Grass grub have always been a “problem” in NZ gardens and lawns. They are c-shaped grubs that live underground, feeding on roots of grasses and other plants/shrubs. There are a number of species in the Beetle Family Scarabeidae (scarab beetles), but the native grass grub, Costelytra zealandica has always been in NZ. It’s traditional habitat and host plants were native grasses, such as tussocks, and they occur at quite high altitudes.
There is no doubt that these beetles considered the new, high-nutrient imported grasses as ice-cream, especially when we started planting whole paddocks full of that stuff! Tiny larvae emerge from eggs and slowly grow larger, shedding their skin as they grow. Each growth phase is an “instar”. Larvae creamy coloured and shaped like the letter C.
Their damage pattern is grasses losing roots and becoming stunted and leaves yellowing – in bad situations these plants die en masse. If you can literally roll the dead grass mat up (as if it were a carpet), your problem is likely grass grub. The beetles emerge in spring and are attracted to bright lights; the beetles feed on foliage of many plants, shrubs and trees. Even succulents are on the menu. But they do love lawns. Some species (on pastures) are tolerant of grass grub infestations: tall fescue, cocksfoot, yorkshire fog, prairie grass, birdsfoot trefoil, phalaris and chicory.
In the old days we used to spray lawns with soil insecticides, such as Diazinon, but apart from the fact this is tricky to obtain for garden use, the grass grubs have shown some resistance to that active ingredient (organo-phosphate). Since the 1990s some biological control mechanisms have been trialled and found to be quite useful:
* The bacterium Serratia entomophila, causes amber disease in grass grub. When grubs are infected they stop feeding in a few days and die rapidly; the bacterium stays in the soil for prolonged periods of time (years!) to kill the next generations.
* Bioshield is the name of the material https://biostart.co.nz/bioshield-grass-grub-liquid/. Spray this according to label recommendations in February, March and the first two weeks of April. That’s your window of opportunity, because the grassgrub larvae are in late second to mid-third instar and rather susceptible to infection.
I realise this information is a bit too late for people’s lawn treatment; this year has been pretty bad in many places in New Zealan.especially Canterbury. The larvae are pretty fat right now (and beyond the third instar) but because of the downpours they have come to the surface of your lawn (to avoid drowning)
Here’s a reasonably good idea: get an old, water-filled heavy roller and squash the living daylight out of them while the soil is still soft and squishy. Not recommended for steep Wellington sections on a distinct hill! Your insurance company will not be impressed.