Studies on the seasonal availability of the infective stages ofNematodirus filicollisandN. spathigerto sheep in New Zealand

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Abstract
The effects of differing seasonal periods of pasture contamination, upon the pattern of Nematodirus larval populations on 2 adjacent paddocks, were assessed over a 2-year period. The results confirmed previous findings that on set-stocked paddocks a relatively small Nematodirus larval peak occurs on pasture in the spring, followed by 1 or 2 larger peaks in summer or autumn. In addition, larvae of Nematodirus spp. were shown to be available in considerable numbers throughout the year. On pasture contaminated in spring and summer, larval peaks in late summer or autumn were exhibited by both N. filicollis and N. spathiger. Evidence is presented indicating that these peaks were the result of eggs deposited during spring and summer of the current season. The larval patterns of the 2 species differed mainly in the numbers of larvae available during winter and spring: whereas the numbers of N. spathiger larvae on pasture during these seasons were negligible, N. filicollis exhibited a spring peak and larvae were available in considerable numbers throughout the winter. The differences in the larval patterns are attributed to the different rates of development of the eggs of the 2 species. The eggs of N. spathiger develop much faster, at all times of the year, than those of N. filicollis. Consequently there was little overwintering of N. spathiger eggs on pasture. In view of the finding that larvae are available in considerable numbers throughout the year, it would appear that host resistance in older animals explains the occurrence of heavy Nematodirus infestations in young lambs only.