Chapter 5: Defoliators
Insect that are feeding on leaves and needles of forest trees is causing serious problems around the world. The most important defoliators are lepidopteran caterpillars, some larvae of Sawflies (Symphyta: Hymenoptera) and adults and larvae of beetles. The other taxa are, of course, of importance, but excluding ungulates (that have their own chapter in this book) their damages have mostly only of local importance.
Insect harm trees from seedlings till the end of life of the tree. There are many potential pests that might limit growth of seedling of the trees – as several species that are not damaging leaves, but seeds and fruits of forest trees. The main pests of seeds are butterflies (Lepidoptera) and beetles (Coleoptera). The larvae (and sometimes adults) of moths, weevils or wood-worms are feeding inside the seeds and fruits. Caterpillars of snout moths (Pyralidae) could be mentioned as an example – they are relatively common inside fruits or cones of forest trees and most of them are well recognizable due to presence of silk cocoons. Southern Pine Coneworm (Dioryctria amatella), known also as pitch moth is causing cone and shoot damages to the pines (Pinus) in North America. One of the most important beetle pests of seeds of broadleaved trees are weevils (Curculionidae) and wood worms (Anobiidae). Acorn weevil (Curculio glandium) has conspicuous adults due to their elongated snout (rostrum), which is longer in females than males and might reach more than one half of the length of an adult. Its larvae eat acorns from the inside.
Now back to the defoliators with the major impact on forest trees. In this case, the most important partitioning will be between defoliators on conifers and broadleaves.
Broadleaved trees do not appear to be too sensitive to the activity of defoliators as conifer trees. The main reason is that they are able to restore their leaves from buds during vegetation season – although a bit different situation could be in the case of evergreen broadleaves that might be more sensitive. The main danger to the broadleaves lies in repetition of damages on leaves during vegetation season (e.g. defoliating moths and following leaf miners). Furthermore, this situation may be worsened by damages on buds cause by several insect taxa – like caterpillars or e.g. adults of click beetles. Of course, there is also rising danger when several damages co-occur – like late frosts during spring time and defoliation.
Coniferous plantations are in possible threat nearly during the whole rotation period, although the most important time is approximately between 30 and 70 years. However, this is also highly dependent with climatic and geographic conditions. There are several taxa that are able to cause damages to the younger stands, but most of them are only an ephemeral problem. As an example may be mentioned is Pine Shoot Moth (Rhyacionia buoliana).
This moth is causing damages or deformations to the buds and shots of pines and was introduced to whole America. The degree of damage to the forest stand, of course, depends on abundance of caterpillars. One of the first indications of high abundance before the needles are eaten or spun is amount of oval excrements on the forest ground. This can be perfectly measured when some light piece of cloth is attached on the forest ground. The second one option is glued strips around the stem that are catching flightless females. One of the most important defoliators in old continent and America are moths from genus Lymantria (e.g. Nun – Lymantria monacha or Gipsy Moths – Lymantria dispar). These species are able to kill healthy trees and several hectares of coniferous (Nun Moth) or broadleaved (Gipsy Moth) forest stands can be killed during a couple of years.
Seedlings and saplings in forest nurseries are also often in threat caused by defoliating insects. Especially, caterpillars or adults of weevils (e.g. Otiorhynchus spp.) are of high importance. However, the protection against them is much simple due to higher accessibility of forest nurseries. Pesticides (namely, insecticides) are one of the most used in the case of some species, although hand collecting as in agriculture for e.g. Colorado potato beetle (Leptinotarsa decemlineata) is also possible. Using of gloves is recommended in these cases, because caterpillars of some moths and especially their setae might cause allergic reactions. One of the important defoliating beetles is the pine weevil (Hylobius abietis) that is a pest that combines damages on both needles and bast and bark of seedlings. Ecology of this species is much more described in the chapter on phloeoxylophages. In this place it will be good to mentioned also aphids and similar insects that suck on the shoots of many tree species. This process weakens especially the seedlings and is able to kill them. The highest damages can be seen in forest nurseries.
Especially more recently, here is relatively many methods how to deal with high abundances of these pests. Probably the most efficient is spraying of some pesticide like pyrethroids – although here is a problem that this total insecticide kills all other insects (including beneficial organisms) presented on sprayed trees. The other are methods of biological control like using of inhibitors of development of caterpillars, use of more selective insecticides, use of fungal or bacterial pathogens – of which Gram-positive bacterium Bacillus thuringiensis is well known as abbreviation Bt- from agricultural (economic) plants.
