Results of a field trial for the chemical control of grass grub (Costelytra zealandica) (White), melolonthinae, scarabaeidae), including information on its capacity for reinfestation

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Abstract
Results are presented of assessments made in 1963 and 1964 of effects of chemical treatments applied in a trial against the common grass grub (Costelytra zealandica (White)) in 1962. Treatment effects were measured by sampling larvae present and by visual assessment of pasture growth. A high negative correlation was consistently found between the two. Weed infestation increased in untreated plots and in the plots of most of the ineffective treatments. Heptachlor at 2 lb/ac gave virtually 100 per cent control in 1963 and 1964, though it did not completely eradicate grubs during the season of application. None of the five DDT formulations used (all at 2 lb p.p.i, DDT/ac) produced any effect significantly different from untreated control plots on this DDT-resistant population. The high level of control given by diazinon at 4 Ib and 2 lb/ac in 1962 continued in 1963 and 1964. It was shown by bioassay that this was not associated with any soil residual activity. The explanation suggested for this continued control without residual toxicity is that most adult female beetles are mated and lay their eggs at the point of primary emergence from the soil. Reinfestation of plots from which grubs have been eradicated is thus minimal from season to season. The known biology of the related melolonthid Phyllopertha horticola (L.) supports this hypothesis.