ARE CHRISTMAS GRUBS PESTS IN SOUTHERN QUEENSLAND SUGARCANE?

IN 2013 AND 2014, we observed numerous cases of larval Christmas beetles, Christmas grubs, as the most prolific or only white-grubs associated with root pruning in sugarcane crops. Currently, responsibility for damage to the root-mass is allocated to true canegrubs, with Christmas grubs’ role regarded as minor or insignificant. We summarised our previously unpublished data for evidence on the group; identification, biology, population dynamics and the efficacy of currently registered canegrub controls against Christmas grubs. Anoplognathus porosus (Dalman) (Coleoptera: Scarabaeidae: Rutelinae) was the most common Christmas grub in southern region canefields. Its life-cycle is mostly annual but a small proportion take longer. Beetles emerge November–January. Larvae seem to, by autumn, remove hair roots, leaving long, bare, primary roots. Later, as larvae feed through the winter months, the entire root mass may be removed. Feeding slows from September as they pupate. In mixed infestations, Christmas grubs seem predominant in winter. Christmas grubs were less readily controlled with registered insecticide treatments than canegrubs. Christmas grub populations were significantly reduced only where insecticide treatments were applied 1–3 months prior to the beetle-flight period. Liquid formulations of imidacloprid gave slightly more protection than granular controlled-release (CR) formulations. Longer-term, pre-emptive protection from Christmas grubs was minimal. Our data do not support (nor disprove) the notion of species interaction, whereby reduction of canegrub populations following insecticide treatment is associated with increased Christmas grub populations.