Epsilon Archive for Student Projects

Abstract

Larvae of stem mining weevils (C. sulcicollis and C. pallidactylus) cause damage inside the stem of winter oilseed rape when feeding. Both weevil species have a single generation annually but they differ in where they owerwinter. C. sulcicollis colonizes the oilseed rape fields in autumn and overwinter there. The weevil can therefore be controlled by insecticide treatment in autumn, whereas C. pallidactylus overwinters in the edge of forests, under bushes or similar and colonizes oilseed rape fields in the spring. C. pallidactylus usually starts to appear in winter oilseed rape fields in the beginning of May in south of Sweden, and a bit later in the middle of Sweden.
The main purpose of this master thesis is to evaluate how stem mining weevils impact the yield of winter oilseed rape. Insecticide treatments in autumn and spring were performed in two different locations with the aim to vary crop injury by the pests and thereby understand the impact of the pests on yield. The field trial study was conducted in the growing season 2022/2023 at two separate fields in the county of Östergötland. Weevil incidence was determined with yellow water traps. Measuring of length of damage inside the stem and grading of damage index was performed in May and June. The damage in May was caused by C. sulcicollis and the damage in June by both weevil species. The next generation of stem mining weevils was counted with the help of emergence traps. Plots in the field trial were harvested separately and analysed for oil content, chlorophyll content, moisture content and thousand seed weight. Profitability calculations were made for the insecticide and control treatments.
The sites of the field trials in this experiment were chosen with the expectation that pest level would be high, which was indeed the case. Especially pest level of C. sulcicollis was very high in the field trials. The results showed a mean yield increase varying between 170 to 180 kg ha-1 in one of the field trials when insecticides were applied in autumn, in spring and as a combination of the two compared to control. The autumn treatment decreased the damage inside the stem caused by C. sulcicollis, however no response on damage to insecticide treatment was observed later in the season when C. pallidactylus is expected to cause damage as well. In the second trial no yield increase or decrease in damage as a result of insecticide treatment was observed. The plant density was low after winter as a result of the lack of growth regulator treatment at this site, which may have contributed to the trial results. The field trials did not show any significant difference between the four treatments in profitability, even though the yield increase in one of the field trials covered the financial costs that spraying entails. Further research is needed to understand the reasons behind the yield increase without stem damage decrease when spring treatment is performed. In addition, more research is needed to evaluate insecticide treatment at different pest levels in order to be able to set profitability threshold values.