Grass Grub is a member of the scarab family. The larvae are whitish/grey, C shaped, segmented, and with a darker head and three pairs of legs on the underside of the body located in the segment behind the head. Newly hatched larvae are about 5mm long and weigh only 2-3mg. When fully grown, third instar larvae are 20-25mm long and weigh 150-200mg. In the third instar the Grass Grub’s black gut can be seen through the translucent body material. Grass Grubs which appear tinged green, purple and yellow are sometimes encountered and are diseased. Adult beetles are shiny, orange-brown in colour, about 10mm long and can be seen flying at dusk in November and December.
Grass Grubs are found throughout New Zealand, but are most likely to be found south of Hamilton at altitudes up to 1200m. Grass Grubs prefer lighter free-draining soils. Grass Grub Life Cycle Under normal climatic conditions the Grass Grub life cycle typically takes one year to complete. In the one year cycle, adult beetles emerge from the soil in spring and they live for 4-6 weeks during which time they mate and lay eggs. In the
North Island flights usually start in early October, but in the cooler regions of Otago and Southland beetles do not usually appear until November or even December. Female beetles are usually mated as soon as they appear on the surface of the pasture and lay most of their eggs close to the point of emergence. For this reason infestations tend to remain localised. However after laying most of her eggs the female may fly off to infest new sites. The eggs hatch 2-3 weeks after being laid and the young grubs feed in the soil on fine roots of pasture plants. Through the summer young grubs moult into the second instar stage and by autumn they have usually reached the third instar which is the final stage of feeding (and the most detrimental to lawns). During the late autumn and winter grubs stop feeding in the top 5cm of soil, and descend further down the soil profile.
Colour of the grub changes from whitish/grey to a creamy yellow during this phase. In the soil profile, the gut of the grub empties and proceeds to make a smooth walled oval cell, which it pupates from in early spring. Following pupation the adult beetle remains in the ground for a few days, to allow the outer skin to harden, before digging its way up to the pasture above. Populations typically grow over a 3-5 year cycle which is followed by a population crash caused by disease build-up in the soil. Within a single paddock however infestations can be at any stage in the cycle. In dry (drought) or cooler areas Grass Grub larvae grow more slowly and can take two years to become adults. In these areas larvae cease to feed over their first winter, and remain in the second instar stage before feeding resumes on pasture through spring, This can cause spring/summer pasture damage.