Chapter 4: Phloeoxylophagous insects
The group of phloeophagous and wood-boring organisms is mainly consisted of insect taxa – for instance, beetles (Coleoptera), hymenopterans (Hymenoptera) or flies (Diptera).However, beetles appear to be the most important taxonomic groups.Phloeoxylophagous insects and especially bark and ambrosia beetles (Scolytinae and Platypodinae) belong to one of the highly studied topics – and not only in forest protection.
Box 4.1: Invisible fly
Many of the well-known phloeoxylophagous insects, like bark beetles, start their feeding on living trees. After some time, they kill the tree and their development occurs in dead wood.There are not a lot of potential pests that are feeding inside the tree, while do not kill any part of it.For example, European pine shoot moth (Rhyacioniabuoliana) left the tree after the feeding lived, but kill the buds and destroy the shoots.Caterpillars of the goat moths (Cossuscossus) feed inside of the stem of deciduous trees, while majority of them were weekend before they attack them.Most clearwing moths (Sesiidae) are relatively hidden and mostly do not kill the tree, but left quite lot decayed fragments of wood inside and their activity could be recognizable from outside.Nevertheless, some species are able to feed and destroy tissues inside the tree, while their activity can be seen till after the felling.
One of the examples is a fly, Phytobiabetulae, which is feeding inside healthy birch (Betulaspp.) trees.Adults of this fly are active in the summer and are quite small (to 5 mm).TiinaYlioja and her colleagues studied this species and found that females oviposit under the bark of new shoots in upper birch crowns and young larvae immediately feed through the wood toward the stem and thus their activity cannot be fairly well visible.Victoria A. Bonham and John R. Barnett found that the galleries running through the wood as dark strips are wounded by the tree in response to the damage and they become sclerified.Larvae create very long galleries (> 10 m) called as pith flecks.Even if this species is quite common it does not indicate significant outbreak periods.Athing which is important is that this species does not have associated secondary species that would prefer the weakened trees.Second is that the damages of this species do not affect physical attributes of the wood, but the wood is not useful for veneer (i.e., low aesthetic effect).It is still not well known which factors influence high larval densities, although trees on fertile soils are significantly more attacked.Furthermore, slowly growing birches are often less damaged.One of the possible options against damages of this fly appears to be the use of mixture of birch trees with other tree species.
Bonham V.A., Barnett J.R. (2001) Formation and structure of larval tunnels of Phytobiabetulae in Betula pendula.IAWA Journal22: 289–294.
Ylioja T.,Rousi M. (2001) Soil fertility alters susceptibility of young clonal plantlets of birch (Betulapendula) to a dipteran stem miner. Écoscience 8: 191–198.
Ylioja T., Saranpaa P., Roininen H.,Rousi M. (1998) Larval tunnels of Phytobiabetulae(Diptera:Agromyzidae) in birch wood. Journal of Economical Entomology 91:175–181.
Ylioja T., Roininen H., Ayres M.P., Rousi M., Price P.W. (1999) Host-driven population dynamicsin an herbivorous insect. Proceedings of the National Academy of Sciences of the USA 96: 10735–10740.
Ylioja T., Roininen H., Heinonen J., Rousi M. (2000) Susceptibility of Betula pendula clones to Phytobiabetulae, a dipteran miner of birch stems.Canadian Journal of Forest Research30: 1824–1829.
In this case, we could divide the forest protection with respect to the place where it is done.Here will be a bit difference in protection of forest nurseries andforest stands.
Forest nurseries are mostly affected by insects damaging seedlings and saplings and control against them is not much different to forest stands in initial stage of forest stands after clear-cut.There are several organisms that might have more important impact on target tree species in forest nurseries than in forest stands. Beds in forest nurseries are often very dense monocultures.Thus, they are much more jeopardized by potential insect pests that are foraging on their bark, bast and wood.Several species are phytophages in their adult age, although their larvae are foraging on roots.An example might be Melolontha chafers– theirgrubs (i.e., larvae) forage underground on roots of disparate tree species. Roots are also damaged by Europeanmole cricket (Gryllotalpagryllotalpa), which is one of the best known and conspicuous orthopteran forest pests. Other species that cause damages are weevils (Curculionidae) that are foraging on seedlings –especially, those emerged from stumps in surrounding clear-cuts.Stems of saplings might be damaged by several species of barkbeetles (e.g. Scolytus spp.) and ambrosia beetles (e.g. Xyleborus spp.). They are creating galleries under the bark of large saplings (bark beetles) or are able to create galleries inside the wood of relatively thin wood (ambrosia beetles).Both these processes mostly cause death to saplings that are dried or broken. Some jewel (Buprestidae) and longhorn (Cerambycidae) beetles cause damages in the nearly same way.
Forest stands with their very long rotation period have potential to host much higher diversity of potential pests.Stands of coniferous trees like Norway spruce (Piceaabies) in Europe might be a good example – afforested clear-cut area is often affected by weevils (e.g.Hylobiusabietis), then in pole stage stand they are affected by small bark beetles (e.g.Pityogeneschalcographus), after this in premature and mature age by large bark beetles (e.g.Ips typhographus) and stocked timber or overmature trees might by attacked by ambrosia beetles (e.g. Trypodendronlineatum).
