Impact of insecticides on oilseed rape bud infestation with eggs and larvae of pollen beetle (Brassicogethes aeneus (Fabricius))

One of the main insect pests in oilseed rape is the pollen beetle (Brassicogethes aeneus (Fabricius), syn. Meligethes aeneus). To maximize efficiency of control of this pest, insecticides are required that ideally, not just prevent yield losses by bud feeding of overwintered pollen beetles, but simultaneously minimize the reproduction of the pest, thereby reducing the size of the following generation infesting next year’s oilseed rape. The neonicotinoid active substance thiacloprid is known to reduce bud infestation with eggs and larvae. However, the mechanisms by which this occurs as well as the effects of other active substances are not known. In this study, the effects of the neonicotinoid insecticide Biscaya (a.i. thiacloprid) and the pyrethroids Mavrik (tau-fluvalinate) and Karate Zeon (lambda-cyhalothrin) applied at the bud stage of winter oilseed rape with recommended field rates on infestation of buds with eggs and larvae of pollen beetles were tested in field trials in Germany in 2013–2015. In additional greenhouse experiments, it was investigated whether effects on bud infestation were caused by lethal effects on pollen beetle or by insecticidal residues on plants causing sublethal effects. In the field trials, application of Biscaya and Mavrik significantly reduced the percentage of buds containing eggs and larvae in contrast to Karate Zeon. In 2014 and 2015, 14 days after application, bud infestation on the main raceme was reduced by 86 and 82%, respectively, in Biscaya-treated plots and by 51 and 71%, respectively, in Mavrik-treated plots compared to the untreated plots. In the greenhouse experiments, the lowest percentage of bud infestation with eggs and larvae was recorded on Biscaya-treated plants whereas on Mavrik-treated plants, there was no significant difference compared with the control. The results of the field trials show that Biscaya and Mavrik reduced oilseed rape bud infestation with eggs and larvae of pollen beetles primarily by lethal effects on overwintered pollen beetles or by repellency. However, Biscaya had additional effects on egg laying, which was supported by the greenhouse experiments.