Examination of factors affecting larval mortality of the black vine weevil, Otiorhynchus sulcatus Fab., on container grown rhododendrons Public Deposited

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  • Two types of exogenous mortality factors, density-independent and density-dependent, were identified for the black vine weevil, Otiorhynchus sulcatus Fab., developing on container grown rhododendrons. The addition of the insecticide acephate, (O.S.- Dimethyl acetylphosphoramidothioate, Orthene®)75S) to the rhizosphere of rhododendrons in which larval O. sulcatus develop was categorized as a density-independent mortality factor. Acephate when applied as a drench, killed both early and late instar larvae. The mode of entry was primarily contact. Application of acephate drenches to control early instar (I-III) larvae prevents fatal injury (girdling) to container grown rhododendrons. A life table was constructed for the black vine weevil on container grown rhododendrons at three densities (beginning with 25, 75, and 150 eggs per plant) in the process of identifying mortality factors. The greatest mortality occurred during the first instar as the larvae become established on the root systems of host plants. At high larval densities (> 4 larvae per plant, in a 23 x 22 cm plastic container) the amount of available stem exoxylary tissue became a density-dependent mortality factor. This is because the larvae require exoxylary tissue to successfully complete development on rhododendron, although they do not feed on it prior to the fourth instar. Therefore, mortality increased with an increase in insect density because of competition for the limited amount of exoxylary tissue. The percent of stem exoxylary tissue removed (consumed) by larvae was dependent on the volume of rhododendron roots. Plants with large (1622 ± 223 ml) root volumes had significantly less (P<0.01) stem exoxylary tissue removed than plants with small (360 ± 265 ml) root volumes.
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