At the end of this introduction to the chapter of defoliators, it would be good to write that some defoliators insects are indexed in the law of the Czech Republic as pests that might cause calamities in forests – namely, Nun moth (Lymantria monacha), The Larch Tortrix (Zeiraphera griseana) and Cephalcia spp.
Figure 5.1. Caterpillar of the Winter Moth (Operophtera brumata) crawling on the stem of oak. This species is abundant in Europe and its adults are active even in the depth of winter. Females are wingless. This species was introduced to the North America in 1930’s. Species is well known from media (e.g. Boston Globe) due to its outbreaks in orchards in U.S.A.
Figure 5.2. Eggs hatched gregariously by the Vapourer (Orgyia antiqua) to the spruce twigs. This species is relatively opportunistic with respect to host trees, because can cause defoliation on both broadleaved and coniferous trees. Females of this abundant species have reduced wings and abundance of Vapourer is often correlated with other insect defoliators.
Example 5.1: Peltonen M., Liebhold A.M., Bjornstad O.N., Williams D.W. (2002) Spatial synchrony in forest insect outbreaks: Roles of regional stochasticity and dispersal. Ecology 83: 3120-3129.
Population densities of forest defoliators mostly go through cycles – this means that very high population densities and thus high damages caused by defoliation activity by caterpillars are followed by decline in abundance and so on. Eastern spruce budworm (Choristoneura fumiferana) is mentioned to be one of the most destructive native insects in spruce and fir forests of the Eastern U.S.A. and Canada. Western spruce budworm (Choristoneura occidentalis) appears to be the most destructive defoliator of coniferous trees in Western part of North America. Larch bud moth (Zeiraphera griseana) is a coniferous defoliating moth that is native to the Europe. Forest Tent Caterpillar Moth (Malacosoma disstria) is a North American species that is distributed mainly in the eastern part of U.S.A. and Canada and defoliating deciduous trees. Gypsy moth (Lymantria dispar) is an alien pest in North America. It is one of the most destructive pests of hardwood trees.
Except of one bark beetle, Peltonen et al. (2002) studied five moths that cause damages to the forest by defoliation. Two of these species were studied in Canada (spruce budworm and forest tent caterpillar), two in U.S.A. (western spruce budworm and Gypsy moth) and one in Europe (larch bud moth). Even if temporal synchrony in several taxa is well known and studied, the situation in spatial synchrony was not well understood. Authors found that spatial synchrony was not directly associated with dispersal capabilities of particular species. The synchrony in outbreaks declined with geographical distance. Geographical variation in local population dynamics force synchrony to decline more rapidly with distance than the relationship with the environment.
Example 5.2: Raimondo S., Turcani M., Patocka J., Liebhold A.M. (2004) Interspecific synchrony among foliage‐feeding forest Lepidoptera species and the potential role of generalist predators as synchronizing agents. Oikos 107: 462-470.
Oaks (Quercus spp.) in temperate zones are one of the most important commercial trees species, because their hardwood is very durable. There are more species of oaks in the Central Europe, while only two or three of them are of higher commercial importance. Native oaks were one of the most abundant tree species in this part of world before human intervention and thus taxa that is dependent on oaks belongs to the one of the richest around the world. Defoliating caterpillars have many types of strategies, while free-feeders and leaf-rollers are probably the most abundant in Slovakia.
Raimondo et al. (2004) studied the synchrony between sympatric populations of two or more species that are defoliators of oaks in Slovakia. They studied ten species of spring foliage feeding caterpillars sampled over a period of 25 consequent years. They found that nearly one third of interspecific pairs were synchronous and the highest correlation was among species exhibiting similar feeding strategies and thus morphologies.
Example 5.3: Payette S., Bhiry N., Delwaide A., Simard M. (2000) Origin of the lichen woodland at its southern range limit in eastern Canada: the catastrophic impact of insect defoliators and fire on the spruce-moss forest. Canadian Journal of Forest Research 30: 288-305.
So called lichen woodland is dominating forest type in boreal parts of Canada. The horizontal structure is highly homogeneous and is rather simple with two dominant strata – black spruce (Picea mariana) in sparse overstory and lichens in understory. This type of forest and its spatial structure is predicted to be highly driven by fires. Spruce budworm (Choristoneura fumiferana) and non-native European spruce sawfly (Gilpinia hercyniae) are one of the most common potential pests in lichen woodlands.
