Pekka Kaitaniemi

Causes of cyclicity of Epirrita autumnata (Lepidoptera, Geometridae): grandiose theory and tedious practice

Abstract Creating multiyear cycles in population density demands, in traditional models, causal factors that operate on local populations in a density-dependent way with time lags. However, cycles of the geometrid Epirrita autumnata in northern Europe may be regional, not local; i.e., successive outbreaks occur in different localities. We review possible causes of cycles of E. autumnata under both local and regional scenarios, including large-scale synchrony. Assuming cyclicity is a local phenomenon, individual populations of E. autumnata display peaks but populations all over the outbreak range fluctuate in synchrony. This concept assumes that the peaks at most localities are so low that they do not lead to visible defoliation and easily remain unnoticed. In this scenario, populations are able to start recovery a few years after the crash, i.e., at the time of the mitigation of detrimental delayed density-dependent factors, such as delayed inducible resistance of the host plant or parasitism. In that case, the same factors that lead to crashes also explain the periodicity of cyclic fluctuations. According to the regional cyclicity scenario, different factors can be important in different phases of the cycle. The key is to identify the factors that tend to produce outbreaks with a periodicity of about 10 years. Initiation of the increase phase seems to coincide with maxima in sunspot activity, but causal connections remain unclear. Climatic factor(s) associated with the solar cycle could contribute to the large-scale geographic synchrony.

Realized fecundity in Epirrita autumnata (Lepidoptera: Geometridae) : relation to body size and consequences to population dynamics

Potential fecundity of Epirrita autumnata females is strongly correlated with pupal weight. Adult feeding weakly increases fecundity but does not eliminate its dependence on body size. Daily numbers of eggs laid are positively correlated with body size. However, night minimum temperature is the main determinant of oviposition rate under fluctuating natural temperatures. Adult mortality was estimated to be low in a subarctic forest in autumn. Based on these results, we constructed a simulation model predicting oviposition rate and realized fecundity under given weather conditions. The model indicates that expected realized fecundity strongly depends on female size also under field conditions. This causes selection for large body (abdomen) size which is a probable reason for lack of fine-tuned adult behaviour in E. autumnata. Factors acting during the adult stage can be considered relatively unimportant in the demography of this species; populations are largely regulated by biotic interactions of the larval stage, involving delayed density-dependence. This may be an explanation for population cycles, characteristic for this species.

Delayed induced changes in the biochemical composition of host plant leaves during an insect outbreak

Abstract In birch, Betula pubescens, herbivore-induced delayed induced resistance (DIR) of defoliated trees may cause a strong reduction in the potential fecundity of a geometrid folivore Epirrita autumnata. In this study, we examined the biochemical basis of DIR in birch leaves during a natural outbreak of E. autumnata. A set of experimental trees was defoliated at four sites by wild larvae in the peak year of the outbreak, whereas control trees were protected from defoliation by spraying with an insecticide. The biochemical composition of leaves was analysed in the following year and, although the DIR response was weak during this outbreak, causing less than a 20% reduction in the potential fecundity of E. autumnata, some consistent relationships between defoliation, biochemistry and pupal mass of E. autumnata suggested a general biochemical basis for the defoliation-induced responses in birch leaves. Total concentrations of nitrogen, sugars and acetone-insoluble residue (e.g. cell wall polysaccharides, cell-wall-bound phenolics, protein, starch, lignin and hemicellulose) were consistently lower, and total concentrations of phenolics, especially of gallotannins and soluble proanthocyanidins, were higher in the leaves of trees defoliated in the previous year than in those protected from defoliation. The capacity of tannins to precipitate proteins correlated with contents of gallotannins, and was highest in defoliated trees. The pupal mass of E. autumnata showed a strong, positive correlation with concentrations of nitrogen and sugars, and a negative correlation with the acetone-insoluble residue and gallotannins in foliage. Correlations with other measured biochemical traits were weak. The correlation coefficients between biochemical traits and pupal mass consistently had similar signs for both defoliated and insecticide–sprayed trees, suggesting that variation in leaf quality due to defoliation in the previous year was based on similar biochemical traits as variation for other reasons. We suggest that DIR is associated with reduced growth activity of leaves, and may be seen as a delay in the biochemical maturation of leaves in defoliated trees. This explains the high concentration of gallotannins in defoliated trees, a characteristic feature of young leaves. However, the lower content of nitrogen and the higher content of soluble proanthocyanidins in defoliated trees are traits usually characterising mature, not young, leaves, indicating defoliation-induced changes in chemistry in addition to modified leaf age. Our results emphasise the importance of understanding the natural changes in chemistry during leaf maturation when interpreting defoliation-induced changes in leaf biochemistry.

