A study to assess the parasitism of insect pests in winter oilseed rape in Belgium: preliminary results


Abstract: A survey of the parasitoids found in commercial winter oilseed rape was initiated in 2012 and 2013 in the South part of Belgium, using both aerial sampling techniques and soil analysis. Fourteen fields located in two distinct areas and with two different tillage regime (normal and reduced or no tillage) were selected for before and just after flowering. Adult parasitoid hymenoptera were weekly sampled over 8 weeks using sweep net. Pollen beetle larvae, Meligethes aeneus (F.) (Col.; Nitidulidae), were collected and their parasitism rate assessed. Samples of soil were taken from 4 fields in 2012 and 8 in 2013 to collect brassica pod midge cocoon, Dasineura brassicae (Winnertz) (Dip.; Cecidomyiidae), and to assess their parasitism. The soils were gently washed into sieves and cocoons were isolated in Petri dishes until midge or parasitoid emergence.The main parasitoid wasps found in the sweep net samples belong to the Tersilochinae family. However, though adults of this family were regularly collected in large numbers, and were synchronized with their host, the parasitism level of the pollen beetle larvae remained low, with many of the fields below 10-15% parasitism. Preliminary analysis shows that there were no apparent differences between the two distinct areas and between the two different tillage regimes. The main explanation of this low parasitism rate could be the high occurrence of the insecticide applications, as most of the farmers regularly applied one or two insecticides during the season: the first to control pollen beetle before flowering and the second to control other insects later (e.g. seed weevil, brassica pod midge). The highest level of parasitism of pollen beetle larvae (43%) was found in an untreated field. The identification of the species is in progress.The analysis of brassica pod midge cocoons showed that the parasitism rate was low in 2012 (0-5%). However, these results were probably underestimated due to a high mortality of the cocoons during the rearing process. If the parasitism rates were expressed on the basis of rearing success (brassica pod midge or adult parasitoid emerged), the parasitism rate reached up to 59.6% in one specific site, with 58.8% due to 4 Ceraphronidae species and 48.6% due to one species, Ceraphron serraticornis Kieffer. In 2013, the parasitism rate was low (0-3.0%), despite a high success in the cocoon rearing process.These results have shown that several species of parasitoid Hymenoptera are present in Belgium, causing in some cases high parasitism levels. A better use of these parasitoid wasps in the biological or integrated control of several oilseed rape pests is possible, but there is a need to focus on improving understanding of the factors that could explain the variability of the parasitism between sites and the actions that could promote the activity of these beneficial insects and protect their existing populations.

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