RE-EVALUATING THRESHOLDS FOR POLLEN BEETLE IN OILSEED RAPE

摘要

Existing thresholds for the treatment of pollen beetle in oilseed rape are in need of re-evaluation in light of the arrival of pesticide resistance in pollen beetle populations in the UK. Pest thresholds should relate the potential for pest damage tothe inherent tolerance of the crop, to ensure pesticides are applied only when necessary. Oilseed rape produces significantly more flowers than the optimum pod number required for potential yield, so there is an excess number of flowers which could be sacrificed to pollen beetle attack without yield loss. An experiment in which a range of pollen beetle populations were inoculated and confined on oilseed rape plants suggested that a single beetle is capable of destroying nine buds. Excess flower numbers(flower number-pod number at harvest) were assessed in a range of hybrid and conventional winter oilseed rape varieties sown at a range of seed rates (10 to 200 seeds/m2). Hybrid, open pollinated and semi-dwarf varieties produced a similar number of excess flowers suggesting they are equally tolerant of pollen beetle, although there were significant differences between specific varieties. Crops with fewer plants/m2 had more excess flowers per plant than more dense crops suggesting that small or 'backward' crops may not be as susceptible to pollen beetle attack as initially thought. The results make it is possible to predict variation in the number of excess flowers per plant within a season from measurements of plants/m2 at green bud, and the likelihood of pollen beetle damage exceeding excess flower number. A conceptual pollen beetle threshold scheme has been proposed in which the pollen beetle threshold is negatively related to plants/m2.