Degree-Day Concept in Development of Infective Larvae of Haemonchus contortus and Trichostrongylus colubriformis Under Constant and Cyclic Conditions

Abstract

SUMMARY If the temperature was held constant, Haemonchus contortus -infective larvae developed from eggs at 20 to 35 C only if the relative humidity ( rh ) was 100% but not if rh was 85%. Trichostrongylus colubriformis larvae developed best at 100% rh , but some developed at 85% rh and 20 and 30 C. Fewer T colubriformis seemed to develop if the temperature cycled between 20 and 35 C than if it was held constant, but reduction was not noticed with H contortus . Few H contortus and T colubriformis -infective larvae developed if the rh was cycled between 70 and 100% or if temperature and rh were cycled than if the temperature and rh were held constant. The degree-day concept appeared to be applicable whether temperature was cycled or not. A certain total amount of thermal energy is needed for larvae to reach the infective stage, and it does not appear to matter whether this energy is obtained continuously or at intervals. For both species, numbers of infective larvae present after different times essentially followed sigmoid curves. Under constant conditions at 100% rh , half of the T colubriformis 3rd-stage larvae were present after 5.75 days at 20 C, 3.1 days at 25 C, 3.0 days at 30 C, and 2.7 days at 35 C; it required 90 to 115 degree days (temperature in C times number of days) for them to develop. Under similar circumstances, half of the H contortus 3rd-stage larvae were present after 7.2 days at 20 C, 4.3 days at 25 C, 3.2 days at 30 C, and 3.5 days at 35 C; it required 95 to 144 degree days for them to develop.