Effects of different insecticide applications on population development of pollen beetle (Meligethes aeneus (Fabricius)) on oilseed rape (Brassica napus L.)

The pollen beetle, Meligethes aeneus (Fabricius) (syn. Brassicogethes aeneus (Fabricius)) is a major pest in the production of oilseed rape, Brassica napus L., in Europe. At high levels of crop infestation, adult beetles can cause severe economic damage by destroying the buds. Chemical control of this pest is biased due to the widely distributed pyrethroid resistance and a decreasing availability of insecticides with different modes of action. Effective insecticides are required not only to avoid yield losses by overwintered pollen beetles but also to minimize the reproduction rate of the beetle, thereby reducing the infestation pressure and the frequency of insecticide treatments in following years. The objectives of the present study were to determine the effects of the systemic neonicotinoid Biscaya (a.i. thiacloprid, 72 g ha-1) and the two contact pyrethroids Mavrik (tau-fluvalinate, 48 g ha-1) and Karate Zeon (lambda-cyhalothrin, 7.5 g ha-1) on population development of pollen beetle. In the years 2013-2015, field trials were established on crops of oilseed rape in the region of Braunschweig (northern Germany) and further locations in Germany. Recommended dose rates of the insecticides were applied at the bud stage, at the beginning of flowering or at full flowering of the crop. The effect of insecticide applications on overwintered pollen beetles was assessed by counting pollen beetles on plants before and up to two weeks after application. Furthermore, the impact of insecticide application at the bud stage on infestation of buds with eggs and larvae was investigated in field trials as well as in greenhouse experiments using plants and pollen beetles collected from untreated and Biscaya- or Mavrik-treated field plots. The effects of insecticides on larval instars and emergence of new generations of pollen beetles were studied in field trials. In addition, the impact of the insecticide treatments on parasitism rates of pollen beetle larvae by Tersilochus heterocerus and Phradis spp. was analysed by collecting samples of larvae from different field trials distributed over Germany. Application of Biscaya and Mavrik during the bud stage significantly reduced the abundance of the overwintered pollen beetles up to seven days after application. In contrast, application of Karate Zeon did not affect beetle numbers on plants. Consequently, the number of buds infested with eggs and larvae was reduced at least for 14 days after application of Biscaya and Mavrik compared to Karate Zeon-treated and untreated plots. Biscaya had a stronger effect on infestation of buds with eggs and larvae than Mavrik. Results of greenhouse experiments indicated that the lower number of infested buds found in Mavrik-treated plots resulted from a reduced number of overwintered pollen beetles, whereas in Biscaya-treated plots in addition to effects on overwintered pollen beetles a further reduction of bud infestation occurred. In all three experimental years the application of Biscaya at the beginning of flowering or full flowering resulted in significantly higher numbers of premature L1-larvae dropping down from the plants to the ground. This effect was only observed in Biscaya-treated plots, with up to 425% more L1-larvae dropping within one week compared to the control. Numbers of mature L2-larvae dropping down for pupation was reduced in plots treated with Mavrik and especially Biscaya. In Karate Zeon-treated plots the number of dropping L2-larvae was even higher than in control plots. In accordance with lower numbers of L2-larvae in Biscaya- and Mavrik-treated plots the number of emerging new generation beetles was reduced. No significant effect of insecticide application on parasitism rates of pollen beetle larvae by T. heterocerus and Phradis spp. was found. The parasitoid T. heterocerus was predominant with parasitized pollen beetle larvae occurring in all field trials and all experimental years, usually not earlier than in BBCH 65. In contrast Phradis spp. larvae were not detected in all field trials in 2015 and not before BBCH 67. Altogether, the results demonstrate that applications of Biscaya and Mavrik cause reductions of pollen beetle population growth. Especially the neonicotinoid Biscaya could form an important part of a resistance management program for controlling populations of pyrethroid resistant pollen beetles. Applications during the bud stage did not only reduce the overwintered pollen beetles, but in addition had sublethal effects on infestation of buds with eggs and larvae. Particularly Biscaya targeted for control of cabbage seedpod weevil or brassica pod midge during flowering has shown long-lasting effects on pollen beetle populations without affecting parasitization of pollen beetle larvae.