Visitation of a specialist pollen feeder Althaeus hibisci Olivier (Coleoptera: Bruchidae) to flowers of Hibiscus moscheutos L. (Malvaceae)1

Journal of the Torrey Botanical Society, Apr-Jun 2005 by Shimamura, Ryouji, Kachi, Naoki, Kudoh, Hiroshi, Whigham, Dennis F

SHIMAMURA, R., N. KACHI (Department of Biological Sciences, Graduate School of Sciences, Tokyo Metropolitan University. Hachioji 192-0397, Japan), H. KUDOH (Department of Biology, Faculty of Science, Kobe University, Kobe 657-8501, Japan), AND D. F. WHIGHAM (Smithsonian Environmental Research Center, Edgewater, MD 21037). Visitation of a specialist pollen feeder Althaeus hibisci Olivier (Coleoptera: Bruchidae) to flowers of Hibiscus moscheutos L. (Malvaceae). J. Torrey Bot. Soc. 132: 197-203. 2005.-We investigated visitation of a bruchid seed beetle, Althaeas hibisci Olivier, to flowers of Hibiscus moscheutos L. (Malvaceae) under natural conditions. We examined movement of the beetle among flowers that were in different developmental stages, the correlation between corolla size and pollen production, and the correlation between beetle density and corolla size. Beetles moved diurnally from wilted Hibiscus moscheutos flowers that had opened on the previous day to newly open flowers. Beetles did not move to unopened flowers as long as petals were covered by sepals. The results demonstrate that the beetles use corollas as a cue to locate flowers. Hibiscus moscheutos flowers produced pollen in proportion to corolla size; therefore, beetles visiting larger flowers had a high probability of finding larger amounts of pollen. We detected positive correlations between beetle density and petal area when beetles were abundant. Although the correlation coefficients were small (r = 0.24-0.32), they were statistically significant or marginally significant. On the other hand, the correlations were not significant when the density of beetles was low suggesting that the preference for larger flowers is not always detectable and its strength varies with beetle density under natural conditions.

Key words: Althaeus hibisci, bruchid beetle, floral attraction, flower size, foraging, freshwater wetland, Hibiscus moscheutos, Maryland, pollen production.

In many plant species, corolla size varies among and within individuals (Bell 1985, Galen 1999, Wolfe and Krstolic 1999). Theoretically, if corolla size correlates with the amounts of resources provided by flowers, then insect visitors should select flowers based on corolla size to reduce the costs of movement and maximize their rates of resource gain. Previous studies have shown that pollinators prefer to visit flowers with larger corollas (Bell 1985, Young and Stanton 1990, Stanton et al. 1991, Johnson et al. 1995).

Corolla size might also be important in determining the preference and behavior of small insect visitors (e.g., thrips, flies, small bees, and small beetles). Small insects are found to serve as pollinators in some systems (Momose et al. 1998a, b; Sakai et al. 1999). As pollen/ovule feeders, small insects may considerably reduce reproductive success of larger flowers (Brody 1992, Mutikainen and Delph 1996). Furthermore, small insects often use flowers as a site of mating and oviposition (Feller et al. 2003).

In this study, we investigated the responses of a bruchid seed beetle, Althaeus hibisci Olivier (hereafter referred to as A. hibisci), to natural corolla size variation in Hibiscus moscheutos L. (hereafter referred to as Hibiscus) flowers. Adults of A. hibisci visit Hibiscus flowers and forage for pollen (Spira 1989, Kudoh and Whigham 1998). Kudoh and Whigham (1998) reported that the density of A. hibisci on H. moscheutos flowers decreased when petal size was artificially reduced to the smallest size class found in natural populations. Under natural conditions, H. moscheutos exhibits continuous variation in corolla size. To understand the impact of A. hibisci on seed production it is necessary to understand how these beetles respond to the natural variation in a floral trait.

To evaluate the effects of corolla size on flower preference by A. hibisci, we addressed the following questions: (1) Does A. hibisci arrive at Hibiscus flowers before or while they are open? (2) Is corolla size used as a cue in flower choice by A. hibiscil (3) Does corolla size correlate with the amount of pollen in Hibiscus flowers? (4) Does the density of A. hibisci in flowers correlate with corolla size?

Materials and Methods. STUDY SYSTEM. Hibiscus moscheutos L. (Malvaceae) is a herbaceous perennial native to freshwater and brackish marshes of eastern North America (Brown and Brown 1984). Plants have few to many upright stems, 1-2 m tall. The flowering season extends from late July to early September, and the flowers are relatively large (10-15 cm across), with white or pink petals that open for a single day (Spira 1989). Flowers open at dawn and close by night (Fig. 1). Time of anthesis may depend on temperature because anthesis is delayed on rainy days and later in the flowering season (Ryouji Shimamura, personal observation). Flowers are pollinated mainly by an anthophorid bee, Ptilothrix bombiformis Cresson, and by a bumblebee, Bombus pennsylvanicus DeGeer (Spira 1989). The flowers are self-compatible, but spatial separation of the stigmas and the anthers prevents automatic selfpollination (Spira 1989).