Heikki Pöykkö

Females and Larvae of a Geometrid Moth, Cleorodes lichenaria, Prefer a Lichen Host That Assures Shortest Larval Period

Abstract Optimal oviposition theory predicts that females prefer a host that assures the highest offspring fitness. I compared the host preference of females and larvae of a lichenivorous moth, Cleorodes lichenaria, and subsequent larval performance on host lichens with laboratory experiments and field observations. Both females and larvae preferred Ramalina fraxinea and Ramalina farinacea over Xanthoria parietina and Parmelia sulcata. Larvae reached the pupal stage faster on Ramalina species than on other lichens, whereas other fitness parameters including larval survival and pupal size did not differ between the lichen species. Interestingly, growth rates before overwintering were higher on P. sulcata and X. parietina than on Ramalina species, whereas after overwintering, the situation was reversed. The results of this study support the preference–performance hypothesis. Females clearly preferred host lichens that ensure the shortest developmental period at the expense of higher growth rates at the beginning of the larval period. Moreover, ecological and/or evolutionary factors in addition to females’ oviposition in host selection of C. lichenaria seem to be important. Because more larvae were found on Ramalina species than expected according to the oviposition pattern of females, final host selection is at least partially determined by larval behavior. It is also suggested that the physical and chemical properties of the hosts may provide a basis for enemy-free space for larvae.

To grow fast or to grow big? Time-limited larvae of Eilema depressum speed up their growth and reduce number of instars

According to life‐history theory, longer development time may result in bigger adults. However, reaction norms describing age and size at maturity often follow an L‐shaped form. This relationship is attributable to the simple notion that slowly growing individuals may not lengthen their development excessively after the maturation decision has been made, for example, when development is time limited in seasonal environments. In arthropods, growth occurs within instars, and thus the optimal growth strategy might be mediated by the phenotypic adjustment of instar numbers. We studied the relationship between age and size at maturity of a lichen‐feeding moth, Eilema depressum (Esper) (Lepidoptera: Arctiidae: Lithosiinae), and the variability of instar numbers in relation to achieved adult body mass and time used for maturation. A positive relationship between age and size at maturity was found across developmental pathways and a negative one within the developmental pathways. Directly developing larvae had higher growth rates, attained smaller pupal mass, and passed fewer instars than larvae maturing after overwintering. Host quality did not affect whether larvae matured during the remaining or the next season. High variation in the number of instars together with variable growth rates indicates high plasticity in adaptation to varying environmental conditions. Our results also confirm previous results that instar number variability may be a key characteristic mediating age and size at maturity in insects.

Egg size and composition in an ageing capital breeder – consequences for offspring performance

1. Egg size is often used as a proxy of egg quality although size and composition may vary, e.g. in insects egg size usually decreases as female ages. Whether this decrease in size reflects reduced concentrations of essential nutrients such as lipids and proteins of eggs laid by ageing females, or does reduced size per se explain often observed lower fitness of later laid eggs is poorly explored.

Egg maturation and oviposition strategy of a capital breeder, Cleorodes lichenaria, feeding on lichens at the larval stage

Abstract 1. Capital breeders rely solely on resources acquired before breeding, whereas income breeders may obtain considerable amounts of resources following the commencement of reproduction. In income breeders oviposition occurs over a longer time period with a small number of eggs ready to be laid at the start of adult life, whereas capital breeders reproduce more rapidly and contain higher numbers of mature eggs at eclosion relative to potential fecundity.

Response of the lichen-eating moth Cleorodes lichenaria larvae to varying amounts of usnic acid in the lichens

Lichens are characterized by a great variety of secondary metabolites. The function of these substances remains partly unknown. In this study, we propose that some of these metabolites may expel insect herbivores. To test this hypothesis, we reared larvae of the lichenivorous moth Cleorodes lichenaria on three selected lichens, Cladonia arbuscula subsp. mitis, Usnea hirta, and Usnea dasypoga. In experimental setup, the secondary metabolite usnic acid was removed from the lichens with acetone prior to feeding, whereas a control was left untreated. On all three lichens, removal of usnic acid from the lichens using acetone significantly prolonged survival of larvae and increased their viability. Larvae reared on control lichens contained significantly more usnic acid than those reared on treated lichens, both in their biomass and their faeces. These results support the hypothesis that usnic acid serves as a repellent against insect feeding, besides its well established functions of UV protection and antimicrobial properties.