Vertical Migration of Trichostrongylid Larvae on Grasses

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. Taylor (1938), Rogers (1940), Kauzal (1941), Dinaburg (1944), Crofton (1948), and Rees (1950) studied vertical migration of infective trichostrongylid larvae on herbage. Their techniques essentially involved placement of known numbers of infective larvae, contained in a known volume of water, on the soil at the base of the test herbage in experimental plots in the field or laboratory. Subsequent recovery of larvae on the test herbage was interpreted to indicate vertical migration of the larvae from the soil surface onto the herbage. However, except for Crofton (1948) and Rees (1950), most of these investigators did not establish the actual height on the herbage the larvae ascended. Furthermore, only in a few cases (Crofton, 1948; Rees, 1950; Rogers, 1940) was the ability of the larvae to migrate vertically on the herbage correlated with environmental factors. The present study was undertaken in order to obtain an extensive series of observations Received for publication 20 August 1963. * From the Department of Veterinary Science, Paper N.S. 400, with the approval of the Director, Wis. Agr. Exp. Sta., supported in part by the University Research Committee. on the actual height of vertical migration by trichostrongylid larvae as well as the portion of the population which does migrate, and to observe the influence of various environmental factors including moisture, temperature, relative humidity, light, type of grass, and the number of grass blades available upon movement of the larvae. MATERIALS AND METHODS The larvae for these studies were obtained from seed calves primarily infected with Haemonchus placei with minor populations of Cooperia onchophora and Trichostrongylus colubriformis. The larvae were cultured in sterilized sphagnum according to the method of Cauthen (1940) and recovered by using Baermann funnels. Differential counts of larvae were not made for individual tests, and the larvae, therefore, have been referred to as trichostrongylid. The larvae were 7 to 20 days old when used in the tests, but in a given trial all larvae were the same age. Grass blades were grown from seeds planted in a light silt soil or a muck soil in paper pots. The soils were sterilized to render them free of nematode life prior to the tests. The grass seedlings emerged in a tuft of blades and were used when 3 inches or more tall. Numbers of larvae used in a test were estimated by dilution counts. By restriction of volume of the suspension, a known quantity of larvae could be placed in position on the soil surface at the