Factors regulating the population dynamics and damage potential of pollen beetle (Meligethes aeneus F.) on crops of oilseed rape

The pollen beetle Meligethes aeneus (Fabricius) (Col.: Nitidulidae) is one of the most important pests of oilseed brassicas in Europe. Due to extensive use of pyrethroids and the limited supply of registered insecticide products pollen beetle populations have built up widespread resistance to pyrethroids in recent years. Alternative integrated pest management strategies are needed to regulate the population growth and damage caused by pollen beetle below threshold levels. In this study, we investigated various factors which might have an impact on population development, behaviour, damage potential and natural control of this pest. The effect of different cultivars of winter oilseed rape, Brassica napus, on the reproduction and population growth of pollen beetle was studied in field experiments over three years. Further, the effectiveness of the alternative insecticides thiacloprid and chlorpyrifos-methyl, applied for control of adult beetles in the bud and early flowering stage of oilseed rape, on population growth and plant damage was tested in these field experiments. As hymenopterous parasitoids can exert substantial natural control of pollen beetle populations, we analysed the impact of site-specific factors and insecticide application on the level of larval parasitism of M. aeneus in a monitoring study, collecting samples from various locations across Germany. In addition, the effect of trap cropping in combination with targeted insecticide treatment on the spatio-temporal distribution of adults within the field was studied as an integrating pest management strategy for pollen beetle. Four Brassica napus (L.) cultivars with different flowering behaviour were grown in field experiments in 2008-2010. The population growth of pollen beetle on these cultivars was determined with respect to the effects of annual weather conditions on the phenology of pollen beetle infestation and plant growth stage. In 2008, the early flowering cultivars `NK Passion` (open pollinated) and `Elektra´ (hybrid) developed more rapidly than the late flowering culivars `Favorite´ (open pollinated) and `Titan´ (hybrid). This delay of flowering by 6 days significantly reduced the pollen beetle abundance and significantly increased the population growth of pollen beetle on cv `Favorite´ compared to cv. `NK Passion´. In 2009 and 2010, when the phenological growth of the cultivars was nearly synchronously, no cultivar effects on population growth could be measured, although the adult beetles significantly preferred the buds of the hybrid cultivars for oviposition. The emergence rate of the new pollen beetle generation in relation to the abundance of second instar larvae migrating to soil for pupation was significantly correlated with the period available for egg laying and larval development in the bud and flowering stage in each year. The application of the organophosphate insecticide chlorpyrifos-methyl and the neonicotinoide insecticide thiacloprid significantly reduced the abundance of adult pollen beetles in two of three years of study. However, although the abundance of adult beetles was only little affected by insecticide treatment in the third year, the number of second instar larvae and of newly emerged beetles was significantly reduced. In 2008, 2009 and 2010, percentage reduction of second instar larvae and emerging beetles by insecticides ranged between 83.2% to 90.6% and between 82.5% to 95.5%, respectively. Although the pollen beetles caused high bud losses in untreated plots, no significant effect of insecticide treatment on yield of oilseed rape was recorded. The parasitism rates of pollen beetle larvae by the parasitoids Tersilochus heterocerus and Phradis spp. (Hym.: Ichneumonidae) varied widely between locations and years in Germany. On the 36 and 42 crops of winter oilseed rape sampled in 2008 and 2009, the total levels of parasitism varied from 1.6% to 55.9% and from 1.0% to 81.3%, respectively. The parasitism was influenced by site-specific factors like geographical region/latitude, proportion of oilseed rape crop and field size. Insecticides applied during the main period of parasitoid activity during the bud and flowering stage of oilseed rape reduced the parasitism rates. Despite the preference of immigrating adult pollen beetles for turnip rape (Brassica rapa), a trap crop border strip of turnip rape had no significant effect on the spatio-temporal within-field distribution of pollen beetles on the main crop of winter oilseed rape, as compared to plots with a border strip of oilseed rape. Higher numbers of beetles remained on turnip rape until the oilseed rape began to flower. Further, insecticide application targeted to the border strip did not significantly reduce the abundance of adult beetles in the centre of the field. Plant damage was not significantly different between the treatments. These insignificant differences may have occurred because the plant growth stage of the turnip rape trap crop was only 3-4 days in advance of the oilseed rape main crop in all three years of the field experiments. In summary, these results present new approaches for alternative control strategies of pollen beetle which might be included in an integrated pest management system. The population growth of pollen beetles can be reduced by growing early flowering cultivars of winter oilseed rape and by targeted insecticide treatments within the bud and early flower stage. However, insecticide applications should be minimised because they can cause high mortality of parasitoids which are substantial biological agents of pollen beetle. The use of turnip rape as trap crop depends strongly on weather conditions.