Small plot mowing trials carried out on central North Island pumice soils have shown that Huia white clover and SI70 tall fescue support high grass grub populations, while the resistant legumes Wairau lucerne and Maku Lotus pedunculayus suppress grass grub numbers to low levels. Measurement of the effects of grass grub on herbage production has highlighted both the susceptibility of white clover, Nui and Ruanui ryegrasses, and browntop to grass grub attack, and the potential of highly productive resistant or tolerant specres (lucerne, tall fescue and to a lesser extent Apanui cocksfoot and Matua prairie grass) for lessening the impact of grass grub on pasture production in this region.

The effect on grain yield of removing the upper, middle or lower third of the leaf canopy, singly or in combination, at silking or 2,4 or 6 weeks later, was measured to estimate the grain losses resulting from armyworm caterpillar damage. Complete defoliation at silking produced a 77% reduction in grain yield which lessened with delay in defoliation towards crop maturity. Less severe defoliations resulted in smaller reductions in grain yield and showed the middle and upper leaves to be more important in grain production than the lower leaves. Generally,!rain yield was found to be remarkably tolerant of post-silking defoliation and yiel losses exceeding 20% were recorded only after two-thirds of the leaves were removed and then only when this included the middle third of the leaves. Consequently, returns from insecticide applications to control caterpillars after silking will decrease with crop maturity, and controls seem unnecessary unless substantial defoliation appears likely.

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A high level of host plant resistance to Argentine stem weevil (ASW) (Hyperodes bonariensis Kuschel) has been recorded under field conditions in a line of perennial ryegrass (Lolium perenne L.) collected from the Takapau district in Central Hawkes Bay. When compared with the commonly sown cultivars of perennial ryegrass, 'Grasslands Ruanui' and 'Grasslands Nui', this ryegrass suffered minimal adult ASW damage, harboured low number of eggs and larvae, and hence suffered low levels of larval damage. Another ryegrass cultivar, Ellett, was also found to be more ASW resistant than Nui but its superiority over this cultivar was less marked than that recorded for 'Takapau'. The production and persistence of Ruanui and Nui swards was severely affected by ASW damage and as a result these became white clover (Trifolium repens L.) dominant whereas swards of 'Takapau' ryegrass persisted as well balanced grass-clover mixtures. The production ofNui and Ruanui ryegrass was similar but was approximately 60% and 50% lower than 'Takapau' ryegrass production in summer and autumn respectively.