Pesticides Research, 91 – Biological Control of Weevils (Strophosoma Melangrammum and S. capitatum) in Greenery Plantations in Denmark

Biological Control of Weevils (Strophosoma melanogrammum and S. capitatum) in Greenery Plantations in Denmark

Summary

Weevils from the genus Strophosoma cause damage in Danish greenery plantations. The crop, noble fir (Abies procera), is used for decoration purposes and has a high economic value. Further, it is the political intention in Denmark to replace chemical control by environmentally sustainable control, including biological control.

The project aimed to evaluate the potential for biological control of Strophosoma melanogrammum and S. capitatum by using insect pathogenic fungi. The following elements were included: 1) bio-assays to select virulent insect pathogenic fungi, 2) molecular characterization of Strophosoma spp., 3) field studies on Strophosoma spp. phenology, 4) field experiments to assess the efficacy of a selected isolate of the fungus Metarhizium anisopliae, 5) non target effects and spatio-temporal distribution of the applied M. anisopliae.

For bio-assays, isolates of M. anisopliae (5), Beauveria bassiana (7), Paecilomyces farinosus (3) and Verticillium lecanii (2) were tested. All isolates of B. bassiana and V. lecanii as well as two of the P. farinosus originated from naturally infected adults of Strophosoma spp. The bio-assay methodology included both dipping adults into a conidial suspension of a fungus, and mixing conidial suspensions into soil, onto which larvae were added. All tested fungi were able to cause mycosis in Strophosoma spp. For adults, the average survival time ranged between 16 and 20 days at 15°C, depending on isolate. For larvae, the median lethal time ranged between 20 and 30 days at 15°C for B. bassiana and M. anisopliae.

Molecular methods were implemented to assist in the identification of Strophosoma spp. larvae. It was possible to amplify and get sequences of part of cytochrome oxidase subunits I and II in mDNA. Consensus was found within each species of Strophosoma and specific primers were designed for both S. melanogrammum and S. capitatum. A combination of three primers gave the best results for the identification of Strophosoma larvae. The method can, with further modification, be used for similar studies within the family Curculionidae.

The study documented that adults of both S. melanogrammum and S. capitatum feed on the greenery stand in spring and autumn. Eggs are laid in the canopy during spring and after hatching the larvae seek to the soil to complete their development. Adult weevils disperse by walking within the stands at distances of several meters per week, while wind dispersal of larvae is apparently of no importance. For both S. melanogrammum and S. capitatum, the development time is in general 15-17 months, although S. capitatum may have a greater flexibility in its development time from egg to adult. The generation initiated in the spring thus emerges in the autumn the following year.

M. anisopliae (isolate BIPESCO 5) was applied against larvae of Strophosoma spp. in summer 2000 and 2001, and it was possible to reduce the target population significantly in the treated plots. The effect was, however, not seen until autumn the following year, due to the long development time of the larvae in the soil: In treated plots, 77 S. melanogrammum per m² emerged versus 102 per m² in untreated plots in spring 2002. For S. capitatum the data were 46 per m² versus 69 per m². In 2001 infections of M. anisopliae were initiated in adult of both Strophosoma species with prevalences reaching more than 80%. Again, the effect on population level was, however, not registered until autumn the year following the application of M. anisopliae.

The non-target effects of M. anisopliae were studied by sampling insects and mites in the treated plots. The results therefore reflect the ecological host range of M. anisopliae (which did not occur in the soil in the plots before treatment). Seven days after treatment, the two sampled insect orders (Hemiptera and Coleoptera) and ticks were found with prevalences of M. anisopliae above 50%, compared to no infection in the untreated controls. Infections in non-targets were found as long as 277 days after treatment. However, the effect on population level of non-target is still unexplored. The persistence of the fungus was also reflected in the fact that conidia of M. anisopliae were documented 418 days after application, by plating a soil suspension onto agar from the treated soil.

Overall, the project brought new insight on the phenology of the weevils. Also it documented that molecular methods can assist in the species identification of larvae. The project was successful to select and test virulent fungal isolates in the laboratory and to show that M. anisopliae was able to infect Strophomosa spp. in the field with a reduction in population size the following year. Non-target effects were observed in the field but were evaluated the effects on the population level are unknown. The project implemented and further developed a range of methods applicable for the tested system and also for other systems. It is concluded that it is possible to develop biological control of S. melanogrammum and S. capitatum.

A development towards commercial usage depends, however, on many factors. Despite the new insight obtained in this study, we stress the following points: Firstly, the phenology of the target insects must be fully understood by studying this in different types of stand and geographical regions. Secondly, the pest status of S. melanogrammum and S. capitatum must be assessed by more monitoring. Thirdly, more extensive field trials must be performed to obtain more data on efficacy. Fourthly, non-target effects should be more studied especially with respect to long-term effects. Finally, the economic aspects of biological control must be evaluated from several points of view: the grower, the producer and distributor of a biocontrol agent, and the society. In the bottom line, the societal benefits should be the main factor to determine development towards practical use.