Vertical Migration of Trichostrongylid Larvae on Grasses

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Abstract
Experiments were conducted in the laboratory to investigate the ability of trichostrongylid larvae to migrate vertically on various grasses under controlled environments. Only a small proportion, 2 to 3%, of the larvae given opportunity to ascend the grass blades, under conditions considered favorable for migration, climbed. Of these, 59.2, 26.7, 9.9, 3.4, and 0.8% were found on the 1st-, 2nd-, 3rd-, 4th-, and above the 5th- inch height level from the soil, respectively. It was concluded that though infective trichostrongylid larvae can climb grasses, the greatest portion remain at the base of the vegetation. The larvae were comparatively more successful on the tall fescue than on the smooth brome. The external morphology of grasses apparently influences migration of the larvae. A film of moisture is obviously advantageous to migration. On an average 0.65% of the test larvae climbed wetted blades while only 0.04% climbed unwetted blades. Upward migration was favored by higher relative humidity. On the average 1.36% of the test larvae were successful at 95% relative humidity while only 0.06% were successful at 56% relative humidity. Lower temperature adversely affected upward migration. Only 0.13% of the test larvae climbed at 40 F while 2.54% did so at 79 to 80 F. A direct influence of light intensity was not clearly established since migration in light and darkness was essentially similar.

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