The Foundation for Arable Research (FAR) is conducting trials to help farmers reduce their reliance on chemicals to control grass grub in arable crops.
Richard Chynoweth from FAR says farmers stand to lose three grass grub products in the next 10 years so the industry needs to think about reducing its reliance on these, particularly products containing Diazanon.
One of the most exciting discoveries made by FAR staff back in 2012 was a larvae of a long-legged fly (Ostenia robusta) feeding on grass grub pupa.
Before this discovery the grass grub’s only known predators were birds, bacteria and a few fungi.
One of the most exciting discoveries made was the larvae of a long-legged native fly feeding on grass grub pupa.
Chynoweth says nothing is known about Ostenia robusta but it is the first insect that might have potential for controlling grass grub through integrated pest management.
“It’s a long way off but it’s very exciting.”
Chynoweth says the fly was found on an average cropping farm growing clover, wheat, radish and ryegrass under a minimum tillage/direct-drill regime. The farm was close to a lake, though, so there is a chance the fly might prefer moist soils.
About the size of a blowfly, it appears to take flight in December and January, although it doesn’t fly much above the crop canopy, preferring to stay close to the ground.
Farmers have been asked to keep an eye out for these flies this season and report any sightings to FAR.
While the fly’s potential as a biological control agent is as yet unknown, GreenSeeker technology gives farmers the opportunity to better target areas for control and therefore reduce chemical use.
A GreenSeeker map can identify areas of grass grub damage and using GPS farmers can then apply inputs or cultivate specific areas.
The damage done to arable crops by grass grub is dependent on both the size of the population and the life-cycle stage of the grubs.
Chynoweth says that in the arable sector most of the damage done by grass grubs occurs during the crop establishment phase. Once established, farmers can use nitrogen to promote tillering to compensate for plant losses.
Crop established into trial plots with populations of 220 grass grubs/m2 showed significant plant losses but wheat plants were able to tiller to compensate for damage.
Yields in the plots that remained untreated were seven tonnes/ha whereas plots planted with treated seed that had subsequent OP treatments yielded 14t/ha.
Cultural control tools may include rolling but Chynoweth says it is the physical abrasion between soil and the grub that does the damage to the grub so the more soil that is moved around the better. This type of control is more likely to be applicable in a pastoral situation than in a cropping one.
The damage done
A grass grub is considered big when it is about the size of a 20 cent piece. Less than 5mm is considered small and it is the smaller, younger grubs that do most of the damage to plants.
Through its life-cycle it goes from being clear to a milky colour – this colour is fat deposits. These fat grubs also tend to be further down in the soil profile than the younger ones as they prepare to pupate. They need a cold winter to pupate and after pupation will not emerge in the adult phase until November.
