In a series of dawn-to-dusk studies, we examined the nature and accessibility of nectar rewards for pollinating insects by monitoring insect visits and the secretion rate and standing crop of nectar in the British native plant species Salvia pratensis, Stachys palustris, S. officinalis, Lythrum salicaria, Linaria vulgaris, the non-native Calendula officinalis, Petunia x hybrida, Salvia splendens, and the possibly introduced Saponaria officinalis. We also compared single with double variants of Lotus corniculatus, Saponaria officinalis, Petunia x hybrida and Calendula officinalis. All the British species studied are nectar-rich and are recommended for pollinator-friendly gardens. They showed maximal secretion rates of about 10-90 mug sugar per flower h(-1), and most had mean standing crops of about 5-60 mug sugar per flower. In all British species studied, the corolla was deep enough for the relatively long-tongued bumblebee Bombus pascuorum, but the shallower flowers of Lythrum salicaria were also much visited by shorter-tongued bees and hoverflies, as well as by butterflies. The exotic Salvia splendens, presumably coevolved with hummingbirds in the Neotropics, has such deep flowers that British bees cannot reach the nectar except by crawling down the corolla. With a secretion rate approaching 300 mug sugar per flower h(-1) and little depletion by insects; S. splendens accumulated high standing crops of nectar. S. splendens, and single and double flowers of the two probably moth-pollinated species Petunia x hybrida and Saponaria officinalis, received few daytime visits despite abundant nectar but Calendula was well visited by hoverflies and bees. We compared single and double variants of Lotus corniculatus, Petunia x hybrida and Calendula officinalis, and also Saponaria officinalis, the last being probably introduced in Britain (Stace, 1997 New flora of the British Isles. 2nd edn. Cambridge: Cambridge University Press). In Petunia, Saponaria and Lotus, double flowers secreted little or no nectar. In Calendula, where doubling involved a change in the proportion of disc and ray florets rather than modification of individual flower structure, double and single capitula had similar standing crops of nectar. Except in Calendula, exotic or double flowers were little exploited by insect visitors. In the exotics, this was probably due to the absence or scarcity of coevolved pollinators, coupled, in double flowers, with the absence of nectar. (C) 2001 Annals of Botany Company.
