Partners in crime – How cabbage seed weevil assists brassica pod midge in damaging pods of winter oilseed rape in fields in Southern Sweden

The brassica pod midge (Dasineura brassicae) has emerged as an important pest causing problems in Winter Oilseed Rape (WOSR) (Brassica napus ssp. napus) production in Southern Sweden during recent years. Adult female D. brassicae oviposit into WOSR pods. After hatching from the eggs, the D. brassicae larvae feed on the inside of the pod, causing deformation and premature opening of the pod, which can result in dramatic yield losses. As the ovipositor of the adult female D. brassicae is too weak to pierce WOSR pod walls, oviposition is mostly done in pre-damaged WOSR pods. Most of these damages that are utilized for oviposition by D. brassicae are caused by the cabbage seed weevil (Ceutorhynchus obstrictus). The weevil feeds on the WOSR pods, thereby causing damages that facilitate oviposition by D. brassicae. Pest control measures are therefore directed against C. obstrictus as the brassica pod midge is extremely difficult to control. More information about the phenology and the combined effect of the two insect species is needed in order to create pest control as sustainable as possible. In this study, C. obstrictus and D. brassicae were monitored during six weeks from May until June 2019 in 20 different WOSR fields around Scania, the southernmost province of Sweden. Different active (visual count) and passive (yellow pan traps, yellow sticky traps) monitoring methods were used to assess the abundance of C. obstrictus and D. brassicae in the 20 WOSR fields and the effect of their presence on the amount of pod damage by D. brassicae. Trap samples were collected weekly and analyzed in the laboratory. At the end of the study period, pod damage was assessed in each field. The number of captured insects of the two species was very low in comparison with previous studies from the last two years, so was the percentage of pod damage. Phenology of C. obstrictus deviated from the expected pattern: Usually the cabbage seed weevil can be observed in a WOSR field earlier in the season than the pod gall midge. In this study C. obstrictus captures peaked 2-3 weeks later than D. brassicae captures, presumably creating less oviposition possibilities for D. brassicae. Nonetheless, the brassica pod midge could benefit from the presence of the cabbage seed weevil – significant correlations between abundance of C. obstrictus monitored in the field border and pod damage caused by D. brassicae inside the field were found. Insecticide treatment showed no effect as the amount of pod damage inside an insecticide-free control zone did not differ from the amount of pod damage outside the insecticide free control zone. Nonetheless, insecticides had been used by the farmers, indicating the need for development of more refined and more rapidly available monitoring and decision tools for farmers to improve IPM strategies for pest control of C. obstrictus and D. brassicae and to reduce insecticide use.Skidgallmyggan (Dasineura brassicae) har nyligen blivit en viktig skadegörare på raps (Brassica napus ssp. napus) och har de senaste åren orsakat stora skador i höstrapodlingar i Södra Sverige. D. brassicae-honor lägger ägg i höstrapsskidor och de nykläckta larverna äter på insidan av rapsskidan. Larvernas gnag påverkar rapsskidan så att den blir deformerad och spricker i förtid. Detta kan resultera i stora skördeförluster. D. brassicae-honans äggläggningsrör är för svagt för att genomborra rapsskidans vägg och äggen läggs därför oftast i redan skadade rapsskidor. De flesta av dessa skador på rapsskidor som kan utnyttjas för äggläggning av D. brassicae-honor är gjorda av blygrå rapsviveln (Ceutorhynchus obstrictus). När viveln äter på rapsskidorna uppstår gnagskador som kan gynna äggläggningen av D. brassicae. Växtskyddsåtgärder riktas därför mot C. obstrictus, eftersom skidgallmyggan är extremt svårkontrollerad. Det behövs mer kunskap om fenologin och den kombinerade påverkan av de två insektsarterna för att kunna utforma västskyddsåtgärder så hållbart som möjligt. I denna studie blev C. obstrictus och D. brassicae övervakad i 20 olika höstrapsfält över hela Skåne under 6 veckor från maj till juni 2019. Olika aktiva (räkna vivlar på plantan) och passiva (gulskålar och gula klisterskivor) övervakningsmetoder användes för att uppskatta abundans av C. obstrictus och D. brassicae i de 20 höstrapsfälten och för att undersöka hur abundans av de båda insektsarterna hänger ihop med omfattningen av skador på höstrapsskidor orsakad av D. brassicae. Fällfångster samlades veckovis och räknades på labb. I slutet av undersökningsperioden genomfördes en skadegradering i varje fält. Antal fångade insekter från de två undersökta arterna samt omfattningen av skador på höstrapsskidor orsakad av D. brassicae var väldigt få i jämförelse med studier från de två föregående år. C. obstrictus fenologi avvek från det förväntade mönstret: Vanligtvis observeras blygrå rapsvivel i ett höstrapsfält tidigare på säsongen än skidgallmyggan. I denna studie nådde fångsterna av C. obstrictus sin topp 2–3 veckor senare än D. brassicae fångster. Förmodligen förvärrade detta möjligheter till äggläggning för D. brassicae. Skidgallmyggan verkar ha kunnat dra nytta av blygrå rapsvivelns närvaro ändå: signifikanta korrelationer hittades mellan förekomsten av C. obstrictus i fältkanten och skador på höstrapsskidor orsakad av D. brassicae inuti fältet. Kemisk bekämpning med insekticider verkar inte ha gett effekt, eftersom skadorna på rapsskidorna inuti och utanför en insekticidfri kontrollruta inte skiljde sig åt. Att kemisk bekämpning med insekticider ändå har utförts av jordbrukarna tyder på att övervakningsmetoder kan behöva förfinas för att ge jordbrukarna beslutsunderlag för att förbättra IPM strategier mot C. obstrictus och D. brassicae och minska insekticidanvändningen

