Factors influencing rain splash dispersal of infective larvae of Ostertagia ostertagi (Trichostrongylidae) from cow pats to the surroundings

Laboratory experiments were conducted to investigate rain splash dispersal of infective Ostertagia ostertagi larvae (L3) from cow pats to the surroundings during simulated rainfall. Simulated rainfall on dry cow pats resulted in dispersal of only very few L3. When cow pats were watered prior to the experiments, many infective larvae were stimulated to migrate up onto the surface of the pats, where they might be hit by water drops. When pre-watered pats were hit by falling drops, a considerable proportion of L3 was translocated. More than 90% of the translocated larvae were transported passively by splash droplets and only a minor part of the larvae migrated actively in water films or were transported passively by water run-off to the soil surrounding the pats. A reduction of the fall height of the simulated raindrops resulted in a significant reduction of the number of translocated L3. When cow pats were hit by drops of diameters between 2 and 5 mm, it was found that small raindrops were just as effective as large drops in splash dispersal of L3, provided the amount of rain was the same; but individual small drops were less effective in this respect than individual large drops. When splash droplets could travel freely, most of the splash-dispersed L3 were found within a horizontal distance of 90 cm from the pats; this applied for all sizes of drops. Most splash droplets carrying L3 were travelling within a vertical distance of 30 cm from the ground.