1 Introduction
1.3 IMPORTANT INSECT HERBIVORES OF THE BOREAL FORESTS IN
1.3.1 Biology of insect species
The European pine sawfly, Neodiprion sertifer Geoffroy (Hymenoptera: Diprionidae) and the Large pine sawfly, Diprion pini L. (Hymenoptera: Diprionidae) are severe pests of Scots pine (Pinus sylvestris L.) and outbreak in cycles. N. sertifer females usually lay 6–8 eggs in each of approximately 10–12 pine needles from late August to October and the eggs overwinter in needles. Larvae start to feed on mature previous-year needles of Scots pine during the first half of the summer season. Larvae pupate in soil in July and new adults emerge in August and September. D. pini has different timing to its life cycle. Adults overwinter as pupae in soil and emerge from soil from May to July. Adult females lay eggs on pine needles in June and July, and larvae hatch 3–4 weeks later. D. pini larvae consume all pine needle generations in the latter half of the growing season until they pupate from August to September.
Both species of diprionid sawflies produce only one generation per year and male larvae have five and female larvae six instars during their development (Lyytikäinen-Saarenmaa 1999;
Lyytikäinen-Saarenmaa and Tomppo 2002; Kurkela et al. 2005).
The great web-spinning pine sawfly, Acantholyda posticalis (Matsumura) (Hymenoptera: Pamphiliidae) is a harmful insect
species of the pine forests of Asia, and central and eastern Europe (Vapaavuori et al. 2010; Voolma et al. 2009). A. posticalis females lay eggs on needles of young and mature Scots pine in June. Larvae cut the needles of all needle generations and feed on them inside a silken web that they make close to the branch bark. Larvae feed on needles for three to four weeks from June to July, and at the fourth larval instar stage they move to the soil where they stay for two to five years before a short pupation.
Larvae of this species pass through five or six instars (depending on the sexes). Adults emerge from the soil from May to early July, but due to the long developmental period in the soil they are of different ages (Voolma et al. 2009).
The European spruce bark beetle (Ips typographus L.
Coleoptera: Curculionidae) is the most severe pest of Norway spruce (Picea abies K.) in the whole of Europe and Eurasia (Chinellato et al. 2014; Öhrn et al. 2014). Adults of I. typographus attack the thick bark of the Norway spruce trunk in May and June, females lay eggs in feeding galleries under the bark surface and the larvae develop and feed on the phloem underneath it (Cognato 2015). Larvae pupate in their feeding galleries and emerging adults make exit holes in the bark in August and September and then fly away after emergence.
Adults dig holes in soil litter for over-wintering during colder seasons. I. typographus usually attack bark of weakened or dying spruce trees such as wind fallen trees. After population density increases, adult beetles are capable of attacking heathy trees in the surrounding area (Weed et al. 2015).
1.3.2 Insect outbreaks and climate change
There have been serious outbreaks of the two species of diprionid sawflies (Neodiprion sertifer and Diprion pini) in the pine forests of northern Europe, which have resulted in tree growth reduction, timber losses and substantial economic losses (Lyytikäinen-Saarenmaa 1999; Lyytikäinen-Saarenmaa and Tomppo 2002; Kurkela et al. 2005). A large-scale outbreak of D.
pini occurred in Scots pine forests of Finland during the period 1997–2001. The outbreak affected approximately 500,000 ha of
the forest area in western, eastern and southern parts of the country (Lyytikäinen-Saarenmaa and Tomppo 2002). Heavy defoliation by N. sertifer and D. pini reduced tree growth in Scots pine stands by 38% and 94%, respectively, whereas the rates of tree mortality following outbreak by each species were respectively 4% and 30% (Lyytikäinen-Saarenmaa and Tomppo 2002). Outbreaks of D. pini are more severe than outbreaks of N.
sertifer due to its feeding on all-generation needles of pine and having the potential to fully defoliate the trees. Therefore, D.
pini outbreak has to be considered as a potential threat to both young and mature Scots pine forests of Scandinavia and other boreal regions. Sawfly performance may increase in northern Fennoscandia with increases in outbreaks on Scots pine projected to occur with climate warming (Niemelä et al. 2001).
Furthermore, primary damage caused to pine trees by sawfly outbreaks pose a risk to trees of secondary damage by other insects, e.g. bark beetles (Lyytikäinen-Saarenmaa and Tomppo 2002).
The web-spinning pine sawfly Acantholyda posticalis has recently undergone mass outbreak in Scot pine forests of Siberia, Estonia (Voolma et al. 2009) and Finland (Vapaavuori et al.
2010). The mass outbreaks of A. posticalis defoliated about 250 hectares of Scots pine stands growing on sandy soil on the Estonian island of Saaremaa in 2008 (Voolma et al. 2009). A.
posticalis outbreak occurred over an area of about 200 ha of Scots pine forest in western Finland during the dry summer of 2006. It caused serious damage to 30 ha of pine stands, and the seriously affected area was further increased in the summer of 2009, resulting in trees dying in up to 100 ha of the forest area (Vapaavuori et al. 2010). The northward spread of this sawfly species from, for example, Estonia (in the south), and covering the entire boreal forest zone is expected with current and projected global warming (Vapaavuori et al. 2010).
Outbreaks of the European spruce bark beetle (Ips typographus) have been found to increase at southern latitudes and with low elevation, and also with increasing summer temperature in Europe (Chinellato et al. 2014). I. typographus is a
tree-killing bark beetle species which has the potential to undergo large-scale mass outbreaks. Less than 1% of over 600,000 bark beetle species (Curculionidae: Scolytinae) worldwide undergo broad-scale outbreaks (Raffa et al. 2008).
Global warming is the key driver of extreme weather events (droughts and storms) that result in an increased number of weaker, stressed and wind thrown trees in forest sites and largely increase the population of I. typographus by providing them necessary resources such as a food, shelter and suitable breeding materials. The total volume of Norway spruce trees killed by I. typographus outbreaks during the period of 1960–
2009 in Sweden was about 9 million m3, and the largest I.
typographus outbreak in Sweden during the period (1971–1981) killed about 4.5 million m3 of those spruce trees (Kärvemo and Schroeder 2010). The ‘Gudrun’ storm of southern Sweden in 2005 felled 50–75 million m3 of conifer trees (Bengtsson and Nilsson 2007); this was the largest single cause of timber damage in the country in the 20th century and resulted in a massive outbreak of I. typographus in the Swedish forests. I.
typographus outbreaks are likely to increase in the boreal forest region with more severe forest damage predicted to occur with increases in global warming and the frequency and intensity of storms (IPCC 2014).
Temperature plays a key role in species composition and population dynamics of forest insects. Temperature influences insect populations by either directly affecting their phenology, behaviour, abundance and distribution or indirectly via other climate-induced changes mediated by other climate parameters (Bale et al. 2002). On the other hand, sensitivity of species to warming increases with trophic level, and higher level species benefit more from climate warming than insect herbivores, and this in the long-term may result in a balanced ecological response to climate warming (Berggren et al. 2009). Climate warming causes periodic increases in insect populations and enhances their outbreak potential (Dale et al. 2001). Populations of insect species adapted to the high latitude zone, mostly covered by boreal forests, have big advantages with climate
warming due to increases in their fecundity rate, generations per year and survival, and by reduction in their mortality rate with warmer winters (Stange and Ayres 2010). Warming causes northward shift in the outbreak areas of forest defoliators due to a positive growth response of the boreal conifer treeline trees with warmth (Wilmking et al. 2004; Soja et al. 2007). Future outbreaks (for the period 2080–2100) of spruce budworms Choristoneura fumiferana Clem. (Lepidoptera: Tortricidae) are predicted to last for approximately 6 years longer and to cause 15% more defoliation than the current outbreaks (Gray 2008).
European spruce bark beetles (I. typographus) are expected to benefit from predicted climate warming with increases in their geographic range, number of generations per summer and number of intensive outbreaks (Jönsson et al. 2009). The attack rates of I. typographus have been found to increase with summer temperature in Europe (Chinellato et al. 2014). An increase in temperature of 5oC increases the outbreak risk of N. sertifer in boreal pine forests due to a shortening of the larval development period by 37–41% (Kollberg et al. 2013). There is a likelihood of ‘damage chains’ in the ecosystem food-web, e.g.
the defoliation of pine trees by N. sertifer makes trees more susceptible to the attack by pine shoot beetles (Tomicus spp.) (Björkman et al. 2011).
1.4. BIOGENIC VOC EMISSIONS IN THE CONTEXT OF BOREAL