The Grass Grub

Te Ao Hou, December 1962, Page 59

The Grass Grub It is important to understand the life cycle of the grass grub. The adult beetle, commonly known as the brown beetle, is on the wing in November to early January. Eggs large and white are laid at least 3 inches in the soil and the young grubs hatch out in 7–16 days to start feeding on the roots of pasture plants. These grubs grow over the summer, autumn and spring, feeding all the time, and remain within 2–3 inches of the top of the ground until September or October, when they burrow to a depth of from 4–10 inches to pupate—a new generation of beetles appearing in November to start another cycle. Most farmers are familiar with the type of damage done to pasture by large numbers of grass grubs. In such circumstances most of the grass is killed and the dead turf can be rolled up like a carpet. This type of damage is so obvious that no effort is required to persuade the farmer to use an insecticide to bring the grubs under control. Recent investigations, however, carried out by the D.S.I.R., which involves the sampling of 70 paddocks in the North and South Islands, showed that whereas in grub-free pastures the leaf length could be as much as 6–8 inches in 4–6 weeks, areas of the same paddocks harbouring 1–9 grubs per square foot had leaf lengths of only 1–3 inches. The average reduction was over 3 inches in any 4–6 week period between the end of January and the end of August. The pastures at no stage showed what farmers usually consider to be typical grass grub damage. This rather startling discovery emphasises the necessity to treat pastures with D.D.T. where grubs are known to exist in the district even though no damage is visible on a farm. If D.D.T. is properly used it will proof the pasture against grass grub damage. One pound of 100 per cent D.D.T. per acre should be used where no evidence of grub can be seen but where there is a likelihood that they do exist. In other words this dressing will be an insurance and should be used particularly where new pastures are sown down. Where a pasture has been attacked and grubs are working actively the application rate should be doubled to two pound of 100 per cent D.D.T. per acre. During March and April farmers are advised to go over the farm with a spade digging bits of soil to see if any grubs are present. It should be remembered that they will be working in the top 2 inches of soil and that they will be very small at that time of the year. D.D.T. prills have proved an efficient insecticide and make it possible to apply accurately a comparatively small amount of D.D.T. required per acre without first having to mix the insecticide with fertiliser. The recommended rat of application in 16 lb of prills per acre, which contains 2 lbs of 100 per cent D.D.T. But good results have been recorded from the use of 12 lb prills, that is, 1 ½ lbs of 100 per cent D.D.T., and this lesser amount may be an economic proposition. The total cost including application should not exceed 25/- per acre.