Effect of time of sowing on Argentine stem weevil (Hyperodes bonariensisKuschel) damage in autumn-sown Tama ryegrass

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Abstract
A field trial is described which tested the laboratory observation that Argentine stem weevil appears to go into a state of facultative diapause during winter. Tama ryegrass was sown in an area of heavy weevil infestation on 23.1.78, 15.2.78, and 8.3.78. The crop that emerged from the 23.1.78 sowing was severely damaged by adult feeding and larval mining. The subsequent sowings showed sharp reductions in damage. This result supports the concept that the end of egg-laying is photoperiodically controlled and therefore predictable. By interpolation it can be estimated that this event occurs at the end of the first week of March. A corresponding sharp reduction in adult feeding suggests that the jotensiey of adult feeding damage declines rapidly after egg-laying because of reduced metabolic demand. Chlorpyrifos and phorate were used to estimate damage and were not tested as control measures themselves. Since egg-laying declines rapidly at a known time, the timing of sowing of short rotation ryegrass crops for winter greenfeed could now be far more accurate. Furthermore, greater precision should now be possible in the selection and timing of insecticidal treatments throughout the Canterbury region.

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