Impact of pine looper defoliation in Scots pine

Widespread defoliation of forests caused by insects or fungi cause economic losses throughout the world. Successful outbreak management involves cost/benefit estimation and requires knowledge of potential yield losses. Currently, such knowledge is scarce. This thesis evaluates the significance of single-year defoliation by the pine looper moth (Bupalus piniaria L.) (Lepidoptera: Geometridae) and secondary attack by the pine shoot beetle (Tomicus piniperda L.) (Coleoptera: Scolytinae) for yield losses in Scots pine (Pinus sylvestris L.). In a seven-year study, tree mortality and growth losses were quantified after a B. piniaria outbreak with a subsequent T. piniperda infestation at Hökensås in 1996. Secondary attack by T. piniperda was also studied in a Scots pine stand at Fredriksberg, infested by Gremmeniella abietina (Lagerberg) Morelet. in 2001. Tree mortality at Hökensås mainly occurred in areas with an average defoliation intensity of 90-100%. Tree mortality averaged 25%, and the “defoliation threshold” for tree survival was found to be 90% defoliation. Tree mortality peaked two years after the defoliation event, which coincided with high levels of pine shoot beetles. Involvement of T. piniperda in tree mortality increased with time since defoliation, and tree susceptibility to beetle attack increased with increasing defoliation intensity and decreasing dominance status of trees. As foliage of trees recovered, fewer trees were susceptible to beetle attack, and ultimately, beetle attacks ended as breeding substrate was depleted. Tree susceptibility to beetle attack was a function of tree vigour and beetle density. Growth losses at the Hökensås site were a combined effect of pine looper defoliation and shoot pruning by T. piniperda. Radial, height and volume growth losses were proportional to defoliation intensity. Whereas radial growth was little affected by beetle-induced shoot pruning, height growth was severely affected by beetle-induced damage to leading shoots. Growth losses alone, excluding tree mortality, were large enough to economically justify control of the pine looper outbreak had the outbreak been prevented in 1996. At Fredriksberg, T. piniperda colonised trees that would have survived the damage caused by G. abietina. However, trees with 90-100% foliage losses died because of the damage caused by G. abietina alone, and tree survival seemed to require that trees retained at least 20% of full foliage

The role of local space charge concentrations in producing branched tree structures

Electrical trees are branched damage structures produced in polymeric insulation subject to high divergent fields. The density of branching ranges from a sparse form like a tree in winter to a dense compact form like a bush. This variation in form is significant as the bush structure occurs at higher voltages but grows slower. We present here a deterministic model for the formation of electrical trees based on damage produced by charges injected into the polymer from discharges taking place within the gas-filled tubules of the tree. A number of processes within the mechanism cause the space charge fields to fluctuate chaotically, and this is held to be responsible for the branching that is observed. Different tree shapes are found depending on whether or not injected/extracted charges reach a kinetic energy high enough for damage only at a few tree tips or everywhere around the tree periphery

The advantage of the extremes: tree seedlings at intermediate abundance in a tropical forest have the highest richness of above-ground enemies and suffer the most damage

1. Tropical forest tree diversity has been hypothesized to be maintained via the attraction of density responsive and species-specific enemies. Tests of this hypothesis usually assume a linear relationship between enemy pressure (amount of damage and enemy richness) and seedling or tree density. However, enemy pressure is likely to change nonlinearly with local seedling abundance and community scale tree abundance if enemies are characterized by nonlinear functional responses. 2. We examined the abiotic and biotic factors associated with richness of above-ground enemies and foliar damage found in tree seedlings in a tropical forest in Puerto Rico. Rather than identify specific enemies targeting these seedlings, we used damage morphotypes, a paleo-ecological method, to derive a proxy for enemy species richness. 3. We found that the relationships between local and (conspecific seedling density) community scale (conspecific basal area of adult trees) abundance and both richness of above-ground enemies and foliar damage were hump-shaped. Seedlings of tree species existing at intermediate levels of abundance, at both local and community scales, suffered more damage and experienced pressure from a greater diversity of enemies than those existing at high or low densities. 4. We hypothesized that greater damage at intermediate abundance level could arise from a rich mixture of generalist and specialist enemies targeting seedlings of intermediate abundance tree species. Consistent with this hypothesis, we found that generalist enemies were more diverse on species at rare or intermediate abundance relative to common tree species. However, specialist enemies showed no significant trend across tree species abundance at either the local or community scales. 5. Synthesis. Our results suggest that interspecific variation in tree species abundance leads to differences in the magnitude and type of damage tropical tree seedlings suffer. This variation leads to a nonlinear, hump-shaped relationship between species abundance and enemy damage, highlighting fruitful directions for further development of species coexistence theory