The phloeoxylophagous insects can be divided into those that are attacking bast (e.g. bark beetles of Ips spp.), wood (e.g. ambrosia beetles of Platypus spp.) or both bast and wood (e.g. many Cerambycidae).Their damages are different and also the dieback of trees is mostly much quicker in the case of bark beetles than ambrosia beetles.Bark beetles that are feeding in bast appear to be more important, because many species of them have more than one generation per vegetation season in lower altitudes and thus they are able to cause serious damages to the stands more times per year.It is also known that bark beetles are more important in boreal and temperate forests and their impact is replaced by ambrosia beetles when closing to Equator. In tropical forests, bark and ambrosia beetles can have overlapped generations through the year.The impact of insects that are feeding in both bast and wood appears to be lower in temperate and boreal forests, although some alien species might have very high local impact on native ecosystems and also on target trees within harvested woodland landscapes.Presently several jewel beetles and longhorn beetles are of high importance, e.g. in eastern states of the U.S.A.One of serious threatsis also association of phloeoxylophagous insects with alien associates like in elm bark beetles and symbiotic fungi or association of some longhorns with nematodes.
The active forest protection against phloeoxylophages is mostly based onchemical and mechanical approaches, while biological control is also, but still marginally, used.As an example of chemical protection in forest nurseries is the use of pheromone traps – even if sometimes un-effective.The mechanical approaches are, for instance, application of bark traps on weevils or burning of material that was attacked by pests. One of the important beetles in forest nurseries and clear-cuts is the pine weevil (Hylobiusabietis), which is a pest that combines damages on bast and bark of seedlings.Forest stands have again more complex system in protection against phloeoxylophages.The main used approaches are pheromone baited traps and tree traps, or combination of them – like tripod logs with pheromone and furthermore, with application of contact insecticide.All these kinds of traps are often used for monitoring of actual population densities of phloeoxylophagous insects.Presently, there are highly selective pheromones for particular insect species. Releasing of natural enemies is an example for biological control against phloeoxylophages.
All these systems have, of course their drawbacks like relatively short intervals of control of pheromone baited traps – because the trapping success of target insect species is lowering with rising time interval and number of non-target and often beneficial insects is also rising in traps.In the case of highly effectivetrap trees, it should be mentioned need for early deactivation of this trap trees, which means peeling of the bark during the time of last larval stages, spraying by insecticides immediately before emerging of immature beetles or covering by net.Tripodsmight be nonselectivemethod– many beneficial insects are killed.And finally, biological control may fail due to poor adaptation of natural enemies.
Figure4.1.Last generation of the year of the Spruce Bark beetle (Ips typographus).Callow (i.e. immature) adults are mostly at the end of former larval galleries.Some part of them will overwinter under the bark, while majority will emerge and overwinter in forest soil.
Examples:
Example 4.1.Karvemo S., Van Boeckel T.P., Gilbert M., Gregoire J.C., Schroeder M. (2014) Large-scale risk mapping of an eruptive bark beetle–Importance of forest susceptibility and beetle pressure.Forest Ecology and Management318: 158-166.
Attacks by spruce bark beetle (Ips typhographus) on Norway spruce (Piceaabies) forest stands can cause huge economic losses in Europe.This bark beetle is one of the most important forest pests in Europe.Its population densities have increased during the recent years.Except of mountainous and boreal parts of its distribution (i.e. native distribution of Norway spruce), has this species more generations per season.
Karvemo et al. (2014) made the risk-rating models for damages caused by this bark beetle.They studied this species in temperate part of Sweden and used predictors like wood volume of Norway spruce and Birch per hectare, mean tree height, distance to the clear cuts, presence of bark beetle infestation spots and number of infestation spots in the neighborhood.They found that increasing volume of the Norway spruce was the only one significant predictor that positively influenced relative risks of infestation by the spruce bark beetle.They also found that the trees killed by the spruce bark beetle in managed forest landscapes are distributed in many small infestation spots.
Example 4.2.Lubojacky J., Holusa J. (2013) Comparison of lure-baited insecticide-treated tripod trap logs and lure-baited traps for control of Ips duplicatus (Coleoptera: Curculionidae).Journal of Pest Science86: 483-489.
Double-spined bark beetle (Ips duplicatus) is presently spreading in Europe.The species has nearly the same requirements as the most important pest of Norway spruce– Ips typhographus.With respect to forest protection against bark beetles, mostly two measures are used –pheromone baited traps or pheromone baited tripods that are treated by insecticide.Even if, trap trees appears to be the most efficient, they bring relative risk when they are not debarked in the right time before new adults emerged.
