An Estimate of the Relation Between Density of Bertha Armyworm and Yield Loss on Rapeseed, Based on Artificial Infestations12

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Abstract
Regressions of yield on larval density were obtained by caging different densities of laboratory-reared 5th instar Mamestra configurata Walker over commercially grown rape until they completed feeding. In 1974 and 1975, yield loss was 0.325 g/larva and was stable for infestations established on plants at different stages of growth (from ½frac12; full bloom to mature pods with seeds becoming dark green) and for 2 varieties in the 2 growing seasons. In a similar test in 1976, a yield loss of 0.479 g/larva was similar for laboratory-reared 5th instars and for “field-conditioned” larvae that were collected at 5th instar from a fenced field plot where they had developed from the 1st instar. The greater amount of damage per larva in the 1976 test was attributed to drought conditions.