Strategies for nematode transmission: selective migration of Trichostrongylus tenuis infective larvae.

Successful transmission of macroparasites is dependent on exposure of susceptible hosts to free-living infective stages. When these hosts are herbivores that feed mostly on a single food plant then natural selection should favour those infective larvae that selectively ascend this main food plant. Red grouse feed predominantly on heather, Calluna vulgaris, so we predict that the infective larvae (L3) of the caecal nematode Trichostrongylus tenuis selectively locate and ascend heather plants. To determine whether the presence of heather influences the horizontal dispersal of T. tenuis L3 across soil, the movement of L3 across trays of soil with and without heather was investigated in the laboratory. More T. tenuis L3 were recovered from soil when heather was present, implying that larval migration may be influenced by chemical cues produced by heather plants. This was investigated in a second experiment, in which the horizontal dispersal of T. tenuis larvae was examined in the presence of heather and grass vegetation. This trial was repeated with larvae of a second species, Haemonchus contortus, a nematode whose hosts feed on a wide range of grass and shrub species. Significantly more larvae of both nematode species were recovered in the region of the heather than the grass or controls. This implies that T. tenuis and H. contortus L3 exhibit selective migration towards heather, perhaps reflecting a general response to plant cues which may be stronger for heather than for grass.