The effect of nectar reduction in Petunia Axillaris on foraging behavior of noctural hawkmoths, observed in laboratory and field behavioral assays

A key component shaping plant-pollinator interactions is nectar. Its volume can regulate the length and frequency of pollination events. Nectar provisioning can be costly for the plant. Once secreted by the nectaries, the sugar-rich solution is usually consumed by a floral visitor and lost for “recycling” within the plant. Nectar reduction should thus be advantageous for the plant: non-secreted carbohydrates can be reallocated within the plant to other structures promoting growth, reproduction or attraction. However, most angiosperms provide nectar. It is assumed that certain pollinator behaviors, so called partner control mechanisms, favor nectariferous over deceptive plants and ultimately prevent the spread of “cheaters”. The partner control mechanisms identified in the context of plant-pollinator mutualisms are <b>avoidance</b> of nectarless species, <b>reduction</b> of <b>drinking time</b> and <b>number of flowers</b> visited on nectarless plants. Not all behaviors are performed simultaneously, and external conditions such as plant density as well as intrinsic factors of the foraging insect can determine to which extent certain behaviors are exerted. In the present study, we analyze foraging behavior of nocturnal hawkmoths on cheating <i>Petunia axillaris axillaris</i> plants under several conditions. The aim of this thesis was to assess which partner control mechanisms are executed by pollinators facing nectarless/low nectar plants. We observed hawkmoth behavior in two field sites and conducted experiments with naïve and experienced hawkmoths <i>Manduca sexta</i> under controlled conditions. We investigated which of the foraging decision rules might potentially reduce the fitness of cheaters and thus limit their spread in a population. In field assays, we observed that the density of naturally occurring <i>P. axillaris</i> plants and the presence of alternative food sources can influence hawkmoth behavior on nectarless Petunias: only when food plants were abundant and dense, pollinators would reduce the number of flowers on nectarless Petunias, whereas in the lower density there seemed to be no selection against cheaters. In learning experiments under controlled conditions, we observed that none of the tested behaviors (reduction of drinking duration on nectarless plants, avoidance of nectarless plants, reduction of number of flowers visited on nectarless plants) were improved over the course of the experiment. However, in all learning trials there was a significant reduction of drinking duration on nectarless plants, indicating that this control mechanism of hawkmoths is always exerted innately. Learning might therefore not be of major importance in discrimination against cheaters in our system. We constructed a plant with extremely high phenotypic similarity to <i>P. axillaris</i> yet only a third of the regular nectar volume (F25). Genotyping of F25 revealed a high genotypic imilarity to its parental plant but failed to answer questions about the genetic background of low nectar volume. The low nectar line was used in behavioral experiments with <i>Manduca sexta</i>. A major goal was to find out how pollination behavior affects female reproductive success of F25. Analogous to previous experiments, we found that the drinking duration was significantly reduced on cheaters. In hand pollination assays, F25 produced significantly more seeds than <i>P. axillaris</i>, however this effect was neutralized when pollinated by <i>Manduca sexta</i>. The benefits of nectar reduction are thus counterbalanced by a change in pollinator foraging behavior. In the future, we would like to assess which other fitness parameters are concerned when a plant ceases its nectar production. Altogether, we were able to show which foraging rules are exerted by hawkmoths on cheating <i>P. axillaris</i> and how one partner control mechanism, namely drinking duration, affects seed set of a plant with reduced nectar offerings. We hope that this work has contributed to answering questions about the costs and benefits of cheating.

Thèse de doctorat : Université de Neuchâtel, 2009 ; Th. 2121