Day-degrees models of nematode development assume that temperature stochasticity has no effect on the development rate of infective stages as long as the mean temperature is held constant. This assumption was tested in this study. Unembryonated Heterakis gallinarum eggs were subjected to nocturnal and diurnal daily temperature cycles at 12 and 17 C. respectively, and embryonation was compared with eggs subjected to similar stochastic daily cycles, in which random normal variations in the temperature were added to the 2 temperatures. The prediction that there is no effect of stochasticity was refuted. Embryonation of eggs subjected to variable daily cycles occurred significantly earlier than that of eggs subjected to deterministic daily cycles, suggesting that stochastic variation in temperature accelerated embryonation even though mean temperatures were the same. These findings show that the development time of H. gallnarum eggs is decreased by stochastic variation in temperature, which may have important implications for the effects of climate change on parasite availability.
Attempts to recover the infective third-stage larvae of Trichostrongylus tenuis from heather (Calluna vulgaris) vegetation in the field are rarely successful because the larvae may be: (i) concealed within heather leaflets; (ii) concentrated in dew droplets which are lost from the plants upon sampling; or (iii) simply highly aggregated in the field. Heather plants were exposed to T. tenuis larvae in the laboratory and kept under suitable conditions for larval migration. Few larvae were found in dew droplets or concealed within heather leaflets; most larvae were recovered from the plant surface. This suggests that the low larval recovery from vegetation in the field simply reflects a highly aggregated distribution of few larvae. In a second experiment, the efficiency with which infective larvae migrate up the structurally complex heather plants was compared with migration up two control plant species with simpler structures: a monocotyledon, wheat, and a dicotyledon, hebe. After constant exposure, significantly more larvae were recovered from heather than from either of the control plants. This implies that the structural complexity of heather causes no problems for the infective larvae.
Summary
Predicting outbreaks of Trichostrongylus tenuis in RedGrouse 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.
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