathogenic nematodes, no specific symbiotically associated bacterium has been isolated from this nematode to date, although growing the nematode with different bacteria alters the resulting nematodes' yield in in vitro cultures and pathogenicity to slugs. At present, the relationship between P. hermaphrodita and bacteria is poorly understood.

Pests Attacked

P. hermaphrodita has been shown to infect and kill a wide variety of pest species of both slugs and snails.

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 slug Arion ater agg. 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.

Relative Effectiveness

Phasmarhabditis hermaphrodita has been used in a number of field experiments in many European countries including the UK, France, the Netherlands, and Switzerland. Crops in which the nematode has been tested include high value horticultural crops such as lettuce and strawberries and low value field crops such as wheat, potatoes and oilseed rape. If applied at a dose rate of 3 x 10⁹/ha the nematode gives equivalent control to methiocarb pellets, a chemical standard. The rapid feeding inhibition caused in slugs by P. hermaphrodita means that the nematode can be applied with the same timing as chemical molluscicides.

Conservation

Little is known about ways to conserve populations of P. hermaphrodita. Practices which typically conserve nematodes within the soil, e.g. reduced tillage and reduced pesticide input, are also likely to favor slugs.

Pesticide susceptibility

Soil insecticides, nematicides and fumigation should not be used when it is desired to conserve P. hermaphrodita.

Commercial availability

Phasmarhabditis hermaphrodita is presently only available in the UK where it is sold to domestic gardeners. It is manufactured by MicroBio Ltd, (Thriplow, Herts, UK) and distributed by Defenders (Ashford, Kent, UK) under the trade name "Nemaslug". However, nematode producing companies in other European states and in the US have expressed interest in producing P. hermaphrodita as a biological molluscicide.

References

Coupland J.B. (1995) Susceptibility of helicid snails to isolates of the nematode Phasmarhabditis hermaphrodita from southern France. Journal of Invertebrate Pathology, 66, 207-208.

Wilson, M.J., Glen, D.M. & George, S.K. (1993). The rhabditid nematode Phasmarhabditis hermaphrodita as a potential biological control agent for slugs. Biocontrol Science and Technology, 3, 513-521.

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