LubojackyHolusa (2013) studied the difference between pheromone baited traps and pheromone baited tripods that are treated by insecticide.They found that commonly used pheromone traps were nearly three times more efficient in captures of the double-spined bark beetle.Furthermore, pheromone traps captured more females than males, while tripods lured and killed males and females in the same ratio.Both types of traps were more efficient during spring months than in summer.Moreover, tripods killed significantly higher number of beneficial and non-target insects than pheromone traps. The use of tripods thus should be omitted in forest protection.
Example 4.3.Zas R., Sampedro L., Prada E., Lombardero M.J., Fernandez-Lopez J. (2006) Fertilization increases Hylobiusabietis L. damage in Pinuspinaster Ait. seedlings.Forest Ecology and Management222: 137-144.
Pine weevil (Hylobiusabietis) has quite complicatedecology.Adults are feeding on bark of young stems and they are partly saproxylic in stumps.Larvae are feeding in fresh stumps and,furthermore,in stems of young seedlings.Adults live for more years and preferred clear cuts with fresh stumps of conifers.One of the best measures in forest protection against this species is to leave the afforestation to the second year, when stumps are more decayed and thus not attractive for the Pine weevil.Maritime pine (Pinuspinaster) is the most important forest tree species in Spanish Galicia.Fertilization is a commonly recommended for second-generation Maritime pine plantations.
Zas et al. (2006) studied damages caused by the Pine weevil in Maritime pine (Pinuspinaster) plantations in Spain.They found that fertilization (combinations of ammonium nitrate, calcium phosphate, potassium sulphate and magnesium sulphate) had a strong and significanteffect on pine weevil damage.The damage by the pine weevil was greater on fertilized plants.The loss on seedlings against unfertilized control was nearly four times higher.Moreover, no significant effect of fertilization on growth of seedling of Maritime Pine was observed. Fertilization in this case did not make any profit and the latter is truth.
Example 4.4. Muller J., Bussler H., Gossner M., Rettelbach T., Duelli P. (2008) The European spruce bark beetle Ips typographus in a national park: from pest to keystone species.Biodiversity and Conservation17: 2979-3001.
The spruce bark beetle (I. typographus) is considered as one of the major pests in European forests.On the other hand, it is known that if the areas with high nature conservation value are attacked by this beetleand are left for succession, the biodiversity often rapidly grows.The bark beetle surely have high potential as ecosystem engineer that change the environment and is creating habitats for many other organisms, furthermore, it also open the canopy of high forests.
Muller at al. (2008) studied this bark beetle in Germany.They compared undisturbed forest stands, gaps created by man andgaps created by spruce bark beetle.Species richness of three studied taxa – bugs (Heteroptera), bees and wasps (Hymenoptera) and saproxylic beetles (Coleoptera) was higher in gaps than in forests. Saproxylic beetles were more species rich in gaps created by bark beetle and the same result was reached for threatened species.Authors concluded that that Ips typographus fulfils the majority of criteria for a keystone species, particularly those that help to maintenance of biodiversity in forests. Nevertheless, the potential risk of damages by this beetle could be hardy accepted in spruce plantations.
Example 4.5.Bertheau C., Salle A., Rossi J.P., Bankhead-Dronnet S., Pineau X., Roux-Morabito G., Lieutier F. (2009) Colonisation of native and exotic conifers by indigenous bark beetles (Coleoptera: Scolytinae) in France.Forest Ecology and Management258: 1619-1628.
Exotic trees are often planted as ornamental plants in non-forest habitats.The growth of some of them indicated that they have high potential for forestry and timber production.North American tree species as Sitka spruce (Piceasitchensis), Eastern white pine (Pinusstrobus), Grand fir (Abiesgrandis), Douglas fir (Pseudotsugamenziesii) and Western red cedar (Thujaplicata) are among them.In any case, exotic trees have some potential from the point of view of native organisms – and also potential pests including bark beetles.
Bertheau et al. (2009) studied colonization of three native conifers – Norway spruce (Piceaabies), Scots pine (Pinussylvestris) and Silver fir (Abiesalba) – and five above mentioned north-American conifers by native bark beetles.The study was done in France.The main aim of study was to estimate factors causing shift of bark beetles onto new host trees.While using window traps, they collected eighteen indigenous and,furthermore, two exotic bark beetles.Surprisingly, all exotic conifer species were colonized by indigenous bark beetles and no significant difference was observed between native and exotic tree species.They found that the ability of indigenous bark beetles to shift onto exotic conifers depend on host tree species, the presence of phylogenetically related native conifers and that of similar resources, in combination with insect host specificity. Conclusion is that trees are under the treat by bark beetles also in non-native areas.
Example 4.6. Ayres M.P., Wilkens R.T., Ruel J.J., Lombardero M.J., Vallery E. (2000) Nitrogen budgets of phloem-feeding bark beetles with and without symbiotic fungi.Ecology81: 2198-2210.
Many bark beetles (Scolytinae) have their own and sometimes species-specific fungal or bacterialassociates.Present research revealed also other organisms like mites, nematodes or protozoans. These associates often help them during the attack and killing of host trees. Some of them help them during the feeding and food digestion. On the other hand, host trees have many types of mechanism that help them against the insect attack.