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Written by Wally Richards.
Gardening Articles for week ending 4th June 2005
Written by Wally Richards.

Native to New Zealand is the Grass Grub, ‘Costelytra zealandica’ which has proliferated since we became a pastoral country. Farmers, greenkeepers and home gardeners are always fighting to control this pest each year and the methods that each group uses varies. A book in my collection called, ‘The Garden Pest Book’ 9by Bruce Chapman & David Penman) first published in 1982 shows that June and July are the most active months of the grass grub. Activity starting in March and building up to June/July then dropping off to finish in September.
Where the adult beetle emerges in October and is active till mid December dependant on area and temperatures. Southland sees emergence in December. The Females mate soon after emerging and lay their eggs back near where they emerged from. (unless distracted by night lights) A female lays about 40 eggs in batches, 70 to 200mm below the soil surface. The larvae hatch in about 20 days dependant on soil temperature.
They start feeding at these low depths and over the months work their way up to the top 20-30mm of soil in the May to July period. During the dry soil times of summer the grubs hibernate till the soil becomes moist with autumn rains. Which places the May to July period as the best time to control the grubs, as they are near the surface.
I believe that the main generation of the pest still follows the above pattern but as we have had warming conditions over the last 30 odd years that there are likely to be grubs in different stages of development in some areas at any one time. Likewise some of the beetle emergence may occur later in the season than the main emergence.
There is only one sensible solution and that is to lift some turf and check for the pest and how many in a square foot. This is done by making four cuts about 80 mm deep with a spade which form a square in the turf. The spade is then placed in one of these cuts and slid under the square to lift the grass, roots and all. You then check the hole made and the turf lifted for grass grubs. If you only find two or three grubs it is not really worth treating that area. If you find a good number in the area lifted then that area should be treated. By lifting turf in different spots you can work out where your problems are. To even simplify matters you should check past problem areas and places that are near windows or lights that are on at night. For instance lawns near street lights will have large populations of grubs where around the back of the house there may be none or only a few. Whatever treatment you decide to use, don't waste your money and time treating areas when there are no grubs of consequence. After checking the square of turf, place back and trample to settle.
There are three products that can be used the most common is the chemical Diazinon which can work well if your lawn soil is not a heavy clay, peaty or has a good amount of humus. Which means only light or sandy soils are suitable for its use when the grubs are near the surface. For all soil types you can use Pyrifos G, a chemical that many green keepers prefer. It is economical too as used at 200g/100sq metres. Evenly apply product across area to be treated either by hand or using a Scotts Handy Green fertiliser spreader on setting 1. Thats 2 grams per Square Metre. The 500 gram pack does 250 Square Metres. It is available from some garden centres. Best applied before rain or water in and re-water 24 hours later before allowing pets and children back onto the treated area.
I do not like using chemical treatments of any type because of the danger to pets, children and wild life. Chemicals such as Pyrifos G that do a good job means that they are also harmful to other living things.
In the past I have suggested a safer method and that is applying Neem Tree Granules at the rate of 50 grams per square metre then watered in. The theory behind this is that as the granules break down they release the properties of the Neem kernel which can be then taken up by the plant’s roots. Pests feeding on the roots or plant can then get some of the Neem into their gut and they would then stop feeding to die later of starvation. The properties can also affect their ability to instar being a growth regulator.
A number of gardeners that have opted for this natural product have reported satisfactory results.
If only problem areas are treated, where grubs are actively near the surface, by any of the products mentioned, then it is economical. To do all your lawn areas can be a waste of money for little benefit.
Another lawn pest that is also active over the May to July period and that is the Porina caterpillar. It also lives under the soil in a burrow and comes out to feed on the base of the grass about dusk to early evening. It can be controlled with the same products which means its best to apply any of them late in the day after having mowed the lawn earlier. Many lawns will also have root nematodes and these are not usually noticed by gardeners until they have treated for them, and then the lawn appears healthier than before. Root nematodes suck goodness from the roots but don't usually kill the plant. Neem has been shown to be very helpful in controlling nematodes.
Some gardeners have found brown patches in their lawns in spring to autumn periods and thought that they had grass grub problems. A number of these are often dry spot caused by thatch buildup and moisture unable to penetrate into the soil below.
By applying Thatch Busta in the spring and autumn to your lawns will keep the thatch problem at bay and the grasses roots will then penetrate deeper into the soil. An application of Gypsum every year will also greatly assist in opening up the soil for healthy lawns. Very dry areas can be treated simply with a dish washing liquid in water to make the soil more porous in dry times.
The main point is don't waste money and only apply whatever treatment you prefer, when the grubs are proved to be there.

Problems ring me at 0800 466 464
Email wallyjr@manawatu.gen.nz

Web site www.gardenews.co.nz

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