27 November 1998

Aphid sprays unwanted

SPRAYING spring beans for black aphid when the current threshold of 5-10% plants infested could be wasting money judging by the results of PGRO and ADAS Wolverhampton trials.

Despite over 30% of stems being infested on three sites and 10% on the fourth, and an average colony size of 500, control of the pest produced no yield response. Nor did the reduction of virus infection in the two trials in which it occurred.

Researchers Anthony Biddle of the PGRO and Bill Parker of ADAS point out that a yield benefit of 3.5% on a 5t/ha crop would be needed to cover the cost of controlling the pest. That ignores the application cost and the yield penalty of wheelings.

According to their findings it seems that modern spring bean varieties can tolerate much higher aphid numbers without yield loss than those grown in the 1970s.

Their advice is to only spray crops on which smothering aphid colonies appear to be developing.