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 larva l counts has been hampered because of the poor recovery of the third-stage infective larvae from heather vegetation. Two possible explanations for thi s poor recovery were examined: (i) larvae are spatially aggregated; (ii) la rvae exhibit temporal variability in abundance and may be missed by samplin g at the wrong time. 2. Heather vegetation was intensively sampled and infective larvae were fou nd to be spatially aggregated. The temporal distribution of T. tenuis also varied, with greater numbers of larvae present on heather sampled in the af ternoon than the morning. 3. In a series of laboratory experiments the diurnal availability of T. ten uis larvae were examined on two plant species with differing physical struc tures: heather and wheat. These trials were tested against infective larvae of Haemonchus contortus, a related nematode known to exhibit diurnal migra tion patterns. Larvae were exposed to diurnal variations in light and tempe rature. 4. Infective larvae of both species were recovered in greater numbers durin g periods when the lights were on. Temperature did not have a significant i nfluence on H. contortus larval availability but greater numbers of T. tenu is L3 were recovered at low temperature (10 degrees C) than at higher tempe rature (20 degrees C). 5. Both temporal variation in the presence of T. tenuis larvae on vegetatio n and the spatial variation in larval distribution may account for poor rec overy of Trichostrongylus tenuis L3 in the field.