The ecology of immature phases of Trichostrongyle nematodes: II. The effect of climatic factors on the availability of the infective larvae of Trichostrongylus retortaeformis to the host

Abstract

1. Eggs and larvae of Trichostrongylus retortaeformis were used.2. The rate of hatching of eggs was shown to be mainly related to temperature. From November to March, when maximum temperatures were below 50° F., there was no hatching. When maximum temperatures of 50–55° F. occurred eggs hatched on or before the fifteenth day, but never during the first 8 days. Eggs hatched in 8 days or less when maximum temperatures of 60–80° F. occurred.3. When the rate of evaporation in the air was high, eggs still hatched and reached the infective stage, the grass blades reducing the rate of loss of moisture from the faecal pellet. Laboratory experiments show that eggs may not develop to the infective stage if the faecal pellets are on a grassless portion of the pasture. This is most likely to occur when the rate of evaporation is high and the temperature low.4. Hatching may be delayed by cold conditions, but some eggs remain viable for long periods and they hatch when the temperature rises. Eggs passed by the host in the autumn can survive a cold winter and hatch in the spring, but eggs passed during the coldest period die.5. During periods when the maximum temperature never exceeded 55° F., little or no migration of larvae occurred. When temperatures rose above 55° F. the number of larvae migrating increased; but rise of temperature was associated with increase in the rate of evaporation. High rates of evaporation reduced the number of larvae migrating on the grass blades.6. Some infective larvae died soon after exposure on grass plots, but a small number survived long periods. In cold weather some larvae were still alive after 20 weeks. A high death-rate occurred in warm weather. A large proportion of the larvae died during periods in which the rate of evaporation was high; in one of these periods 95% of the larvae were dead at the end of 4 weeks' exposure.7. The number of larvae on grass blades of a pasture was shown to be dependent, at any time, upon the climate at that time, and upon past conditions which had influenced hatching and survival: