Mass trapping of the blowfly Calliphora vicina Diptera Calliphoridae in Norwegian stockfish production areas : lure development, damage reduction, and pest species phenology

Abstract

Rødkinnet spyflue (Calliphora vicina) er et alvorlig skadedyr i norsk tørrfiskproduksjon. I dårlige år forårsaker insektene et økonomisk tap som overskrider 10 millioner kroner. Problemene oppstår når fluene legger egg på tørrfisken og larvene utvikles i fiskekjøttet. Fordi larvebefengt fisk ikke er et salgbart produkt, medfører spyfluene et direkte tap for norske tørrfiskprodusenter. For å redusere skadeomfanget har det blitt utviklet feller med lukt og farge som er attraktive for fluene. Fellene har blitt testet i produksjonsområder for tørrfisk over en fireårsperiode. Massefangstforsøkene viser en klar reduksjon i skade sammenlignet med kontrollområder der bekjemping ikke har foregått. 25-50 % av spyflueskadene på tørrfisk kan elimineres ved hjelp av disse fellene.<br> Ved hjelp av adferdsstudier i vindtunnel har attraktive luktstoffer blitt identifisert. Disse enkeltstoffene har videre blitt kombinert for å lage et syntetisk lokkemiddel som er nesten like tiltrekkende som spyfluenes naturlige ressurs – døde dyr. Lokkemiddelet brukes i rusefeller. Spyfluene viser også økt landing på gult og hvitt. Dette har blitt utnyttet i form av limfeller tilpasset forholdene i produksjonsområdene. Bruk av kontrast, i kombinasjon med lukt, kan øke fangsteffektiviteten i forhold til de skadegjørende hunnfluene. Livssyklusstudier i laboratoriet og feltundersøkelser av spyfluer i Lofoten har bidratt til økt forståelse av fluenes overvintringsstrategi og sesongaktivitet. Denne kunnskapen har bidratt til å forstå effekten av massefangst på fluebestandene og til å gi anbefalninger om optimalisering av bekjempingsstrategiene. Ved riktig bruk av de to felletypene holdes arbeidsmengden på et akseptabelt nivå, samtidig som man får god fangsteffekt. Undersøkelsene er gjennomført ved Nasjonalt folkehelseinstitutt og Bioforsk plantehelse.

Calliphora vicina causes yearly losses of 1–2 million € to the stockfish industry in Lofoten, Northern Norway. Production of high quality stockfish takes place from February to May. The dominant blowfly species with adults present during stockfish production are C. vicina and Protophormia terraenovae, but damage is only caused by C. vicina. Preliminary mass trapping tests showed that yellow sticky traps and funnel traps baited with dimethyl trisulphide were functional in the field. Their target specificity with high female catches and favourable cost efficiency indicated that mass trapping can be used to reduce damage from blowflies.<br> To improve the efficiency of traps, the attraction of C. vicina to single synthetic compounds, blends, and authentic odours was investigated in a wind tunnel. The attraction of females was 0–22% for single compounds, 26–64% for synthetic blends, and 58–88% for authentic odours. Dimethyl trisulphide was the most attractive single compound, and significant improvement in attraction was achieved with a three-component lure consisting of dimethyl trisulphide, mercaptoethanol, and o-cresol. Field tests support the results of the wind tunnel study, and a high number of blowflies, consisting of 99.1% female C. vicina, were caught in funnel traps. C. vicina landing behaviour was tested in the wind tunnel using odour in combination with 7 different visual cues representing different levels of realism. Flies showed a significant increase in landing on visual cues providing a vertical contrast with the background, whereas a horizontal contrast was associated with no difference in landing rate compared with treatments lacking visual cues. In a field catch study, blowflies showed a significant increase in landing frequency on traps when a lure was present, and a combination of olfaction and visual input significantly increased female catch ratios.<br> To develop an efficient management program, knowledge of the life cycle and phenology of blowflies in production areas is needed. Field investigations and cohort studies in simulated Lofoten climates indicated that abundance peaks observed in the field in early spring and mid-summer can be explained by a numerically dominant stockfish cohort. Simulation studies suggest that C. vicina may overwinter as a mixed population of larvae, pupae, and adults, and this flexibility favours egg-laying in stockfish. The large proportion of adults overwintering compared to the commonly used larval diapause strategy might be interpreted as an adaptation to exploit the stockfish resource. The majority of female C. vicina that cause damage to stockfish are likely to have developed on fish dried the previous year, and a continuous year-long trapping is recommended to reduce the population.<br> Mass trapping was tested as a potential solution to reduce damage during a 4-year experiment at 10 production sites. At five of these sites, mass trapping was performed, and levels of damage to stockfish were compared with five sites without trapping. The level of larval infestation was significantly reduced in areas with trapping. Olfaction-based traps selectively removed female C. vicina from the damage-causing cohort. Interception traps based on visual cues caught flies early in the spring to prevent damage during the most critical time of drying. Damage reduction of up to 63% was achieved in the last year. Given these effects, the use of trapping in this production system is highly cost efficient.