Spatial aggregation and temporal migration of free-living stages of the parasitic nematode Trichostrongylus tenuis

1. Predicting outbreaks of Trichostrongylus tenuis in Red Grouse from larval counts has been hampered because of the poor recovery of the third-stage infective larvae from heather vegetation. Two possible explanations for this poor recovery were examined: (i) larvae are spatially aggregated; (ii) larvae exhibit temporal variability in abundance and may be missed by sampling at the wrong time. 2. Heather vegetation was intensively sampled and infective larvae were found to be spatially aggregated. The temporal distribution of T. tenuis also varied, with greater numbers of larvae present on heather sampled in the afternoon than the morning. 3. In a series of laboratory experiments the diurnal availability of T. tenuis larvae were examined on two plant species with differing physical structures: heather and wheat. These trials were tested against infective larvae of Haemonchus contortus, a related nematode known to exhibit diurnal migration patterns. Larvae were exposed to diurnal variations in light and temperature. 4. Infective larvae of both species were recovered in greater numbers during periods when the lights were on. Temperature did not have a significant influence on H. contortus larval availability but greater numbers of T. tenuis L3 were recovered at low temperature (10 °C) than at higher temperature (20 °C). 5. Both temporal variation in the presence of T. tenuis larvae on vegetation and the spatial variation in larval distribution may account for poor recovery of Trichostrongylus tenuis L3 in the field.