Targeting biocontrol with the slug-parasitic nematode Phasmarhabditis hermaphrodita in slug feeding areas: a model study

The nematode Phasmarhabditis hermaphrodita was applied to soil in an outdoo r miniplot experiment to protect Chinese cabbage seedlings from damage by t he field slug Deroceras reticulatum. The aim was to investigate the possibi lity of reducing the numbers of nematodes applied by only partially sprayin g soil in the nl ea where slug control was needed. Nematodes sprayed as ove rall applications were compared with band applications along plant rows and spot applications around individual plants in plots with nine or 18 plants . Band and spot applications,were applied at two rates designated the full rate (same number of nematodes per plot as in the overall application) and the area rate (same number of nematodes per unit area comprising 43% (band) and 18% (spot) of the overall application). In plots with 18 plants, where spot-treated plant alternated with untreated plants, no significant differ ence in damage was found between spot-treated plants and untreated plants T his indicates that slugs were not repelled from nematode-treated ni eas and that any effects in reducing slug damage were not clue to repellency. All nematode treatments resulted in a significant reduction in the mean level o f slug damage to seedlings from six or more days after treatment. However; there were significant interactions between nematode treatment, the number of plants per plot, the position of plants within plots (edge or middle) an d time after treatment. The effect of time after treatment was modelled. Th e log time to 50% reduction in slug damage (t(50)) was related to the area treated and the close applied. In plots with band ol spot treatments at the full dose, there was a relatively small increase in t(50) with declining a rea treated. In plots treated with band or spot treatments at the area clos e, t(50) increased consistently with declining relative mea treated The fin al level of damage, expressed as a percentage of damage on untreated plots (P-i), was influenced by both the dose and area treated Final damage was gr eatest on spot-treated plots where half the plants were untreated. We concl ude that partial treatment of soil around all plants to be protected from s lug damage is a potentially valuable method of reducing the overall nematod e dose required for control of slug damage provided that some damage can be tolerated.