Time Approaching To Act On Stem Weevil

Press, Volume XCVIII, Issue 28950, 18 July 1959, Page 8

Time Approaching To Act On Stem Weevil

It will soon be time to treat pastures against damage from the Argentine stem weevil. In the South Island the present recommendation is the application of 2Jlb of DDT or lindane in superphosphate in the last two weeks in Augus't.

The treatment should not be given too early or too late. If it is given too early the insecticide may be washed into the soil where at even an eighth of an inch under the surface it is too deep and there is a chance of the pasture being reinfested by the small brown weevil which does not move much below the soil surface. If the treatment is left too late the insects will begin laying their eggs and at that stage, according to Mr J. M. Kelsey, officer in charge of the Entomological sub station of the Entomology Division of the Department of Scientific and Industrial Research at Lincoln, the pasture may as well be written off as treatments will be ineffective. The point is that grubs hatching from eggs are under cover throughout their larval feeding period and even systemic insecticides which are taken into the sapstream of plants have no effect whatever on larvae. Eggs have been seen as early as August 31, but eggs in any sizeable numbers are not laid until the first two weeks in September. . The weevils can be present in pasture at any month of the year. From the middle of July until the end of August the signs of their presence become more marked though the damage that they do at this stage is not so serious. The weevil attacks the top of the grass leaf eating out the soft part and leaving the cuticle which becomes twisted like a tiny piece of cellophane. This damage can be confused with slug damage but in that

case there will be the tell-tale slime trails. Damage attributable to the weevil is a warning that action must be taken before the end of next month if a pasture is valued. Eggs of a further generation of weevils are laid from about December 17 onwards—these are the insects that do most of the damage in newly-sown pastures—and while the earlier treatment may continue to give control here it may be necessary to repeat the treatment in the South Island in the first two weeks of December. The application of DDT dry In the super is preferred to spraying for in the latter case the spray may stick to the leaves of the herbage and be consumed by stock. As with DDT silper for grassgrub and porina control it is recommended that the insecticide in super should be applied to dry pasture so that it will penetrate down on to the ground. Incidentally work is contemplated to try to work out lower dosages for Argentine stem weevil control. Mr Kelsey has observed that Short rotation and Italian\ ryegrass are equally susceptible to weevil attack, but perennial ryegrass and the new long rotation ryegrass, not yet released to farmers, while also subject to attack, seem to have the ability to throw off its effects. In Mr Kelsey’s view this may be due to a palatability factor.