Juhan Javoiš

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.

Responses of ovipositing moths (Lepidoptera : Geometridae) to host plant deprivation : Life-history aspects and implications for population dynamics

Abstract For three species of geometrid moths with noneruptive population dynamics, physiological and behavioral responses of ovipositing females to larval host deprivation were recorded. Single substrate trials were used. Availability of adult food was also varied to manipulate physiological condition of the moths. Egg production rate decreased only weakly in the absence of host plant, whereas it was strongly influenced by adult feeding. The time from the beginning of the experiment to initiation of oviposition was largely determined by presence of suitable host. This variable, oviposition latency, was only weakly affected by nutritional status of the females. Oviposition rates also remained lower on unsuitable hosts after oviposition was initiated. This indicates that the effect of inferior oviposition substrate was not limited to postponing oviposition. Host deprivation and adult feeding both lead to accumulation of chorionated eggs in abdomens. However, high egg loads of the fed females did not alter their behavior in the direction predicted by optimality models. We conclude that oviposition latency was the only variable in which female responses to unsuitable hosts were sufficiently strong to have potential significance for population dynamics. Latency presumably corresponds to dispersing behavior in nature. Such a response, not having been observed in an outbreaking geometrid studied earlier, may reflect a behavioral mechanism capable of stabilizing local population dynamics. The results are discussed in the light of the gradation from capital to income breeding. In herbivorous insects, oviposition latency may serve as an index of overall sensitivity of ovipositing females to adverse environmental conditions.

Quantifying income breeding: using geometrid moths as an example

Although the general concept of capital vs. income breeding has become widely used in insect ecology, finding easy‐to‐measure indices for quantifying the role of larval‐ vs. adult‐derived nutrients in egg production has remained a challenge. When searching for possible candidates for this task, we evaluated the applicability of three morphometric ratios to be measured on freshly eclosed adults: (1) relative size of female abdomen, (2) sexual size dimorphism, and (3) proportion of volume of mature eggs relative to total volume of female abdomen. We report the values of these indices in five species of geometrid moths [Hypomecis punctinalis Scopoli, Ematurga atomaria L., Selenia tetralunaria Hufnagel, Semiothisa clathrata L., and Epirrhoe alternata Müller (all Lepidoptera: Geometridae)] and compare them to the degrees of income breeding measured directly by oviposition experiments. As a comparison, we also recorded the ovigeny index and the effect of income on lifespan. All morphometric indices varied considerably between the species studied, and, as predicted, the values of all three proposed indices were higher in the more capital‐breeding species. The only exception to this pattern was a low proportion of mature eggs in abdomens in one primarily capital‐breeding species, S. tetralunaria. Unlike other capital breeders, this species also proved to be largely synovigenic, indicating that capital breeding and pro‐ovigeny are not strictly linked. The reproductive traits measured concord with ecological niches occupied by the species studied: the oligophagous meadow dwellers were largely income breeding and synovigenic, whereas the polyphagous forest dwellers were capital breeders. The high and predictable among‐species variation in egg‐production strategies suggests that geometrid moths form a promising target group for comparative studies on respective traits. Simple anatomical indices, in turn, appear to be applicable as easy‐to‐record proxies of egg production strategies.

Degree of specialization is related to body size in herbivorous insects: a phylogenetic confirmation

Numerous studies have suggested a general relationship between the degree of host specialization and body size in herbivorous animals. In insects, smaller species are usually shown to be more specialized than larger‐bodied ones. Various hypotheses have attempted to explain this pattern but rigorous proof of the body size–diet breadth relationship has been lacking, primarily because the scarceness of reliable phylogenetic information has precluded formal comparative analyses. Explicitly using phylogenetic information for a group of herbivores (geometrid moths) and their host plant range, we perform a comparative analysis to study the body size–diet breadth relationship. Considering several alternative measures of body size and diet breadth, our results convincingly demonstrate without previous methodological issues—a first for any taxon—a positive association between these traits, which has implications for evaluating various central aspects of the evolutionary ecology of herbivorous insects. We additionally demonstrate how the methods used in this study can be applied in assessing hypotheses to explain the body size–diet breadth relationship. By analyzing the relationship in tree‐feeders alone and finding that the positive relationship remains, the result suggests that the body size–diet breadth relationship is not solely driven by the type of host plant that species feed on.

Sexual differences in weight loss upon eclosion are related to life history strategy in Lepidoptera.

