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Green Scarab Beetle - Diphucephala sp.
Diphucephala is a genus of the Scarabaeidae family, many whose larvae are commonly known as Chafer grubs. Chafers are the C-shaped white grubs commonly dug up in your garden and usually handed to waiting birds who will readily accept them. Melolonthinae is one of several subfamilies whose larvae most fit the Chafer description and if they escape the birds and other hazards, mature into beetles of shades of brown or green, but Diphucephala beetles are special by sporting a glowing metallic green and like this species, can be quite spectacular.
According to the Australian Faunal Directory there are 68 Diphucephala species and collectively they are distributed Australia wide. The larvae like other chafer grubs burrow into the soil to eat the roots of various plants and some species (not Diphucephala) can occur in vast numbers to become a serious agricultural pest.
Most scarab beetles conduct their activities at night, however the green metallic ones are diurnal (active during the day), which is what you would expect from such a colourful insect, so presumably they must taste awful or look as though they would. I have seen them around Esperance at various times, but locally they are not overly common, which conceivably could reflect the availability of the vegetation the grubs feed upon. This in turn could reflect current burning practises, which being more limited these days than with pre-European aboriginal management, would curtail the growth of some plant species, particularly herbaceous ones.
These beetles are to be found after seasonal rains have moistened sub-surface soil, thereby permitting the beetles to escape their soil encased cocoon and so dig themselves out.