Even at relatively low numbers, they can make a meal of your farmers’ paddocks, particularly in the weeks ahead.
The economic threshold for control of grass grub in newly sown pasture, for example, is as little as three larvae per spade square, or 75 per square metre.
And research shows it only takes 40 porina larvae per sq metre to eat as much pasture as two stock units.
That’s without considering the extra cost of dead pasture plants and weed invasion caused by heavy pest pressure.
On the upside, says technical specialist Cynthia Christie from Nufarm, helping clients avoid such damage this autumn is as easy and cheap as doing some digging when you’re next on farm.
“Monitoring is the secret to success when it comes to controlling both porina and grass grub,” she says.
“And this is an opportunity to add value to your service in another way, too – often farmers don’t know the difference between these two pests, so by doing some surveillance and identification, you can make sure they use the right control programme for best results.”
Porina moth caterpillars are yellow-cream on the underside, and grey-green on the back, with a dark brown-black head.
They grow to 65 mm long, live underground in tunnels 15-50 cm deep, surface feed on grass at night (leaving tell-tale bare
patches) and are rarely seen alive unless they’re dug up.
Grass grub beetle larvae on the other hand are C-shaped, and creamy white in colour with a brown head. They start at about 6 mm long and finish up about 20 mm long when mature.
Typically they are found in the top few centimetres of the soil, where they attack the roots of ryegrass and clover, causing
yellow patches, loss of clover, stunted growth and plant death.
Damaged areas can spread outwards year by year.
Nufarm has several proven insecticide options available if required, Christie says, and the company’s territory managers
are more than happy to work with merchants to establish which are best suited to each farmer’s situation.
“For porina, Dimilin 2L is one of the most cost effective forms of control early in the life cycle.”
This insect growth regulator (IGR) interferes with the caterpillars’ ability to mature. After eating it, they are unable to
moult, then die, usually in their burrows.
The interval between moults is shortest when caterpillars are young, so applying Dimilin 2L early targets them at their most vulnerable stage.
“By ‘young’ we mean Dimilin 2L is best applied before caterpillars are 25 mm long. This is usually about 12 weeks after moth flights have peaked.”
For grass grub, options include Diazinon 20G which can be applied down the spout with seed at pasture renewal, and will control larvae for around six weeks.
Diazinon 20G will also control porina in both new and established pasture but has to be broadcast before rain.
Dew 600 is another option for both grass grub and porina in established pasture, which needs to be applied immediately
before or during rain to ensure the chemical washes into the soil.
For effective population control longer term, suSCon Green remains a preferred choice for new pasture, Christie says.
The tiny green polymer granule, which is drilled down the spout with the seed, is impregnated with chlorpyrifos which is control-released into the surrounding soil for up to three years after application, protecting expensive new grass from being wiped out by grass grub.
These days, more and more farmers are making suSCon an integral part of pasture renewal planning and budgeting, as opposed to a discretionary purchase, she says.
“It’s a given for those guys.
They’re the ones who can see the medium term gains, and who value not having to worry about spraying grass grub every year for three years. It’s a head ache they don’t want.”
Because of its length of activity, suSCon Green is ideal on farms where grass grub has either already been an issue, or is expected to pose a risk this year.
For more detail contact your Nufarm territory manager.