Observations on the vertical migrations of the third-stage larva of Haemonchus contortus (Rud.) on experimental plots of Lolium perenne S24, in relation to meteorological and micrometeorological factors

The cycle of vertical migration of the infective larvae of Haemonchus contortus on experimental plots of Lolium perenne S 24, kept out of doors, has been related to meteorological and micrometeorological factors. 2. The three main factors concerned with vertical migration are temperature, humidity and light intensity. 3. In the majority of cases the greatest number of larvae are on the grass blades in the early morning and in the evening, less being recovered at night and during the middle of the day. The time of the morning maximum becomes progressively earlier passing from winter to summer, and the time of the evening maximum progressively later. The reverse is true in the second half of the year. 4. Low and high temperatures accompanied by low humidity inhibit vertical migration on the grass. 5. High humidity (continuous precipitation) does not prevent migration, the two daily maxima are apparent, but the numbers climbing are less than in the absence of rain.