Maximum-likelihood estimates of the mortality and migration rates of the infective larvae of Ostertagia ostertagi and Cooperia oncophora.

We present an analysis of the survival and migration rates of the infective (L3) stages of Ostertagia ostertagi and Cooperia oncophora. Although the majority of laboratory studies show that the survival of the L3 stage depends upon temperature, moisture and the age of the larvae, a simple mathematical model of larval demography, in which their mortality and migration rates are held constant, provides as good agreement between observed and predicted larval counts as models in which these rate processes are made explicit functions of larval age and microclimate. Maximum-likelihood estimates of larval mortality rates in the faeces and on the herbage are 0.0284/day and 0.00887/day respectively. The average migration rate from faeces to herbage under temperate Northern European conditions is estimated as 0.00884/day. Finally, we discuss the probable scale of L3 larval losses due to desiccation and lavage (active or passive migration into the soil).