A Study of the Effects of Sitona discoideus (Coleoptera: Curculionidae) Larval Feeding on the Growth and Development of Lucerne (Medicago sativa)

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Abstract
(1) Plant growth analysis techniques were used to investigated the continual effects of Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) larval root-feeding on lucerne growth during the 1983-84 season in Canterbury, New Zealand. From data collected at about weekly intervals, continuous time trends and their errors were derived from orthogonal polynomial functions fitted to the natural logarithms of root dry mass and components of foliar growth derived from both affected and unaffected plants. The same approach was taken with total plant mass. (2) The high level of correspondence of estimated S. discoideus-induced yield loss using curves fitted by this technique, to the actual yields measured at mowing for hay, shows that the fitted trends are probably reliable. Indications were that after the first cut for hay, the damaged plants became abruptly dormant with heavier roots than their undamaged counterparts. In general, these plants exhibited many of the characteristics of plants under severe drought stress. (3) It is speculated that the abrupt cessation of lucerne growth observed in this study may somehow be related to the equally abrupt appearance of larval damage thresholds reported elsewhere in the literature.