Time to spray sitona weevil

Press, 3 June 1983, Page 9

Time to spray sitona weevil

The timing of insecticide application for sitona weevil control in lucerne is vital, according to M.A.F. entomologists Dr Steve Goldson and Dr Ray French. Application of one of three suggested insecticides should be carried out this month as the recent flights of weevils from their oversummering resting sites into lucerne stands have now finished, they advise. Such an insecticide application should kill most adults laying eggs in lucerne stands between now and November and thus prevent the lucerne root and nodule systems from being attacked by larvae. This damage becomes most obvious in the spring. At Darfield about 70 incoming adults per square metre have been recently recorded. Previous research has indicated that this population can produce a spring larval population of about 3000 per square metre, which may result in up to 40 per cent yield reduction in the first cut. After landing in the stand, weevils feed actively during egg laying which continues throughout the winter and early spring. Ruth Frampton, a Ph.D student is currently working on the viability of eggs laid now, and over the next five months. Her results after two season’s work indicate that:

• Females continue to lay eggs at varying rates during the winter and into spring. Each female can lay about 1000 eggs. O Eggs laid in late winter and early spring probably contribute most to spring larval numbers. Although Ruth has found that recently-laid eggs brought into warm laboratory conditions hatch in about three weeks it is thought that winter field conditions do not supply enough warmth for hatching and that a prolonged hatching period is associated with high egg mortality. Verification of this hypothesis could mean that in practice insecticide use may be delayed until July or August to coincide with a weedicide application, say the entomologists. The insecticides recommended by the entomologists are Chlorpyrifos, Fenitrothion and Diazinon. • Use Ikg ai/ha with about 300 litres of water or enough to get good leaf and ground coverage. • Spray on as warm a day as possible. • It is probably not necessary to spray lucerne stands over four years old. Most of these more mature stands are less susceptible to larval attack because of their robustness and deeper, more extensive root systems. However, it may be neccessary to consider some of these “older” stands as still physiologically young. A series of droughts for example would affect the stand’s vigour and the root systems may consequently be under-developed and hence still susceptible to larval damage. • Fenitrothion and Diazinon would also reduce any resident porina populations. • All three insecticides would also kill any aphids present. The entomologists stress that there is only one incoming flight of adults each year, in autumn.