Toomas Tammaru

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.

Intraspecific Variability in Number of Larval Instars in Insects

Abstract The number of larval instars varies widely across insect species. Although instar number is frequently considered to be invariable within species, intraspecific variability in the number of instars is not an exceptional phenomenon. However, the knowledge has remained fragmentary, and there are no recent attempts to synthesize the results of relevant studies. Based on published case studies, we show that intraspecific variability in the number of larval instars is widespread across insect taxa, occurring in most major orders, in both hemimetabolous and holometabolous insects. We give an overview of various factors that have been observed to affect the number of instars. Temperature, photoperiod, food quality and quantity, humidity, rearing density, physical condition, inheritance, and sex are the most common factors influencing the number of instars. We discuss adaptive scenarios that may provide ultimate explanations for the plasticity in instar number. The data available largely support the compensation scenario, according to which instar number increases in adverse conditions when larvae fail to reach a species-specific threshold size for metamorphosis. However, in Orthoptera and Coleoptera, there are some exceptional species in which instar number is higher in favorable conditions. In more specific cases, the adaptive value of the variability in instar number may be in reaching or maintaining the developmental stage adapted to hibernation, producing additional generations in multivoltine species, or increasing the probability of surviving in long-lasting adverse conditions.

Capital breeders and income breeders among Lepidoptera : consequences to population dynamics

Among Lepidoptera, populations of some species display outbreaks at irregular intervals, some show regular cycles, whereas densities of most species never reach extremely high values. In recent years, researchers have looked for correlations between life-history traits and different types of population dynamics (Nothnagle and Schultz 1987, Hunter 1991, Hunter 1995a) while others have generalized based on relatively few but well-studied cases (Price et al. 1990, Price 1994). The proximate reasons of changes in population density presumably are in the environment (Ruohomiki and Haukioja 1992) but these kinds of observations may have masked the importance of life-history x environment interactions. Here we emphasize the importance of adult feeding, a correlate of eruptive (sensu Price et al. 1990) and cyclic population dynamics among Lepidoptera which has deserved almost no attention until very recently (Miller 1996). Furthermore, we propose ecological and evolutionary explanations for the correlation.

Determination of adult size in a folivorous moth: constraints at instar level?

1. Reaction norms for size and age at maturity were studied in Epirrita autumnata (Lepidoptera, Geometridae). Growth rates were manipulated by rearing larvae on different levels of food quality and quantity, and instar‐specific final weights and development times were recorded.

Reproductive decisions are sensitive to cues of life expectancy: the case of a moth

Life history theory predicts that reproductive effort should increase as life expectancy decreases. Empirical evidence in support of this hypothesis is still fragmentary. We measured the effects of different potential indexes of mortality risk on oviposition in a geometrid moth Scotopteryx chenopodiata L. We conducted two experiments: in one we manipulated mortality risk by clipping wings or depriving the moths of food; in the other we studied the effect of incidental injuries, and varied host quality and food availability. Physical injuries led to a higher oviposition rate and shorter latency (i.e. a shorter discrimination period before oviposition) on both a high-ranked and a low-ranked host. High mortality risk was associated with a shorter latency to oviposit on the low-ranked, but not on the high-ranked host. These effects remained significant when we controlled for the influences of estimated adult age and eggload. Insects deprived of food as adults increased oviposition activity in the first experiment but not in the second. Our results suggest that certain cues of an adverse environment, as well as the individuals terminal condition, can increase reproductive effort and reduce host selectivity in insects.

Achieving high sexual size dimorphism in insects: females add instars

Abstract 1. In arthropods, the evolution of sexual size dimorphism (SSD) may be constrained by a physiological limit on growth within each particular larval instar. A high SSD could, however, be attained if the larvae of the larger sex pass through a higher number of larval instars.

Cascading effects of variation in plant vigour on the relative performance of insect herbivores and their parasitoids

Abstract 1. Consequences of variation in food plant quality were estimated for a system consisting of two monophagous noctuid herbivores and three ichneumonid parasitoids.

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.

Components of male fitness in relation to body size in Epirrita autumnata (Lepidoptera, Geometridae)

Abstract. 1. The effect of body size on different components of male fitness was studied for Epirrita autumnata, a geometrid known for its eruptive population dynamics. Body size is the main determinant of female fecundity in this species.