Skip to main content
Cows in pasture

Spadework and chemistry take on grass grub and porina

ADAMA recommends Diazol® 800 and sampling ahead of tough season
Cows in pasture

Doug Speers, from crop protection company ADAMA New Zealand, reckons nothing beats boots on the ground for keeping ahead of threats to crops. The experienced Commercial Manager for Central and Lower North Island says that’s important right now, when the dual threats of grass grub and porina may be significantly heightened and there’s still time to hit back.

Doug says reports, particularly from Canterbury, of large flights of grass grub beetles back in early summer could point to a spike in insect populations putting valuable pasture at risk. Strong pasture growth in areas up and down the country to date has also provided favourable conditions for both pests and plentiful feed for young grubs.

Doug’s approach to the menaces is matter-of-fact. “Dig a spade square and if there are one or two grubs, do something about it.” That “something” is Diazol 800 Insecticide (Diazol 800). Diazol 800 contains diazinon, a non-systemic organophosphate (OP) insecticide, which acts on the nervous system of grass grubs and porina grubs, killing by contact or ingestion.

Pasture pests not to be underestimated

Doug warns the potential for damage shouldn’t be taken lightly. Grass grub and porina can reduce pasture dry matter production by up to 4,300 kg/ha. Even low levels of the grubs can make a significant impact.

He cautions against attributing poor pasture growth under dryland farming systems to drought. “People sometimes blame a lack of moisture when pastures fail to thrive. But it is not till they start digging that they see that grass grub or porina are the real issues.”

While often mistaken for each other, grass grub and porina have quite distinctive features. Grass grub larvae feed on roots in the top 60 mm of the soil, with damage indicated by patches of suppressed growth, typically with yellowing of the leaf tips. The grass grub larvae are horseshoe shaped and whitish with 6 legs and a brown head for Costelytra zealandica, and a darker head for the Tasmanian grass grub Aphodius tasmaniae hope.

In contrast, porina feed above ground on leaves. Their activity and damage are characterised by patches of shorter pasture and increasing areas of bare ground. They feed from late evening and leave worm-like granular casts.

Effective control with Diazol 800

Doug says for effective grass grub control, Diazol 800 should be applied through to April, when grubs are small and near the surface and before damage becomes severe. The pasture should be closely grazed.

For best results, apply when foliage is wet, and rain or irrigation is imminent. It is critical that at least 13 mm of rain, or irrigation, follows to wash Diazol 800 into the soil where it will do its work. If it dries on the pasture, efficacy is reduced.

For porina, Diazol 800 should also be applied to closely grazed pasture. A higher rate is recommended when larvae are numerous, or pasture is very dense. The best control can be achieved by spraying in the early evening on warm nights, even if there has been some rain.

When applied as per label directions, there is no stock withholding period for Diazol 800.

For more information on Diazol 800 and how to combat grass grub and porina, click here.