Testing the enemies hypothesis in forest stands: the important role of tree species composition

Numerous studies conducted in agro-ecosystems support the enemies hypothesis, which states that predators and parasites are more efficient in controlling pest densities in polycultures than in monocultures. Few similar studies, however, have been conducted in forest ecosystems, and we do not yet have evidence as to whether the enemies hypothesis holds true in forests. In a 2-year study, we investigated whether the survival of autumnal moth (Epirrita autumnata) larvae and pupae differs between silver birch monocultures and two-species mixtures of birch with black alder, Norway spruce and Scots pine. We placed young larvae on birch saplings and monitored their survival until the end of the larval period, when we checked whether they had been parasitized. After the larvae had pupated, pupal survival was tested in a field trial. In 2002, the larvae disappeared earlier and their overall survival was lower in birch–pine mixtures than in other stand types. In 2003, survival probability was lowest in birch–pine stands only during the first week and there were no differences between stands in overall survival. Larval parasitism was not affected by tree species composition. Pupal weight and pupal survival were likewise not affected by stand type. Among the predators, wood ants were more abundant on birches growing in birch–pine mixtures than in other stand types probably because colonies of myrmecophilic aphids were common on pines. In contrast, spider numbers did not differ between stand types. Ant exclusion by means of a glue ring around the birch trunk increased larval survival, indicating that ants are important predators of the autumnal moth larvae; differences in larval survival between stands are probably due to differential ant predation. Our results provide only partial support for the enemies hypothesis, and suggest that it is both tree species composition and species diversity which affect herbivore survival and predation.

Oviposition Choices of Epirrita autumnata (Lepidoptera: Geometridae) in Relation to Its Eruptive Population Dynamics

We found that oviposition behaviour is indiscriminative in Epirrita autumnata (Lepidoptera, Geometridae), a species known as a periodical pest of birch forests in north-emmost Europe. Oviposition in E. autumnata was not suppressed by low host quality, injured host leaves, or the presence of conspecific eggs or sex pheromone. Oviposition was not limited to host plants, whereas physical characteristics of the substrate were decisive in oviposition site selection. Therefore, the oviposition behaviour of E. autumnata is typical for an outbreak species and certainly contributes to the eruptive population dynamics of the species. Young larvae are responsible for finding a host plant but their capacity for host selection is rather restricted ; larval dispersal cannot compensate for indiscriminative oviposition. Indiscriminative oviposition should not be seen as a phylogenetic constraint in E. autumnata ; more probably, it results from selection in favour of large body size in females which reduces their flying ability.

Density and performance of Epirrita autumnata (Lepidoptera : Geometridae) along three air pollution gradients in northern Europe

1. Larval density, size of pheromone-trapped adult males, and survival and parasitism rates in the laboratory of field-collected larvae of Epirrita autumnata were investigated along three pollution gradients: the surroundings of smelters at Monchegorsk and Nikel in north-western Russia and a factory complex at Harjavalta, south-western Finland. 2. Along the gradients, the emissions are qualitatively similar, consisting mainly of Cu, Ni and SO 2 , but the amounts emitted vary. Cu and Ni concentrations in birch (Betula pubescens) leaves, a common diet of E. autumnata larvae, were monitored and used as general indices of pollution. 3. High Cu and/or Ni concentrations in birch foliage, and/or associated changes in other pollutants, were associated with detrimental effects on E. autumnata performance. They occurred when concentrations of foliage Cu and/or Ni exceeded 20-30 μg g -l . 4. In the Harjavalta and Monchegorsk gradients, crossing the above threshold values in heavy metal concentrations in the foliage, larval density and survival increased significantly with distance from the smelters. No pollution-related trends with distance were found either for male size or for larval parameters in that section of the Nikel gradient where foliage heavy metals were below this threshold value. 5. Parasitism rates were not associated with pollution, indicating that parasitoids were not differently sensitive to pollutants than their herbivorous host. 6. No indication was found of a positive effect of moderate pollution on E. autumnata and heavy pollution had detrimental effects. The results do not suggest that the species might become a severe pest in polluted areas outside its natural outbreak range.

Temporal and spatial variation of larval parasitism in non-outbreaking populations of a folivorous moth.

Abstract In order to assess the role of parasitoids in the regulation of non-outbreaking populations of Epirrita autumnata, a geometrid lepidopteran with outbreaking populations in northern Europe, we examined the temporal and spatial variation of larval parasitism in southwestern Finland during 6 successive years. The study was carried out on two spatial scales, among trees within sites of about 1 ha and among sites separated by distances of 2–10 km, using experimental and observational approaches respectively. The overall percent parasitism was independent of host density on both spatial scales, while temporally it fluctuated only little. Of the two main parasitoids, the commoner one, Protapanteles immunis, showed a variable response to host density on the larger spatial scale and negative density dependence on the smaller scale. Temporally, parasitism caused by this species was independent of host density. Another parasitoid, Phobocampe bicingulata, showed positive density dependence on the smaller spatial scale and had a variable response on the larger scale, but exhibited negative density dependence over time. The results of this study caution against drawing conclusions concerning population regulation on the grounds of spatial density dependence alone. Larval parasitoids apparently do not maintain low densities in the E. autumnata populations studied. However, they may suppress E. autumnata densities to a level low enough for density-dependent mortality factor(s) to become regulating. Among other mortality factors of E. autumnata, pupal predation has been found to be temporally positively density-dependent.

