Larvae hatch from eggs from October to December and develop through three instars (stages). The small first and second instar feed on roots and soil organic matter, doing little damage. By autumn, they have developed into the larger and most damaging third instar, feeding one to 3cm below the ground on plant roots.
Grass grub larvae prefer white clover, but eat most other species, too.
Populations typically grow over a three to five-year cycle, followed by a population crash due to disease build-up in the soil.
Within a single paddock, infestations can be at any stage in the cycle.
The best way to find grass grub is by digging. Grubs are translucent, 25mm long, with six legs and a honey brown head. They are usually curled in a C shape and become active only when disturbed. The grass grub life cycle typically takes one year, but in cooler regions they can take two years to develop.
One-year-old grass grubs pupate in spring, emerging as adult beetles which fly in October to December, starting the larval stage again.
In dry or cooler areas, larvae cease to feed over their first winter. They resume feeding on pasture through spring, rather than pupating, and can cause spring pasture damage.
If establishing new pasture into a paddock with grass grub problems, an insecticide or appropriate seed treatment is recommended.
Grass grub primarily feed in autumn, but control may be needed for spring sowings in areas where grass grub has a two-year cycle.
Cultivation usually drastically reduces grass grub larval numbers, but also disrupts natural diseases, allowing a rapid resurgence of the population over the next two to three years.
Direct-drilling techniques are often used in grass grub-prone areas to maintain more stable diseased populations and damage levels. Control of grass grub with insecticide has had mixed results, as soil-dwelling insects are relatively difficult to kill. Best success is through early applications of chemical in February to March, when grass grub are smaller.
Mob stocking and heavy rolling when soil is damp can reduce grass grub populations, but effects are variable.
A bio-pesticide is also available for grass grub control. – Supplied Agriseeds, and Country-Wide.
