GRASS GRUB MENACE

Press, Volume LXXXIX, Issue 27010, 9 April 1953, Page 12

GRASS GRUB MENACE

HIGHER POPULATIONS THIS YEAR

ADVICE ON APPLICATION OF D.D.T. Populations of both grass grubs and caterpillars in areas so far inspected were higher than last year when damage was so extensive, said Mr J. M. Kelsey, officer in charge of the entomological station of the Department of Scientific and Industrial Research at Ashburton, in a statement. He re- : minded farmers that damage by grass i grubs and oxycanus (porina) caterpillars had now reached a critical stage wherever these pests were present in average to high numbers. Mr Kelsey said that vigorous and dense pasture cover in most districts gave the impression that there was as yet no damage but it would be found on testing that mfich of the cover could be pulled because grubs had destroyed the roots. The subterranean caterpillar was also feeding vigorously at present and thinning out the aerial parts of pasture at ground level though the longer cover tended to mask their activity. In stating that populations of both grass grub and caterpillars on areas so far inspected were higher than last year, Mr Kelsey said that the severity of the attack would, however, largely depend on the presence or absence of rainfall and irrigation. If the average Canterbury yearly rainfall of about 26 inches fell evenly during the period from February to August the grub damage would be offset to some extent by the formation of new root systems, for though top growth of pasture was not evident from late May to mid-August pasture root growth continued in Canterbury at even the lowest soil temperatures. When, however, rainfall was low from February to August—particularly from early February to May—there was not sufficient soil moisture to promote new root growth of grub-damaged pasture, with the result that it dried up or was lifted*by grazing stock. In some districts, said Mr Kelsey, there would be severe grub damage later than August. This was thought to be due to grubs hatching from eggs laid by late-emerging beetles at the end of January and in February. These late larvae often caused extensive damage to young lucerne stands. Mr Kelsey said it was often thought that the subterranean caterpillar would cause most injury when there was no rainfall to promote pasture growth but this was not so. Soil moisture was apparently just as necessary for caterpillar development as it was for pasture because damage—unless populations averaged 20 or more to the square foot—was seldom extensive until after heavy rains from midFebruary onwards. After light showers there was an increase in damage for only a few days while north-west winds restricted major caterpillar activity until the next rains. There would also be some areas where there would be caterpillar damage after the end of August. This was due to lateflying moths of the commonest species of oxycanus- and also to the presence of other oxycanus species that sometimes caused trouble.

Effect of Rainfall Many farmers who applied D.D.T. last year had been disappointed at the lack of early control, he said, as conditions had prevented the D.D.T. from being spread through the soil by groundwater. That it was purely a matter of adequate rainfall had been proved time and again by the excellent control obtained on D.D.T.-treated irrigated land. “D.D.T. can be applied at any time,*’ said Mr Kelsey, “but it is considered that best results will be secured by applying it now before soil temperatures fall so low that top growth ceases for the winter.” The research station at Ashburton recommended the following alternatives. for applying D.D.T.—super or lime mixtures (they were based on the fact that the insecticides must reach the soil surface for best results):—(l) D.D.T. should be applied to close-grazed, dry pastures; or (2) if pasture is not dry apply D.D.T. to close-grazed pasture, but stock should be kept off it until at least half an inch of rain has fallen in not more than 24 hours. The long pasture at present to be seen throughout Canterbury fell outside the specifications of the first recommendation. said Mr Kelsey, but it was obvious that D.D.T. would have to be applied to it in any case this year. The test for dry pasture was to throw a handful of D.D.T.-super over the pasture to be treated, and then brush it with the finger-tips. If the dust fell off the pasture on to the ground the area was ready to treat. It was suggested that D.D.T. could be applied to the dry, long pasture using sacking or Canvas curtains round the topdresser to ground level to keep dust down, and dragging a chain or brush broom behind the machine to remove the dust as much as possible from the pasture. Mr Kelsey said that where there was a light drizzle or dew after the application, and the D.D.T. dried out on the pasture leaves, it might adhere so strongly that it would require two t° three inches of very heavy rain to dislodge it. “The present heavy rains are excellent for washing D.D.T on to the ground,” said Mr Kelsey, but it will be extremely difficult on most farms to get pastures closely grazed, as very few farmers have sufficient stock to do this because of the dense cover everywhere. This being so, farmers who have to treat wet long pastures can only hope for a continuation of the heavy rains to wash the D.D.T. off. Should, however, rain cease before the D.D.T. is removed from pasture, farmers should on no account graze stock on it if they wish to control the grubs.”