Reproductive biology of Vincetoxicum rossicum (Kleo.) Barb. (Asclepiadaceae), an invasive alien in Ontario1

Journal of the Torrey Botanical Society, Jan-Mar 2004 by St Denis, Melissa, Cappuccino, Naomi

Discussion. Vincetoxicum rossicurn has flat, regular dull-colored flowers and open nectar, characteristic of fly-pollinated plants (van der Fiji 1961). Of the flowers that we monitored daily in the field, 25-50% had at least one pollinarium removed. We rarely observed visits by insects other than ants. Flies large enough to carry pollinaria were not observed; smaller flies, such as gnats, that had been trapped by a pollinarium, were occasionally seen. It is unlikely that V. rossicum pollinaria are dislodged when brushed by neighboring plants swaying in the wind. The pollinaria are difficult to remove unless a thin object, such as an insect appendage, is drawn up the interstaminal slit. The lack of observation of floral visitors contrasts with Christensen's (1998) observations of flies and wasps visiting V. rossicum, although she notes that the diversity of floral visitors in some swallow-wort stands is impoverished. Lumer and Yost (1996) reported that V. nig rum flowers in southern New York State were visited by several dipteran species and that they had a fetid odor, consistent with pollination by flies. Vincetoxicum rossicum flowers, on the other hand, have a sweet, fruity odor. It is possible that some floral visitors to V. rossicum are nocturnal; however, the odor is quite pronounced during the daytime hours.

Flowers of isolated plants were more likely to have been visited than plants in dense monocultures. Natural plants in dense stands may be more stressed by competition than isolated plants, and produce a less attractive floral display, as was suggested by the slightly but non-significantly smaller number of flowers per plant in the monocultures. To avoid any bias caused by differences in plant quality, we supplemented our observations on natural plants with those on potted plants placed in large, dense patches, in small (5-plant) patches and in grass-dominated fields. As was the case for the observations on naturally growing plants, this experiment revealed a positive effect of isolation on the removal of pollinaria. This positive effect of isolation was unexpected, since isolated plants are often reported to receive fewer visits from pollinators (Jennersten 1988, Aizen and Feinsinger 1994, Kunin 1993, Steffan-Dewenter and Tscharntke 1999). The lower visitation rate in dense monocultures of V. rossicum might be the result of a dilution effect. If relatively few potential visitors per unit area are presented with a large number of flowers, the probability that an individual flower will be visited is low. Single plants or small patches surrounded by grass, on the other hand, may be the only source of nectar in a large area, and the few flowers will not be passed up by foraging generalists seeking nectar.

In our field experiment using potted plants, 25% of the flowers produced fruits. This fruit-set was considerably higher than that of other asclepiads, even those given fertilizer as was done with our plants (Wyatt and Broyles 1994, Queller 1985, Willson and Price 1980). Our hand-pollinated greenhouse plants had even higher fruit-set (mean for all treatments except emasculation = 75%). In the field experiment, visits by potential pollinators were not necessary for fruit-set, but having been visited at least once increased the probability that a flower would produce a mature fruit. In addition, fruits produced by visited flowers contained seeds with more embryos. Results of the hand-pollination experiment, however, did not support this finding; flowers "pollinated" by the dissecting needle were no more likely to produce a fruit than the unmanipulated controls and their seeds were not more likely to be polyembryonic. It is possible that the positive association between visits and fruit set in the field was a spurious one, perhaps driven by pollinators being attracted to larger flowers, which may be more likely to produce fruits with a higher proportion of polyembryonic seeds. It is also possible that there may be inbreeding depression following selfing that is only expressed under field conditions.