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Biological Control of Weevils (Strophosoma melanogrammum and S. capitatum) in Greenery Plantations in Denmark 1 Introduction1.1 Background 1.1 BackgroundInsects in forestry have predominantly been controlled by means of chemical insecticides. The extensive use of chemical insecticides has, however, caused problems for the environment, the major concerns relating to side effects on non-target organisms and percolation into the ground water. This has led to an increasing interest and need for more environmentally friendly alternatives such as biological control. In Danish forestry the economically most important insect problems and consequently the most intensive use of chemical insecticides occur in the production of Christmas trees and decoration green (Kirkeby-Thomsen and Ravn, 1997; Ravn, 2000). Abies nordmanniana (Spach.) and Abies procera Rehder are the dominant tree species in this production today. No total estimate of the cultivated areas exists, but for A. procera the area is at least 9.300 ha, while for A. nordmanniana it is at least 21.500 ha (Christensen, pers. com.). The greenery has a high export value, exceeding 165 mio USD per year (Mainz, 2003). The market demands a very high product quality, and no damages from feeding of insect pests are accepted. In the Danish greenery production, weevils (Coleoptera: Curculionidae) are frequently occurring insect pests. In particular, two species from the genus Strophosoma, the nut leaf weevil S. melanogrammum Förster and S. capitatum Degeer, are economically important pests (Harding, 1993; Kirkeby-Thomsen and Ravn, 1997; Thorbek and Ravn, 1999; Ravn, 2000). The damage is caused by the adult weevils feeding on the needles (Fig. 1.1). The weevils feed on current-year needles as well as elder needles. Although weevil damages are observed in the whole canopy, the damages are most pronounced in the top of the crown, where also the needles of the leader are frequently heavily grazed upon. The damage may result in economically significant losses for the growers.
Figure 1.1: Damage on needles caused by Strophosoma spp. (from Sedlag and Kulicke, 1979). Both Strophosoma species are polyphagous and inhabiting woodland. The adults have been reported to feed on important forest trees such as pine, spruce, larch, douglas fir, true firs, beech, birch, mountain ash and oak (Grimm, 1973; Szmidt and Stakowiak, 1980; Ehnström, 1985; Palm, 1996; Urban, 1999), but also on hazel and herbaceous understory vegetation (Wagner and Schaefer, 1987; Parry et al., 1990). Needles, leaves and buds are the preferred food, but in case of shortage of foliage, the weevils also feed on bark of young shoots. In European forestry, Strophosoma spp. are generally regarded as insect pests in newly established cultures of conifers and deciduous trees as eg. beech and oak. In Denmark, however, in recent years weevil damages in stands of noble fir and nordmann fir appear to have become an increasing problem in the production of decoration green (Harding, 1993; Harding et al., 1998; Harding and Ravn, 1999; Thorbek and Ravn, 1999). S. melanogrammum seems to be the most common species in the Danish greenery plantations (Thorbek, 1998), whereas S. capitatum is considered of major importance in most other European countries. S. melanogrammum and S. capitatum occur together in the stands. The adults are morphologically very similar, brown and covered by greyish hairs and scales, but are easily separated in the adult stage by the presence of a naked black stripe (“melanogram”) on the elytra in S. melanogrammum (Fig. 1.2). Populations of S. melanogrammum consist solely of parthenogentic females, whereas both males and females are known for S. capitatum (Sedlag and Kulicke, 1979; Palm, 1996)
Figure 1.2: Drawings of the economically most important pest weevils in the Danish greenery production: Strophosoma melanogrammum and S. capitatum (from Sedlag and Kulicke, 1979) In both species, the adults have two periods of feeding activity: One during spring prior to oviposition and one during late summer-autumn (Grimm, 1973; Schauermann, 1973; Sedlag and Kulicke, 1979; Palm, 1996; Thorbek and Ravn, 1999; Urban, 1999; Harding et al., 2002). The larvae develop in the soil, where they are believed to graze on roots (Palm, 1996). However, only little is known about the biology of the larvae. Curculionid larvae are difficult to identify and within the genus Strophosoma, species identification is not possible (Hansen, 1964; Scherf, 1964). Therefore, great uncertainty exists about the development time of the two Strophosoma species, although this information is crucial for expectations to efficacy of control of the weevil populations. Reports range from 3-5 months (Schindler, 1974; Palm, 1996; Urban, 1999; Thorbek and Ravn, 1999) to 12-17 months (Schauermann, 1973; Sedlag and Kulicke, 1979) from oviposition to emergence of the offspring. Control of insect pests in the production of greenery was earlier based on the use of chemical pesticides. However, from 2003 no chemical pest control is allowed in state forestry (The Danish Environmental Protection Agency, 1998). For privately owned forests there is a political wish from the state authorities to phase out chemical pesticides (Ravn, 2000). Biological control using natural enemies of the pest populations may thus provide an alternative to conventional chemical treatment. Only few records exist concerning natural enemies of Strophosoma spp. Parasitation of the adult beetles by braconid wasps and parasitic flies have been reported as well as predation by ground beetles and spiders (Schindler, 1964; Grimm, 1973), and eggs parasitized by a so far unidentified braconid have occasionally been found (Harding et al., 2002). Yet, parasitation levels appear to be very low and of no major importance to the population levels. Until recently, no documentation of naturally occurring entomopathogenic fungi has been recorded. Lately, however, the entomopathogenic fungi Beauveria bassiana (Bals.) Vuill., Paecilomyces farinosus (Holm ex. C.F. Gray) Smith & Gray and Verticillium lecanii (Zimmermann) Viégas have been recorded to regularly kill adult weevils (Vestergaard et al., 2000; Eilenberg et al., 2003). No records exist concerning natural enemies of larvae, but it is believed that entomopathogens may play an important role in the regulation of the larvae, since they are known to be an important mortality factor for many other soil-dwelling larvae including Curculionidae (Rath, 1992; Marchal, 1977; Verkleij et al, 1992; Zimmermann, 1993; Keller et al., 1999, 2000; Vestergaard et al., 2000). Microbial control by entomopathogenic fungi may therefore be a possibility to minimise the damage caused by S. melanogrammum and S. capitatum. 1.2 Project objectivesThe aim of the project was to clarify the potential of insect pathogenic fungi for microbial control of Strophosoma spp. Emphasis was on the potential of using fungi belonging to the class Hyphomycetes. A number of biological parameters of the weevils and the fungi have been studied to assess the feasibility of a variety of possible control strategies. 1.2.1 Specific project objectivesThe specific objectives were as follows:
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