Abstract
In a semi-field net cage experiment set up in three growing seasons (2015–2018) at Julius-Kühn Institute Braunschweig, Germany, the influence of different release times of adult cabbage stem flea beetles, Psylliodes chrysocephala (early September, mid-September, early October) and different beetle densities (6.7, 13.3 and 20 beetles/m2) on reproduction and damage potential in winter oil seed rape was investigated. In parallel to the cage trials, the number of deposited eggs was assessed in small boxes exposed in the field. Beetles released in early September laid significantly more eggs compared to beetles released early October, with a maximum of 270 eggs per female. The early release of beetles resulted in significantly higher numbers of larvae in autumn, but this differed significantly between the years. Warm autumn or winter conditions led to continued egg laying and development, whereas cold conditions stopped the reproduction. Significant yield losses (25%) and significant plant losses during winter were only found in 2016/17 in treatments with an early release of high numbers of beetles (20 beetles /m2). The German damage threshold of 3–5 larvae per plant was reached when 13 beetles/m2 were released before 20th September and 20 beetles per m2 after 20th September. Data of this study can be used for a better damage prediction and a more targeted beetle control.
Highlights
The cabbage stem flea beetle, Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae), is one of the most important autumn pests in winter oilseed rape (Brassica napus L.) (Nilsson 2002; Alford et al 2003)
The time of immigration, the start of oviposition and the occurrence of first larvae depend on autumn temperatures (Mathiasen et al 2015b; Johnen and Meier 2000; Bergermeier 1992; Bonnemaison and Jourdheuil 1954; Johnen 1997)
In 2015/16 and 2016/17, oviposition was rare before onset of October and the peak of egg deposition was found during October (Fig. 2)
Summary
The cabbage stem flea beetle, Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae), is one of the most important autumn pests in winter oilseed rape (Brassica napus L.) (Nilsson 2002; Alford et al 2003). The adult beetles immigrate to the fields between beginning of September and end of October (Hoßfeld 1993; Johnen and Meier 2000; Conrad 2019). Entomology, Georg August-Universität Göttingen, Göttingen, Germany the feeding damage of adult beetles is usually less important compared to the damage caused by larval feeding (Schulz and Daebeler 1984; Johnen 1997). In years with dry weather conditions, when plants grow very slowly, feeding damage by adults is important. The neonate larvae bore into the petioles and stems of the plants and feed till they pupate in the soil (Schulz 1983; Nilsson 2002). The time of immigration, the start of oviposition and the occurrence of first larvae depend on autumn temperatures (Mathiasen et al 2015b; Johnen and Meier 2000; Bergermeier 1992; Bonnemaison and Jourdheuil 1954; Johnen 1997)
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