Payette et al. (2000) studied the dynamics of black spruce dominated sparse woodlands of boreal Canada. They tested the hypothesis that the lichen woodland is a regressive type of more humid spruce–moss forests that were affected by fire disturbances. Lichen spruce and spruce moss stands that grew under similar soil conditions were compared using tree size, tree ring patterns, and macrofossil analysis of organic soil. The most important result was that all the macrofossils of plants that were buried in the organic part under the charcoal layer corresponded to a moss forest and included head capsules of the spruce budworm or European spruce sawfly. Thus, defoliating activity of insects together with fires could degrade the forests – in this case from humid moss to sparse lichen woodland.
Example 5.4: Meigs G.W., Kennedy R.E., Cohen W.B. (2011) A Landsat time series approach to characterize bark beetle and defoliator impacts on tree mortality and surface fuels in conifer forests. Remote Sensing of Environment 115: 3707-3718.
As defoliators are important forest disturbance agents, mapping of their effects on mortality of trees and forests represents an important issue in forest protection. Remote sensing provides information about objects without making physical contact. The use of aerial sensor technologies to detect and classify objects on Earth is the major method. Most studies of remote sensing have focused on single species or single locations and not changes to the ground.
Except of the impact of bark beetle, Meigs et al. (2011) studied the impact of defoliation by western spruce budworm (Choristoneura occidentalis) in U.S.A. using remote sensing of forests. Many types of forests were studied ranging from mesic mixed-conifer to xeric lodgepole pine. They found that sites damaged by the western spruce budworm appeared to show a consistent temporal evolution of long-duration loss of vegetation followed by its recovery.
Example 5.5: Kyei‐Poku G., Gauthier D., Van Frankenhuyzen K. (2008) Molecular data and phylogeny of Nosema infecting Lepidopteran forest defoliators in the genera Choristoneura and Malacosoma. Journal of Eukaryotic Microbiology 55: 51-58.
As defoliators are causing damages to the forest ecosystems, their population densities are naturally controlled by their enemies. Obligate intracellular parasite (or pathogen-they are presently listed as fungi, but they were formerly listed as primitive eukaryotes) species from genus Nosema are recently highly studied, especially that they can cause serious damages to the domestic bees. As mentioned above in this chapter spruce webworms (Choristoneura spp.) are causing serious damages to North American forests and especially coniferous are the most jeopardized, while forest tent caterpillar (Malacosoma disstria) is a defoliator of broadleaved forests.
Kyei-Poku et al. (2008) studied species from genus Nosema in five moth pests in U.S.A. Spruce budworm (Choristoneura fumiferana), jack pine budworm (Choristoneura pinus) and western spruce budworm (Choristoneura occidentalis) are defoliators of coniferous forests, and large aspen tortrix (Choristoneura conflictana) and forest tent caterpillar (Malacosoma disstria) are pestic agents to the broadleaved trees. These species were collected from various locations across North America. Authors found that each moth species tended to have its own species-specific Nosema species.
Example 5.6: Vanhanen H., Veteli T.O., Paivinen S., Kellomaki S., Niemela P. (2007) Climate change and range shifts in two insect defoliators: Gypsy moth and nun moth-a model study. Silva Fennica 41: 621-638.
The Nun Moth (Lymantria monacha) and the Gypsy moth (Lymantria dispar) are one of the most important defoliators of coniferous and broadleaved forests in the central Europe, respectively. Ongoing global change in climate might cause range shifts of many taxa and potential pests are not an exception. Ectothermic animals, like insects, are expected to shift their distribution ranges northwards toward presently less climatically suitable places in Northern Hemisphere.
Vanhanen et al. (2007) studied possible range shifts of Nun and Gypsy moths within the change in recent climatic scenario. They used life cycle requirements of particular species for modelling, because this measure reflects probability of a viable population to exist at a certain location. They modelled three degrees in increasing of temperatures. The possible climate warming shifted the northern boundary of the distribution area for both studied species by 700 km northward. Furthermore, the shift of southern distribution limit was retracted northwards by 100–900 km.
Example 5.7: Scriber J.M. (2004) Non-target impacts of forest defoliator management options: Decision for no spraying may have worse impacts on non-target Lepidoptera than Bacillus thuringiensis insecticides. Journal of Insect Conservation 8: 243-263.
