The impact of invasive alien species on native species is of increasing global concern. Invasive plants have various negative effects on natives through competition; however, relatively little is known about competition for pollination. The relationship between Japanese native dandelions (Taraxacum spp.) and invasive congeners may be a typical case of such an interaction. For example, native dandelions are being replaced by invasive congeners, especially in urban and suburban areas of Japan. To explain this phenomenon, we hypothesized that when natives are mixed with attractive invasives, natives may suffer from reduced seed set because invasives deprive natives of pollinators or because pollinators frequently move between species, resulting in interspecific pollen transfer. To test this hypothesis, we studied the effect of the invasive dandelion T. officinale on the pollination and seed set of the native T. japonicum using artificial arrays of monospecific and mixed-species plots as well as natural populations. Taraxacum officinale attracted more pollinator visits, perhaps because it produced more nectar than T. japonicum. The number of pollinator visits to T. japonicum was reduced when the congeners were grown together, and pollinators moved frequently between the two species. The proportion of seed set for T. japonicum was reduced in the presence of T. officinale in both artificial arrays and natural populations. These results support our hypothesis that interspecific competition for pollination plays an important role in the recent replacement of native dandelions by invasive congeners in Japan. Because invasive dandelions are apomicts, negative effects are incurred only by sexual natives. Thus, this system can be recognized as a rare case of interspecific interaction through pollination.

Assessing pollinator importance of each floral visitor to a plant species is a key to understanding plant–pollinator interaction. The present study examined visitation frequency, pollination efficiency, and pollinator importance of the full range of floral visitors to Geranium thunbergii natural population, by measuring seed‐set. During 2 years of observations, the flowers were visited by at least 45 insect species belonging to four orders. Among the main 22 visitor species, 11 species belonging to three orders (Hymenoptera, Diptera, and Lepidoptera) acted as the efficient pollinators. In both years, Hymenoptera, especially bees, was the most important pollinator to G. thunbergii. Thus, the flowers could be considered as bee‐pollinated. However, the most important species were not constant between years. The study also documented that the efficient pollinators have larger body sizes. The dish‐shaped floral morphology, taxonomically diverse pollinators, and temporal change in the most important pollinators indicate that G. thunbergii–pollinator interaction is a rather generalized system. The results suggest that casual observations of visitation, or even precise measurement of pollinator importance in a single season is insufficient to identify important pollinators.

SUMMARYLearning plays an important role in food acquisition for a wide range of insects and has been demonstrated to be essential during flower foraging in taxa such as bees, parasitoid wasps, butterflies and moths. However, little attention has been focused on differences in floral cue learning abilities among species and sexes. We examined the associative learning of flower colour with nectar in four butterfly species: Idea leuconoe, Argyreus hyperbius, Pieris rapae and Lycaena phlaeas. All butterflies that were trained learned the flower colours associated with food. The flower colour learning rates were significantly higher in I. leuconoe and A. hyperbius than in P. rapae and L. phlaeas. Among the four species examined, the larger and longer-lived species exhibited higher learning rates. Furthermore, female butterflies showed a significantly higher learning rate than males. This study provides the first evidence that learning abilities related to floral cues differ among butterfly species. The adaptive significance of superior learning abilities in the larger and longer-lived butterfly species and in females is discussed.Key words: associative learning, forewing length, innate colour preference, Lepidoptera, lifespan. THE JOURNAL OF EXPERIMENTAL BIOLOGY3811 Learning rate differences in butterflies Idea leuconoe (Butler) (Danaidae)We used laboratory-reared individuals from the Itami City Museum of Insects, Itami, Osaka, Japan. Butterflies were originally collected in Ishikawa, Uruma, Okinawa, Japan. One to three larvae were reared in a 500ml or 860ml transparent plastic cup on fresh leaves of Parsonia alboflavescens (Dennstedt) Mabberley at 22-25°C and 15h:9h L:D in an incubation room. Argyreus hyperbius hyperbius (Linnaeus) (Nymphalidae)Adult females and larvae were obtained near the Nara campus of Kinki University, Nakamachi, Nara, Japan, from July to August 2004. Adult females were then allowed to oviposit eggs. About 10 larvae were reared together on fresh leaves of several Viola species in 450ml transparent plastic cups at room temperature (range of daily mean: 24-30°C) and natural daylength (range: 14h:10h L:D to 15.5h:8.5h L:D) within the laboratory. Pieris rapae crucivora (Boisduval) (Pieridae)Adult females and larvae ware collected near the Nara campus of Kinki University from September to October 2004. Adult females oviposited eggs, and the larvae were reared on fresh leaves of Brassica oleracea L. and Raphanus sativus L. in 450ml transparent plastic cups (10 larvae per cup) at room temperature (14-29°C) and natural daylength (12h:12h L:D to 14h:10h L:D) within the laboratory.

This study examines the role of learning and memory in the butterfly Pieris rapae crucivora Boisduval during foraging for flowers. In an outdoor cage with 6 flower species, P. rapae showed various visiting patterns: some visited only one species, while others visited several species in a day. The foraging process for flowers of Erigeron annuus (L.) Pers. could be divided into two successive steps: (1) landing on the nectaring caputs, and (2) finding the source of nectar in the caput. Butterflies learned to proceed through the two steps more efficiently with successive attempts: they gradually decreased landings on nectarless caputs and probings on the nectarless petals of ligulate flowers respectively. As a result, handling time per unit caputs became shorter, and apparent rewards per unit time, i.e. the efficiency of collecting nectar, increased. In addition, once learned, P. rapae could remember a rewarding flower color for 3 days, which was not interfered with by learning another flower color. This indicates that P. rapae keeps memory for a period longer than 3 days, and that they can remember at least two flower species as suitable flower resources. Furthermore, data indicated that they sometimes can apply the foraging skills obtained on other flower species to a novel one. These abilities could enable butterflies to easily switch flower species, or to enhance labile preference. It has been known that P. rapae also shows flower constancy, which may be due to memory constraints. Therefore, they may appropriately use two foraging tactics: visit consistency and labile preference, to get enough nectar according to their circumstances.

Learning plays an important role in food acquisition for a wide range of insects. To increase their foraging efficiency, flower-visiting insects may learn to associate floral cues with the presence (so-called reward learning) or the absence (so-called non-reward learning) of a reward. Reward learning whilst foraging for flowers has been demonstrated in many insect taxa, whilst non-reward learning in flower-visiting insects has been demonstrated only in honeybees, bumblebees and hawkmoths. This study examined both reward and non-reward learning abilities in the butterfly Byasa alcinous whilst foraging among artificial flowers of different colours. This butterfly showed both types of learning, although butterflies of both sexes learned faster via reward learning. In addition, females learned via reward learning faster than males. To the best of our knowledge, these are the first empirical data on the learning speed of both reward and non-reward learning in insects. We discuss the adaptive significance of a lower learning speed for non-reward learning when foraging on flowers.

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The lifetime mating frequency of female butterflies is believed to be dependent on the reproductive status of the males which they have mated. This report assesses those status using Pieris rapae L. Multiple mating females mated males with a short time interval after the last mating or males with many mating records. Such males, like small ones, produced small spermatophores during copulation, which may have resulted in high mating frequency of those females. The males with short time interval after the last mating or those with many mating records also showed a long mating duration. Alternative interpretations of the adaptive significance of this behavior for males are discussed.

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