Shine light on pest problem

00:57, Dec 03 2012
Grass grub
SET TRAPS EARLY: Grass grub can be tackled before they become a problem by targeting the beetle.

Gardeners are always complaining about their lawns being damaged by grubs such as grass grub, black beetle grubs or porina caterpillars.

If the grubs are not eating the roots of the grasses, the porina are eating at the base of the grass causing bare patches in lawns.

When any of the above pests are active you will have starlings and black birds ripping your lawn apart to obtain a morsel of food.

This is especially so at this time of the year when the birds are nesting and seeking food for their hungry young.

I had a call this week from a gardener thanking me for the idea of catching porina moths in a light-water trap. The trap, which I will explain later, was a result of information that I received from a farming gardener from Taranaki. So I can't take credit for its invention.

I asked whether the gardener meant grass grub beetles not porina moths. But no, he meant the moths - he was catching lots every night using the trap.


He said it must be helping with the porina problem and that grass grub beetles had not yet emerged in his area.

I had never figured the trap would be a moth catcher but then the porina moth is fairly large and if it hit the pane of glass at speed it would probably fall into the water trap below.

The grass grub is a native grub that grows into an adult beetle (Costelytra zealandica) and has become a menace to both lawns and pasture. Before the pioneers converted native bush and forest to grasslands, the native grubs feasted happily on tussock and native grasses without managing to upset the balance of nature. Once our forebears opened up hectares of pasture grasses, the populations of grass grubs exploded - not really surprising, as each female beetle can lay 100 or more eggs in one month. These grass grubs are found only in New Zealand and are a good example of how the changes man makes to the environment end up altering the delicate balance of flora and fauna, resulting in major long-term problems.

Grass grub adults emerge in October, and are active until about mid-December, depending on weather conditions and location in New Zealand. The cooler the temperature, the later they emerge. The adults will start to emerge in mild conditions. When the soil temperature reaches about 10 degrees Celsius they mate, fly, eat and lay eggs between dusk and early evening. As they tend to fly towards light, you will know it when they hit your lighted window panes.

This attraction for the light has become one of our best weapons in controlling the pest in its adult stage. You can set up a grass grub beetle trap by placing a trough, such as the one used when wall-papering, directly underneath a window near a grassed area. Fill the trough with water to about two-thirds of its capacity, then place a film of kerosene on top of the water. Put a bright light in the window; the beetles fly towards the lit window, hit the glass and fall into the trough. The kerosene acts as a trap, preventing the fallen beetles from climbing out.

You can extend this method to areas away from the house by using a glass tank, such as an aquarium. Place the empty tank into a tray with centimetres of water (and the kerosene), and position a light inside the glass tank. By adding a sheet of ply or something similar over the top of the tank, you will ensure that the light shines only through the sides of the waiting water and kerosene.

It is better to use a dome-shaped battery-powered light rather than an ordinary torch as the bigger light makes the trap more effective.

If the tray and tank are raised off the ground and placed on something such as a table, you will get an even better result.

However you set up your beetle trap, this is a good method to dispose of the pests. Run this system from just before dusk to about two to three hours after sunset.

We know now how to make the grubs' preference for light work against them, but light can also work in their favour.

If you have un-curtained windows in rooms which are lit at night, you will find grass grub beetles attracted to the area during the early hours of the evening. Street lighting is probably the worst offender, and people with areas of lawn near street lights often find those are the parts worst-affected by grass grubs.

Problems? Phone me on 0800 466 464 or email

The Southland Times