Sitona weevil needs attention

Press, 18 May 1984, Page 16

Sitona weevil needs attention

Large yield losses can be averted if farmers spray now to control sitona weevil in lucerne stands. The best time to spray is from now until the end of May, according to an M.A.F. scientist based at Lincoln College, Dr Stephen Goldson. “The adult weevils fly into the lucerne stands between early March and late May,” he said. “Spraying now will kill most of the adult weevils before they

lay their eggs. But farmers shouldn’t be surprised to find small populations of larvae in the spring in spite of spraying. Some of the early flying adults will have already laid some eggs before the spray but a population density of 700 to 800 ■per square metre is probably required before yields are noticeably affected.” The recommended spray is chlorpyrifos at the rate of o.Bkg of active ingredient per hectare.

Dr Goldson has shown that yield losses of between 40 and 60 per cent can be prevented if sitona weevils are kept out of immature lucerne stands. “Most of the damage is caused by the sitona larvae feeding on the roots,” said Mr Goldson. “However, adult sitona feed on the leaves, leaving C-shaped notches. This damage is most obvious in December to January when a new generation of adults is emerging,” he said. After 10 to 20 days feeding, the adult weevils fly out of the lucerne to their oversummering sites, usually in places like the base of fence posts or in shelter belts. They remain dormant until between March and May when they fly back into the stands. They lay eggs throughout the winter until dying off between October and December. Larval populations peak in late November.

“As the eggs hatch, the tiny larvae burrow into the soil where they feed inside the root nodules or on the root hairs of the lucerne plants,” said Dr Goldson. “By this stage, control measures are impractical because the insect is undergrouitf and can’t be

“Mob stocking in winter has little effect on the spring larval populations and could damage the plants.” Immature stands are most at risk, probably because the root hairs and nitrogen fixing nodules are nearer the surface, which allows large numbers of larvae to get established. High larval numbers result in sickly plants, stunted growth and sometimes yellowing of the foliage. “These symptoms can be confused with poor rhizobial innoculation, nutrient deficiency and plant stress,” said Dr Goldson, “which is why larval damage is difficult to identify. We are also concerned about the long term effects of larval damage. “Repeated feeding on the roots could lead to something like bonsai lucerne,” he said. “Although plant density may be satisfactory, yields are sometimes disappointingly low in some older stands.

“It is possible that repeated sitona damage for several seasons could reduce the ability of the plant to produce. We are looking at this problem at. the moment.” <