BIOTROPICA 37(3): 356–362 2005 10.1111/j.1744-7429.2005.00047.x Pollination Biology of the Exotic Rattleweed Crotalaria retusa L. (Fabaceae) in NE Brazil 1 Claudia M. Jacobi 2 Departamento de Biologia Geral/ICB, Universidade Federal de Minas Gerais, Brazil Av. Antˆ onio Carlos 6627, 31270-901 Belo Horizonte, MG, Brazil Mauro Ramalho and Maise Silva IBIO-Universidade Federal da Bahia, Brazil ABSTRACT The rattleweed Crotalaria retusa was introduced in Brazil from Africa, and combines a series of characters that have ensured its establishment in NE Brazil. We focused on its reproductive biology and pollinator behavior to explain its reproductive success. We performed manual pollination and germination experiments, and monitored the behavior of C. retusa’s main pollinators in monospecific plots, and in mixed plots where C. retusa occurred together with two congeners, Crotalaria pallida and Crotalaria lanceolata. Crotalaria retusa is self-compatible and capable of automatic selfing. Inbreeding depression was expressed at the level of percent seed germination, but not seed set. Few insects visited the inflorescences. Legitimate pollinators were two large carpenter bees, Xylocopa frontalis and Xylocopa grisescens which, together, accounted for more than 90 percent of the visits. The former foraged on C. retusa exclusively and has low pollen spread potential. The latter flew longer distances between plants and visited fewer flowers per inflorescence, potentially increasing the extent of pollen carryover, but at the risk of increasing heterospecific pollen transfer, because it visits other Crotalaria species during the same foraging bout. The different foraging strategies, allied to morphological disadvantages represented by pollen overlap on X. grisescens’ body, may partially explain the much lower seed germination observed in C. pallida and C. lanceolata. These results are consistent with the hypothesis that a reduction in flower constancy may significantly depress viable seed set by increasing the chances of self-pollination. RESUMO A leguminosa Crotalaria retusa, vulgarmente conhecida como chocalho ou xique-xique, foi introduzida no Brasil pela ´ Africa, e combina uma s´ erie de caracter´ ısticas que facilitaram seu estabelecimento no nordeste do Brasil. Focalizamos aspectos da biologia reprodutiva e do comportamento dos seu polinizadores para explicar seu sucesso reprodutivo. Para isto realizamos tratamentos de polinizac ¸˜ ao manual e germinac ¸˜ ao de sementes, e monitoramos o comportamento dos principais polinizadores em ´ areas monoespec´ ıficas e em ´ areas onde C. retusa ocorria junto a duas congˆ eneres, C. pallida e C. lanceolata. C. retusa ´ e autocompat´ ıvel e capaz de autofertilizac ¸˜ ao espontˆ anea. Ocorreu depress˜ ao endogˆ amica no n´ ıvel de porcentagem de germinac ¸˜ ao de sementes, mas n˜ ao no n´ umero. Pouco insetos visitaram as inflorescˆ encias. Os polinizadores leg´ ıtimos foram duas abelhas de grande porte, Xylocopa frontalis e X. grisescens que representaram mais de 90 por ciento das visitas. O primeiro forrageia exclusivamente em C. retusa e tem baixo potencial de dispers˜ ao pol´ ınica. O segundo voa a distˆ ancias mais longas e visita menos flores por inflorescˆ encia, potencialmente aumentando a extens˜ ao de dispers˜ ao do p´ olen, mas com o risco de ampliar a transferˆ encia de p´ olen heteroespec´ ıfico, pois visita outras esp´ ecies de Crotalaria durante o vˆ oo. As diferentes estrat´ egias de forrageio, junto com a sobreposic ¸˜ ao de p´ olen no corpo de X. grisescens, explica em parte a baixa taxa de germinac ¸˜ ao de C. pallida e C. lanceolata. Os resultados s˜ ao consistentes com a hip´ otese de que uma reduc ¸˜ ao na constˆ ancia floral pode deprimir significativamente a produc ¸˜ ao de sementes vi´ aveis, ao aumentar as chances de autopolinizac ¸˜ ao. Key words: Crotalaria; exotic weed; floral biology; flower constancy; rattleweed; Xylocopa. THE COLONIZATION ABILITY OF A PLANT SPECIES depends on com- bined aspects of its reproduction and life history attributes. Growth form, longevity, size, mating system, floral allocation strategies, seed set and germination, as well as modes of dispersal, phenotypic plas- ticity, and tolerance to environmental changes are among the most important characteristics correlated with successful establishment in new environments (Baker 1974). A decisive factor for the success of many of these attributes is the presence of suitable pollinators in new environments, either to effect pollination in the case of self- incompatible plants, or to reduce the levels of inbreeding in self- fertile species. Inbreeding depression is a widespread mechanism to prevent self progeny, reflected in the poor performance frequently observed in the progeny derived from self or close relative crosses (Lande & Schemske 1985, Husband &> Schemske 1996, Richards 1 Received 6 February 2003; revision accepted 27 November 2004. 2 Corresponding author; e-mail: jacobi@icb.ufmg.br 1997, Husband & Gurney 1998, Culley et al. 1999). In many cir- cumstances, however, self-fertilization is not so strongly prevented, or is even favored by selective forces. The higher the level of uncer- tainty of cross-pollination, the stronger the effect of reproductive assurance in favor of self-fertilization (Ramsey & Vaughton 1996, Barrett et al. 1997, Petanidou et al. 1998). The supply of suitable pollination services grants good levels of both pollen quantity and quality. Pollination services depend on aspects such as the degree of floral constancy of pollinators, a be- havior that is still the focus of ample debate regarding its causes and consequences, but is generally associated positively with reproduc- tive success (Waser 1986, Kwak & Bergman 1996, McLernon et al. 1996, Chittka et al. 1999). Many legumes (Fabaceae) are among the most widely spread exotic plants and, besides presenting self-compatibility, they benefit from either exotic (Stout et al. 2002) or native pollination services (Etcheverry et al. 2003), although the rates of spread within the family vary considerably (Wu et al. 2003). In this study, we will 356