Kõdra- ja varre-peitkärsaka kahjustuse ja parasiteerituse tase talirapsil

Abstract

Raps (Brassica napus L.) ehk õlikaalikas on tänapäeval maailmas üks tähtsamaid õlikultuure (Hatzig et al., 2018). Rapsi kasvupindala laienemine on suurendanud ka ristõielistele taimedele spetsialiseerunud putukate arvukust ja levikut. Käesolevas töös käsitletakse rapsi kahjuritest kõdra-peitkärsakat (Ceutorhynchus obstrictus (Marsham) syn. C. assimilis (Paykull)) ja varre-peitkärsakat (Ceutorhynchus pallidactylus (Marsham)), kes Eestis kahjustavad peamiselt talirapsi. Rapsil kontrollitakse kahjurite arvukust peamiselt sünteetiliste putukamürkidega (Heimbach, Müller, 2013; Zimmer et al., 2014), mis aga hävitavad lisaks kahjuritele ka teised lülijalgsed, sh kahjurite looduslikud vaenlased (Jonsson et al., 2008; Kovács, 2018). Kiletiivalised (Hymenoptera) parasitoidid on peitkärsakate ühed kõige olulisemad looduslikud vaenlased. Kõdrapeitkärsaka kolm peamist parasitoidi on: Trichomalus perfectus (Walker), Stenomalina gracilis (Walker) ja Mesopolobus morys (Walker) (Haye et al., 2015; Ulber et al., 2010). Antud töö eesmärgiks oli välja selgitada kõdra- ja varre-peitkärsaka kahjustuse suurus talirapsi põldudel ja kas seda mõjutas kaugus eelmise aasta talirapsi põllust. Selgitati kui suur oli kõdra-peitkärsaka parasiteerituse tase ja kas seda mõjutas kaugus eelmise aasta rapsipõllust. Lisaks selgitada, milline oli kõdra-peitkärsaka parasitoidide liigiline koosseis ja kas eelmise aasta rapsipõllu kaugus mõjutas nende arvukust ja koosseisu. Lähtuvalt töö eesmärkidest püstitati hüpoteesid: (i) kõdra- ja varre-peitkärsaka kahjustuse taset ei mõjuta kaugus eelmise aasta rapsipõllust; (ii) kõdra-peitkärsaka parasiteerituse tase on suurem nendel talirapsi põldudel, mis asuvad lähemal kui 500 meetrit eelmise aasta talirapsi põllust; (iii) kaugus eelmise aasta talirapsipõllust mõjutab kõdra-peitkärsaka parasitoidide liigilist koosseisu. Kokku oli 12 katsepõldu, millest kuus asusid lähemal ja kuus kaugemal kui 500 meetrit eelmise aasta talirapsi tootmispõllust. Leiti, et kõdrapeitkärsaka kahjustuse taset mõjutas kaugus eelmise aasta talirapsi põllust, kuid parasiteerituse taset ei mõjutanud. Domineerivaks kõdra-peitkärsaka parasitoidiks oli T. perfectus, kuid leiti ka S. gracilis’e ning M. morys’e isendeid. Varre-peitkärsaka kahjustuse suurust mõjutas oluliselt kaugus eelmise aasta talirapsipõllust. Põldudel, mis asusid lähemal kui 500 meetrit eelmise aasta talirapsi põllust oli kahjustuse tase oluliselt kõrgem kui põldudel, mis asusid kaugemal kui 500 meetrit eelmise aasta talirapsi põllust. Insektitsiidide kasutamine põldudel mõjutas oluliselt positiivselt kõdra- ja varrepeitkärsakate kahjustuse taset ning oluliselt negatiivselt kõdra-peitkärsaka parasiteerituse taset.

Oilseed rape (Brassica napus L.) is one of the most important oil crops in the world (Hatzig et al., 2018). The increase in the cultivation area has also increased the abundance and distribution of oilseed rape pests. Cabbage seed weevil (Ceutorhynchus obstrictus (Marsham) syn. C. assimilis (Paykull)) and cabbage stem weevil (Ceutorhynchus pallidactylus (Marsham)), are two abundant and important oilseed rape pests and mainly harm winter rape oilseed rape in Estonia. Pest management in oilseed rape is mainly based on the use of synthetic insecticides (Heimbach, Müller, 2013; Zimmer et al., 2014), which in addition to pests, also kills the natural enemies of pests (Jonsson et al., 2008; Kovács, 2018). Hymenopteran parasitoids are one of the most important group of pest’s natural enemies. There are three main parasitoid species of cabbage seed weevil: Trichomalus perfectus (Walker), Stenomalina gracilis (Walker) and Mesopolobus morys (Walker) (Haye et al., 2015; Ulber et al., 2010). The aim of this work was to find out the extent of damage done by the cabbage seed weevil and cabbage stem weevil on the winter rape and whether it was affected by the distance from the previous year's winter oilseed rape fields. In addition, to find out the parasitism rate of the cabbage seed weevil and whether the distance of the last years fields affected it. In addition, whether the species composition of parasitoids was affected on the distance from the previous year's winter oilseed rape fields. Based on the aims there were three hypotheses: (i) the damage rate of the cabbage seed weevil and cabbage stem weevil is not affected by the distance of the previous year oilseed rape fields; (ii) the parasitism rate of cabbage seed weevil is greater in those winter oilseed rape fields that are located closer than 500 m from the previous year's winter oilseed rape field; (iii) the distance from the previous year's oilseed rape fields affects the species composition of parasitoids. In total, there were 12 experimental fields, 6 of which were located closer than 500 meters from the previous year's winter oilseed rape fields and 6 were further than 500 meters from the previous year's winter oilseed rape fields. We found that the cabbage seed weevil damage rate was affected by the distance from last year's winter rape field, but the level of parasitism was not affected by the distance of last year's winter oilseed rape fields. Trichomalus perfectus was the predominant parasitoid but also S. gracilis and M. morys were found. The damage rate of the cabbage stem weevil was significantly influenced by the distance from the previous year’s winter oilseed rape fields. The damage rate was significantly greater in fields that were closer than 500 meters from previous years winter oilseed rape fields compared to the fields that were more than 500 meters away from previous years winter oilseed rape fields. The use of insecticides in the fields significantly reduced the damage rate of the cabbage seed weevil and cabbage stem weevil and significantly negatively affected the parasitism rate of cabbage seed weevil.