Given that immature and adult insects have different life styles, different target body compositions can be expected. For adults, such targets will also differ depending on life history strategy, and thus vary among the sexes, and in females depend on the degree of capital versus income breeding and ovigeny. Since these targets may in part be approximated by loss of substances upon eclosion, comparing sexual differences in such losses upon eclosion among species that differ in life history would provide insights into insect functional ecology. We studied weight loss in eclosing insects using original data on pupal and adult live weights of 38 species of Lepidoptera (mainly Geometridae) and further literature data on 15 species of Lepidoptera and six representatives of other insect orders, and applied the phylogenetic independent contrasts approach. In addition, data on live and dry weights of pupae of four species of Lepidoptera are presented. We documented that Lepidoptera typically lose a large proportion (20-80%) of their pupal weight upon adult eclosion. Sexual differences in weight loss varied between absent and strongly male biased. Most of the weight loss was water loss, and sexual differences in adult water content correlate strongly with differences in weight loss. Using feeding habits (feeds or does not feed as an adult) and female biased sexual size dimorphism as measures of degree of capital breeding, we found that the difference among the sexes in weight loss tends to be more pronounced in capital breeding species. Additionally, females of more pro-ovigenic species (large proportion of eggs mature upon emergence) tend to have higher water contents. Our results suggests that metamorphosis is generally facilitated by a high water content, while adults excrete water upon eclosion to benefit flight unless water has been allocated to eggs, or is treated as a capital resource for adult survival or future allocation to eggs.

Oviposition in an eruptive moth species, Yponomeuta evonymellus, is insensitive to the population density experienced during the larval period

We studied if the population density experienced during larval development affects the reproductive schedule of Yponomeuta evonymellus L. (Lepidoptera, Yponomeutidae), a moth species characterized by outbreak population dynamics. More specifically, we predicted that reproduction would be delayed to facilitate emigration from sites with suboptimally high densities of conspecifics. We manipulated larval densities in the laboratory, as well as in those pupae collected from the extremes of natural densities. As the response, we also recorded the timing and sizes of egg clutches laid. The results did not support our initial predictions: the timing of oviposition was not dependent on larval growth conditions. This apparent lack of adaptation might be related to the loss of ‘memory’ during metamorphosis in holometabolous insects. However, oviposition schedules were also only minimally sensitive to elevated adult density. An inability to respond to high larval densities may contribute to the outbreak dynamics in this species.

A two-resource model of terminal investment

The most widely known theoretical basis for the hypothesis of terminal investment is the classic model by George C. Williams (1966). Although this model predicts that reproductive effort (i.e. the proportion of available resources devoted to reproduction) increases with decreasing reproductive value, it implies that reproductive allocation in absolute terms should remain stable. This contrasts with the empirical evidence on terminal investment reported to date: the vast majority of positive case studies report an increase in some aspect of reproductive allocation in absolute terms. Also, a substantial number of studies have failed to record terminal investment, despite expectations. Here, I present a simple conceptual model which explains such results. I argue that to explain terminal investment, an organism’s reproductive capacity must not be considered as a common pool of resources (often described by the term ‘reproductive value’), but as a set of different resources which are not easily convertible to each other, and should be exhausted in balance. Thus, if one resource accidentally decreases, in response, the others must be expended at higher rate. To test this model, each reproductive allocation should be measured in a more specific currency (or currencies) than traditional ‘reproductive effort’. The model is consistent with both the positive and the negative case reports on terminal investment.

An ordination of life‐histories using morphological proxies: capital vs income breeding in insects

Predictive classifications of life histories are essential for evolutionary ecology. While attempts to apply a single approach to all organisms may be overambitious, recent advances suggest that more narrow ordination schemes can be useful. However, these schemes mostly lack easily observable proxies of the position of a species on respective axes. It has been proposed that, in insects, the degree of capital (vs. income) breeding, reflecting the importance of adult feeding for reproduction, correlates with various ecological traits at the level of among-species comparison. We sought to prove these ideas via rigorous phylogenetic comparative analyses. We used experimentally derived life-history data for 57 species of European Geometridae (Lepidoptera), and an original phylogenetic reconstruction. The degree of capital breeding was estimated based on morphological proxies, including relative abdomen size of females. Applying Brownian-motion-based comparative analyses (with an original update to include error estimates), we demonstrated the associations between the degree of capital breeding and larval diet breadth, sexual size dimorphism, and reproductive season. Ornstein-Uhlenbeck model based phylogenetic analysis suggested a causal relationship between the degree of capital breeding and diet breadth. Our study indicates that the gradation from capital to income breeding is an informative axis to ordinate life-history strategies in flying insects which are affected by the fecundity vs. mobility trade off, with the availability of easy to record proxies contributing to its predictive power in practical contexts.

A comparative perspective on longevity: the effect of body size dominates over ecology in moths

Both physiologically and ecologically based explanations have been proposed to account for among‐species differences in lifespan, but they remain poorly tested. Phylogenetically explicit comparative analyses are still scarce and those that exist are biased towards homoeothermic vertebrates. Insect studies can significantly contribute as lifespan can feasibly be measured in a high number of species, and the selective forces that have shaped it may differ largely between species and from those acting on larger animals. We recorded adult lifespan in 98 species of geometrid moths. Phylogenetic comparative analyses were applied to study variation in species‐specific values of lifespan and to reveal its ecological and life‐history correlates. Among‐species and between‐gender differences in lifespan were found to be notably limited; there was also no evidence of phylogenetic signal in this trait. Larger moth species were found to live longer, with this result supporting a physiological rather than ecological explanation of this relationship. Species‐specific lifespan values could not be explained by traits such as reproductive season and larval diet breadth, strengthening the evidence for the dominance of physiological determinants of longevity over ecological ones.