Entomopathogenic nematodes for biological control of codling moth

Entomopathogenic nematodes are often found naturally infecting codling moth larvae. The effect of an autumn treatment with S. feltiae on the fruit damage in the following summer was evaluated by treating 4 different apple orchards in October 2004 and 2005 at application rates of 3.75; 2 and 1.5 billion nematodes in 4000 l / ha. In three of the treated orchards, one treated with 3.75x109 nematodes/ha the other two treated with 2e9 nematode/ha, reduction in fruit damage was around 50%. In the most heavily infested orchard, which was treated with 1.5x109 nematode/ha only 33% reduction in fruit damage was achieved. Compared to previous studies, this was the first assessing the effect on the fruit damage in the summer following the treatment rather than assessing the mortality of sentinel larvae fixed to the treated tree trunks

Associational resistance and associational susceptibility: specialist herbivores show contrasting responses to tree stand diversification

Heterospecific neighbors may reduce damage to a focal plant by lowering specialist herbivore loads (associational resistance hypothesis), or enhance damage by increasing generalist herbivore loads (associational susceptibility hypothesis). We tested the associational effects of tree diversity on herbivory patterns of the tropical focal tree Tabebuia rosea in an experimental plantation setup, which contained tree monocultures and mixed stands. We found higher herbivore damage to T. rosea at higher tree diversity, indicating that T. rosea did not benefit from associational resistance but rather experienced associational susceptibility. The specific consideration of the two dominant insect herbivore species of T. rosea, the specialist chrysomelid Walterianella inscripta and the specialist pyralid Eulepte gastralis, facilitated understanding of the detected damage patterns. Tree diversity exerted opposite effects on tree infestation by the two herbivores. These findings point to resource concentration effects for the chrysomelid beetle (favored by tree monoculture) and to resource dilution effects for the pyralid caterpillar (favored by tree mixture) as underlying mechanisms of herbivore distribution. A strong contribution of the pyralid to overall damage patterns in diversified stands suggests that associational susceptibility may not necessarily be related to higher abundances of generalist herbivores but may also result from specialized herbivores affected by resource dilution effects. Thus, the identity and biology of herbivore species has to be taken into account when attempting to predict damage patterns in forest ecosystem

How expensive is vole damage?

Vole species, especially Arvicola terrestris and Microtus arvalis cause significant economical damage in organic pomiculture by gnawing the root system of trees. The importance of voles as pest organisms is well known. Nevertheless, the estimation of financial loss caused by voles is difficult for German fruit growers. We conducted a survey among organic fruit growers to get data on kind and amount of annual damage. Using the available publications and official statistics we calculated economical values of organic apple trees for different types of orchard processing and tree ages. Furthermore we calculated the number of trees lost due to voles in one year

Discharge-generated electrical fields and electrical tree structures

The discharge-avalanche (D-A) model for electrical tree propagation in polymers is founded entirely upon basic physical concepts. Electrical discharges in an existing tree structure are taken to raise the electrical field in the polymer both along the discharge path and particularly at the tree tips. As a result of the field increase, electron multiplication avalanches occur within the polymer causing damage, possibly through ionisation of polymer molecules, which is accumulated over a period of thousands (or more) cycles and eventually leads to a tree extension of limited size. The assumption that the damage produced in an avalanche is proportional to the number of ionisations allows the model to be expressed quantitatively in terms of material properties: such as the ionisation potential, I; the impact-ionisation length parameter λ; the critical number of ionisations for tree extension Nc; discharge features such as the number of 1-electron initiated avalanches per half cycle, Nb ; and the potential difference ΔV between the start and end of the avalanche over a distance Lb

Arbuscular mycorrhizal fungal diversity and natural enemies promote coexistence of tropical tree species

Negative population feedbacks mediated by natural enemies can promote species coexistence at the community scale through disproportionate mortality of numerically dominant (common) tree species. Simultaneously, associations with arbuscular mycorrhizal fungi (AMF) can result in positive effects on tree populations. Coupling data on seedling foliar damage from herbivores and pathogens and DNA sequencing of soil AMF diversity, we assessed the effects of these factors on tree seedling mortality at local (1 m2) and community (16 ha plot) scales in a tropical rainforest in Puerto Rico. At the local scale, AMF diversity in soil counteracted negative effects from foliar damage on seedling mortality. At the community scale, mortality of seedlings of common tree species increased with foliar damage while rare tree species benefited from soil AMF diversity. Together, the effects of foliar damage and soil AMF diversity on seedling mortality might foster tree species coexistence in this forest

Electrical tree structures generated by the ab-initio discharge-avalanche model

Initially the discharge-avalanche model for electrical tree formation treated the local Poisson fields as being derived from the applied Laplace field via a modification factor that was regarded as a random variable in time and space. Recently we have removed this approximation by calculating the local Poisson fields from the space charge arrangement produced by discharges in the tree tubules and avalanches in the surrounding polymer. This “ab-initio” version of the model has now been fully implemented. The ac-cycle is divided into 16 time segments. In each time segment tube-discharges are allowed to occur if the potential difference along a tube is greater than an onset threshold. Positive space charge is regarded as being deposited on the tube walls in the form of positive ions. Negative charge is allowed to penetrate the polymer where it can be used to initiate avalanches and thereby generate damage. The avalanches compete with a field dependant mobility in rearranging the charge around the tree tips