In large parts of Europe, successful regeneration of coniferous forests by planting depends on the suppression of damage caused by the large pine weevil, Hylobius abietis (Linnaeus, 1758) (Coleoptera: Curculionidae) 1–3. This pest is especially damaging in young forest stands 2,4.
Hylobius abietis adults are attracted to the fresh clear-cut by the odours emitted from damaged, dying or newly dead (cut) conifer trees providing the breeding substrate 5 ; they remain at the site through the entire season and also overwinter at the site 6,7. The individuals of the parent generation can survive several years and continue to lay eggs also the years following harvest and the new generation weevils emerge as adult weevils several years later after cutting 8.Feeding of parental beetles to get reproductively mature when arriving to the clear-cut and losing their flight muscles and between mating on mature trees does not cause any economic damage, but beetle’s feeding on planted seedlings causes severe economic damage 9, because feeding may cause girdling and often the death of coniferous seedlings 7,10,11. A single weevil can damage several seedlings 12. Larvae hatch soon after oviposition and develop under the bark of the above-mentioned material 13. Development requires 1 to 3 years depending on the conditions 10,14, but recent studies indicate that climate change (mainly increasing temperatures) may increase the feeding rate and shorten the weevil’s life cycle 15,16,17. Maturing weevils also feed on the bark of coniferous seedlings 18,19. Hylobius abietis adults cause high levels of seedling mortality and economic losses 20. In Europe, H. abietis causes annual damage of almost 120 million EUR 21.
The depending on population density and weather conditions, H. abietis can kill 50 to 100% of seedlings at a site 11,22,23. Current management relies on silviculture, feeding barriers, and insecticides 20.
The use of synthetic insecticides is effective and relatively cheap 24. However, because of adverse environmental effects, insecticide use will be increasingly restricted 2,19,24,25,26 and replaced by various physical feeding barriers 27. Of the 405 million conifer seedlings in Sweden in 2020, for example, 50% were protected with stem coatings, 47% were unprotected, and only 3% were protected with insecticide 28.
Physical barrier systems (glue/sand/wax coatings, paper/plastic guards, shields, collars, clipstops, etc.) are alternatives to synthetic insecticides. Substantial research on physical barriers has been conducted mainly in Scandinavian countries 3,27,29,30,31, but also in the UK 11,24,32,33,34, and Slovakia 19,21,35,36,37,38.
A relatively new method for the physical protection of coniferous seedlings against the feeding damage caused by H. abietis is the application of Conniflex. In this method, the lower 60% of the seedling stem is protected with a coating containing fine sand embedded in an acrylate dispersion; application of Conniflex is an effective and environmentally friendly alternative to insecticide treatments 3,31.
In Europe, wax (names during development: Bugwax, Ekovax, KVAAE) has been used to protect seedlings against H. abietis for the last 10–15 years, although experiments and development have been ongoing since the 1990s 39. This wax is made from natural materials. It does not contain any insecticide or fungicide and is harmless to insects and animals living in the forest. Wax is fully biodegradable, non-reactive, non-toxic, and insoluble in water 40. The wax is significantly more elastic than normal wax, provides a physical barrier, reduces emission of volatile attractants, and thereby greatly reduces feeding by H. abietis 19,41. In Slovakia, C type of wax (Norsk Wax) has been used to protect seedlings since 2013. The melted wax is manually applied to seedling stem or seedlings are inserted into a “fountain machine” that applies the wax; the wax is applied from the root collar to 15–20 cm above the root collar 19,35. Under ideal conditions, the wax protects the seedlings for about 1.5 to 2.0 years. That this treatment can be as effective as insecticide application has been demonstrated in Sweden 27,30,42 and Slovakia 19,21,36,37,38, 43.
Vermifix is a glue that was developed to protect trees against creeping insects and for use in various glue traps 44. Vermifix (Papírna Moudry s.r.o., Czech Republic) has recently been tested for control of H. abietis on conifer seedlings 19,21,38. In general, the coating of stems with glue or wax has been found to reduce H. abietis damage and in most cases provided control that was not significantly different from that provided by an insecticide treatment 21,38.
Other potential alternatives are application of antifeedant compunds, the plant hormone methyl jasmonate (MJ), or natural product insecticides (NPIs). The study of Azeem 45 suggested that compounds produced by plants that are not hosts of H. abietis might be used to protect seedlings against H. abietis feeding. Unelius 46 stated that research is needed to find compatible combinations of coating material and antifeedants that can protect seedlings against H. abietis feeding for two seasons without harming the seedlings. Recent research suggested that an MJ treatment may protect coniferous seedlings against insect herbivory 47,48. NPIs are chemicals derived from plants or microorganisms. Willoughby 34 described the effects of azadirachtin extracts from neem trees, pyrethrin extracts from the Dalmatian chrysanthemum, and even sheep fat on H. abietis feeding, but none of them led to protection of seedlings.
Clear-cutting followed by planting is the generally used method for the regeneration of coniferous forests in northern Europe 3,49. In contrast, common Slovak forestry management is based on natural regeneration, i.e., a shelter-wood management system 50 that normally results in a great number of naturally regenerated trees 51. Under such conditions, pests such as H. abietis are not very significant. However, the annual occurrence of wind damage 52, particularly in mountainous spruce regions, results in cleared areas that resemble areas subjected to clear-cut forest management. In these areas, the abundance of stumps and harvest residues provide breeding habitat and food for H. abietis 18. Wind disturbances are also regularly followed by outbreaks of bark beetles (especially the spruce bark beetle, Ips typographus; 53), which produce additional material suitable for the development and maintenance of large populations of H. abietis. Damage to seedlings (mainly of spruce) by H. abietis is especially severe in national parks (at elevations up to 1,200 m a.s.l., pers. observ.), where the use of insecticides for seedling protection is forbidden 21,38. Therefore, researchers have recently investigated non-chemical alternatives for protecting coniferous seedlings against H. abietis feeding 18,19,21,36,37,38,54,55.
In the current study, we conducted a 3-year experiment to compare various chemical and non-chemical treatments for protecting seedlings against H. abietis. This experiment was conducted at a site in a mountainous region with high H. abietis abundance and at a nursery site where H. abietis was absent. We evaluated the efficacy, cost, and environmental effects of these treatments. Finally, we provide recommendations for H. abietis management under Central European conditions.