Forest protection shifts from non-selective pesticides to still more and more selective matters. Even, if this shift appears to be highly environmentally friendly, there are several non-pestic and thus non-target taxa that might be very sensitive even to highly selective insecticides. Spraying of microbial pesticides such as those with strains of Bacillus thuringiensis is often mentioned as highly selective and influencing only lepidopteran pests. On the other hand, this insecticide might affect the whole Lepidoptera and thus also those that are under the threat from the nature conservation point of view.
Scriber (2004) described study from laboratory and field studies that were conducted to determine non-target impacts of Bacillus thuringiensis on native Lepidoptera in North America. He found that no spraying may have worse impacts on non-target Lepidoptera than insecticides with Bacillus thuringiensis. He further concluded that the important concept that must be maintained is that all pest management programs have some risk of negative non-target impacts.
Example 5.8: Holusa J., Kuras T. (2010) Diurnal behaviour of Cephalcia lariciphila (Hymenoptera: Pamphiliidae): Relation to climatic factors and significance for monitoring. European Journal of Forest Research 129: 243-248.
The Web-spinning Larch Sawfly (Cephalcia lariciphila) is a pest that defoliates European larch (Larix decidua) and could be regarded as its specialists. This species can also reach outbreak levels. Even when European larch is most probably non-native tree to the most of Europe, this tree is of high commercial importance due to quality of wood. Repeated defoliation of larches might result in a decrease of annual growth ring formation and trunk thickness and decrease might reach 70%.
Holusa & Kuras (2010) studied the day activity of this defoliating sawfly in the Czech Republic. The species start its activity during mid-April and last specimens flew at the beginning of June. Adult occurrence typically lasted about 20 days. The species diurnal activity was highly associated with temperature in interaction with humidity – adults were most active at 16 °C with 20% air humidity. The flight activity starts at 10 AM and ended at 5 PM.
Questions for exam:
5.1. The activity of defoliators might cause dramatic changes in forest ecosystems and also commercial losses. Majority of defoliating insects in temperate forests are discriminating between coniferous and deciduous (broadleaved) trees. Which trees – conifers or broadleaves – are more sensitive to the defoliation?:
5.2. The species richness of caterpillars (larvae of Lepidoptera) depends on more factors in the conditions of temperate zone. The main ones are suitable climate, historical distribution of host tree or leaf palatability. Which tree species appears to be the most species rich on moths (Lepidoptera) in conditions of temperate central Europe?:
5.3. Many threats to the forests are highly interconnected. Interaction of two or more disturbance agents might cause hard change in spatial structure of the forest type and in some cases might cause its degradation. Which interacting factors appear to be the major disturbance threats in boreal forests?
5.4. There are many techniques how to map the damages caused by insect pests. We could divide them easily to those that are done physically in forest stand (e.g. tree traps) and those that are done without physical contact (e.g. observation from airplane). What is one of the mostly used modern techniques made without physical contact, while mapping the distribution of damages caused by defoliators?:
5.5. Caterpillars (larvae of Lepidoptera) in many cases cause damages to the forest trees, because they reduce the photosynthetic activity of trees and, furthermore, they are able to kill trees. Their population densities are often reduced by natural enemies and the most well-known are those that are conspicuous (e.g. large carabid beetles) or well visible (e.g. woodpecker birds). There are several taxa that might highly influence the activity and also fecundity of many forest pests, even if they are not conspicuous. Obligate intracellular parasites from genus Nosema are from the point of view of taxonomy:
5.6. There are many species of defoliators in central Europe. While many of them are of less importance, there are two lepidopteran taxa from the same genus (family of Tussock moths, Lymantriidae) that have caused many outbreaks in their native distribution area and at least in one case also in North America. One of them damages coniferous, while the second one broadleaved forest trees. What are these two most important defoliators of the central European forests?:
5.7. Forest protection uses many types of pesticides and insecticides are those that kill insects. One of the main problems is that many non-target species might be negatively affected. Thus, one of the solutions seems to be use of highly selective insecticides. What type of insecticides appears to be the most environmentally friendly against lepidopteran defoliators?:
5.8. The most important defoliators in forests are caterpillars of Lepidoptera. On the other hand, there are several other taxa that can cause damages to the leaves or needles of forest trees. Which taxa (except of Lepidoptera) appears to be important defoliator?: