Sitona weevil a problem in lucerne

Press, 1 June 1981, Page 9

Sitona weevil a problem in lucerne

Lucerne stands may be infested with the sitona weevil at this time of the year. The presence of the adult weevils will be indicated by the scalloping of the leaves and where numbers are high the effect can be quite spectacular. Young stands are particularly susceptible.

Firm recommendations cannot be made at present for the control of sitona weevils or larvae pending the assessment of trials in progress, but according to Mr T. E. T. Trought, a scientist with the Ministry of Agriculture's researeh division at Lincoln, mob stocking on trial-size areas has shown that weevil numbers (larvae) have been reduced sharply.

The intention is also to apply chemicals against the adults'at about this time of the year and in this case either fenitrothion, diazinon, or chlorpyrifos (Lorsban) can be used at Ikg per hectare active ingredient. With the first two there is a slight risk of yellowing of the foliage but it is short-lived and does not appear to harm the plants. The other chemical does not cause yellowing but is more expensive. The use of these materials will also kill aphids and

make a later spraying or mob stocking against aphids unnecessary.

However, at this stage it cannot be said that if spraying is not done there will be any yield loss. Autumn-sown lucerne is. however, at particulr risk because the plants are young and susceptible to damage by the adult weevils now, so .it is recommended that au-tumn-sown stands be sprayed now.

The principle of controlling the weevil now is to prevent large larval numbers in the spring, which could cause severe denodulation and induce an apparent nitrogen deficiency in the spring growth of the plants.

The weevils are now in the adult stage and are laying eggs which will hatch in the spring.

Because it is not possible to make firm control recommendations now. it is suggested that if farmers are prepared to spray now against the weevils they should leave three well marked plots of. say. 20 metres by 20 metres unsprayed in their stands and get in touch with Mr Trought so that he can examine these areas in November for larval infestation.