Well I was in the ballpark with white grubs/scarabs which isn't saying much since that is a big category
We actually don't get many Maladeras in the US, and just looked up references in my books, and only one Maladera seems to be here located on the Eastern side, but with the same habits as all white grub species that love turf grasses as host areas if they can find them, but can really decimate roots/seedlings of so many flowers, veg and weeds.
In one my organic solutions book for white grubs in general, cultural solutions are crop rotation and poly-cropping (which of course you are/will do anyhow on your plot so that is moot) and the only other solution is biological control - beneficial nematodes. My other book noted that if you have 10 or more grubs per sq. foot that is considered a serious infestation (as I would imagine any grub on that scale). This book recommends milky spore consistently for 2 seasons due to the beetle's life cycles. Apply the milky spore in late spring or fall when temperatures are at least 70F/21C and you know the grubs are present. This book also recommends beneficial nematodes.