Observations were made on vertical migration patterns of Haemonchus contortus infective larvae on Cynodon dactylon (bermudagrass) and Paspalum notatum (bahiagrass) pastures under summer climatic conditions typical of East Texas. Ten thousand H. contortus infective larvae (L3) were introduced to 100 cm^-2 subplots of each pasture species within a plot area of 1 m^-2. Plots were inoculated with larvae by applying them to the soil or mat beneath the vegetation. Herbage from the inoculated areas was harvested on five sampling days over a time span of 21 d. L3 recoveries were observed and recorded each day on four herbage strata viz. 0 - 5, 5 - 10, 10 - 20 and >20 cm from ground level. The log transformed larval recovery data were analyzed for effect of day, stratum, and day x stratum interaction for each grass species during two separate experimental periods. Precipitation, relative humidity and temperature during the study were subjected to correlation and multiple regression analysis with the larval counts. Significant (p ≤ 0.0017) differences were found for the effect of day on larval recoveries. No significant differences were detected for stratum or day x stratum interaction effects. A high positive correlation (≥ 0.93) between rainfall and total average daily larval counts was apparent. The multiple regression analysis did not show significant results for any of the climatic factors tested. This study showed that the H. contortus infective larvae can survive beyond 21 days in the soil and infest pasture grasses when the climatic conditions are favorable. Avoiding use of H. contortus contaminated pasturelands in summer at the onset of rainfall following a dry spell may effectively reduce worm loads in susceptible animals. Total larval recovery of H. contortus in this study was greater in bahiagrass than bermudagrass.

KEY WORDS: Haemonchus contortus, Vertical migration, Cynodon dactylon, Paspalum notatum, Infective larvae.