21/3/19 - With a view to achieving quality winter feed for grazing, now is the time to get on top of grass grub and porina. Even relatively low levels of infestation can be a significant problem with recorded dry matter losses as high as 4,300 kg/ha.
Arable crops, particularly winter sown cereals, can be affected if planted into paddocks suffering grass grub damage.
Both grass grub and porina prefer to lay on taller vegetation, which protects the eggs. As a result, hay paddocks, grass seed paddocks and tall pasture are most likely to be infested.
In some regions (notably Canterbury) unseasonably wet and humid conditions in December have created excess feed and resulting thatch, providing near perfect conditions for the pests.
ADAMA National Sales manager Blair Hopkins says Diazol 800 Insecticide (Diazol 800), which contains diazinon and is a non-systemic organophosphate (OP) insecticide, is effective in controlling the pests. It acts on the nervous system of grass grub and porina, killing them by contact action or ingestion.
What to look for?
While Diazol 800 is the solution, Blair says diagnosis of the pest population burden in pastures is still down to old fashioned spade work.
“What’s best to do is to sample. With grass grub, check out any suspect paddocks in late summer or early autumn, by digging squares out of the ground. Eight grubs per 20 cm square and there’s likely to be a problem.”
Grass grub larvae feed on roots in the top 60 mm of the soil. Damage is indicated by patches of suppressed growth, typically with yellowing of the leaf tips. Also, clumps of pasture are pulled up by grazing sheep or cows. The larvae are horseshoe shaped and whitish in colour with 6 legs and a brown head. Mature third instar grubs are about thumb-nail size. The grass grub life cycle is typically one year.
Porina feed above ground on leaves. Patches of shorter pasture, with increasing areas of bare ground, indicate porina damage. Porina caterpillars feed from late evening and leave worm-like, granular casts. The caterpillars live in vertical burrows and are 15 mm to over 50 mm long, dark greenish-grey on top and cream underneath.