Friends with benefits? - Does gut microbiota of Spodoptera littoralis affect insecticide resistance and are there any costs of insecticide resistance development?

The insect gut microbiota has many important functions for insects such as detoxification of host plant toxins. Recently there has been a growing interest on the effect of insect gut microbiota on insecticide resistance development. Insecticide resistance is a growing concern for food security and sustainable agriculture. More knowledge about the relationship between gut microbiota and insecticide resistance development might help to gain more insight into the ecology behind resistance development and to refine pest management strategies. In this thesis I aimed to understand if gut microbiota can affect insecticide resistance, if there are any costs of resistance development and if gut microbiota can mediate such costs as well as any potential consequences of pesticide exposure on insect life history traits. To answer these research questions, I tested how the gut microbiota of a Cypermethrin-resistant and a susceptible Spodoptera littoralis lab strain affected survival of exposition with the insecticide Cypermethrin. The larvae had either been treated with antibiotics (Streptomycin + Ampicillin) prior to the exposition experiments or not, and thus had either a reduced or intact gut microbiota. The larvae that had been treated with antibiotics prior to the insecticide exposition continued to receive antibiotics after exposition as well. Following the exposition experiment I observed life history traits of the insects for the rest of the insect generation and recorded larval growth rate, larval development time, pupation rate, pupal weight, pupal development time, eclosion rate, survival until adulthood and female adult life span. Furthermore, I performed an oviposition experiment to measure female fecundity. The results showed that survival of insecticide exposition was higher for the resistant strain and for larvae with damaged gut microbiota from both the resistant and the susceptible strain. Insecticide resistance did not seem to depend on detoxification through resistant gut bacteria. Insecticide exposition had a negative effect on larval survival but increased larval growth rate, pupal weight, and fecundity. Thus, consequences of insecticide exposure might be long lasting and reach beyond and arise later than the initial survival following exposition. The resistant strain had shorter larval and pupal development time and increased pupation rate, but lower larval growth rate, pupal weight, fecundity, and survival until adulthood compared to the susceptible strain. Thus, resistance development seemed to create fitness costs for resistant insects. Gut microbiota seemed to have a mediating effect on the costs of resistance as well as on the consequences of insecticide exposition. My results thus indicate that gut microbiota is not contributing to Cypermethrin resistance of S. littoralis larvae. Instead, insecticide resistance may increase if pathogenic gut bacteria are removed. My results indicate further that both insecticide exposure, insecticide resistance and gut microbiota presence could have positive or negative effects on S. littoralis larvae depending on life stage and whether traits are involved in growth or survival. Implications of these results for pest control and further research are discussed