Effects of autumn temperature and oviposition date on timing of larval development and risk of parasitism in a spring folivore

Epirrita autumnata (Lepidoptera, Geometridae) is a periodically fluctuating spring folivore which overwinters in the egg stage. The timing of its larval development can be highly variable, and the synchrony of larvae with parasitoids may affect their risk to be parasitized. We estimated the effects of both oviposition date and site on the timing of larval development by allowing experimental eggs, which were laid on various dates during the oviposition period, to overwinter both above and below snow level, and on both the northern and southern sides of tree trunks. Accumulation of thermal sum was used to quantify the effect of oviposition date on the timing of egg hatch. To assess whether the timing of larval development affected the risk of parasitism, the larval stages susceptible to parasitism and the length of the parasitoid flight period were detennined by exposing host larvae to parasitoids. Unlike previously assumed, the thermal sum required for egg hatching began to accumulate already in autumn, and autumn and spring temperatures additively determined the timing of egg hatch. The first-laid eggs accumulated 70% of the thermal sum required for hatching in autumn whereas the last-laid eggs accumulated less than 3%. Accordingly, because the oviposition period of E. autumnata is long and its timing is variable between years, the period of egg hatching also is long and can be variable between years. However, the flight periods of most parasitoid species exceeded the spread of hatching times and developmental stages of larvae. Only during years of low parasitoid density may the variability in egg hatching times create a temporal refuge for some larvae, contributing to the increase of E. autumnata populations.

Simulating source-sink control of carbon and nutrient translocation in a modular plant

Abstract Modular structure has important consequences for the control of internal resource translocation in plants, and for the decisions concerning growth of plant parts. Some plant parts may act as partially autonomous functional modules with their own resource supply and localized control of growth. Resources for growth are obtained from sources (e.g. photosynthesizing leaves) and are translocated to sinks (e.g. growing meristems). Resource translocation between sources and sinks is determined by the sink strength of growing meristems, which involves a concentration gradient from sources to sinks. If many sinks compete for a limited resource supply, then more resources are proportionally allocated to the strongest sinks. Herbivory and other factors that affect sources or sinks may cause local changes in the allocation of resources. This paper presents a simulation model that takes into account modular plant structure and its consequences for source-sink dynamics and growth. In simulations, Scots pine ( Pinus sylvestris L.) is described as a data structure tree, where the tree nodes represent individual pine shoots and the links between the nodes serve as routes for carbon and nitrogen translocation. A practical property of this approach is the partial autonomy of pine branches. Each simulated shoot has its own sink strength and physiological processes determining its resource intake. Resource translocation between the shoots of the simulated tree is controlled by the sink strengths of the existing translocation routes. To demonstrate the applicability of this simulation approach for questions concerning source-sink dynamics, we have simulated some herbivory treatments, local defoliations and debuddings of pine shoots, and compared the results with corresponding field experiments. The qualitative outcomes of simulated and experimental treatments follow the same general patterns.

Systematic within-tree variation in mountain birch leaf quality for a geometrid, Epirrita autumnata

Abstract. 1 We studied within‐tree variation in leaf quality of the mountain birch, Betula pubescens ssp. tortuosa, for larvae of the autumnal moth, Epirrita autumnata. 2 The purpose of the study was to determine the possible occurrence of systematic differences in larval growth on short shoot leaves (i.e. leaves of the same age): among leaves facing in different compass directions, between leaves of lower and upper branches, among leaves on different positions within a branch and among leaves of different sizes within a short shoot. We also measured larval growth between short shoot and long shoot leaves (i.e. between leaves of different age). 3 The larvae grew best on leaves on the north side of trees and most poorly on south side leaves, the east and west sides being intermediate. Leaves from the upper branches supported larval growth better than leaves from the lower ones. The larvae grew better on the smallest leaf of each short shoot and were able to utilize it more efficiently than the two larger leaves. Short shoot leaves from the basal and middle parts of the upper branches of the trees were of better quality for the larvae than short shoot leaves from the tip part of the branches. The larval growth rate did not differ between short shoot and long shoot leaves. In general, within‐tree variation in the larval growth rate was lower than variation among different trees. 4 Damage to leaves can decrease leaf quality for herbivores in the same year (rapidly inducible responses) or the following year(s) (delayed inducible responses). Our results show that systematic within‐tree variation in larval growth can be as great as the effects of rapidly inducible responses and that variation among individual trees can be as great as the mean effects of delayed inducible responses.