Honey bees are the dominant diurnal pollinator of native milkweed in a large urban park

Abstract

In eastern North America, the field milkweed, Asclepias syriaca L. (Asclepiadaceae), is used in planting schemes to promote biodiversity conservation for numerous insects including the endangered monarch butterfly, Danaus plexippus (Linnaeus) (Nymphalidae). Less is known about its pollinators, and especially in urban habitats where it is planted often despite being under increasing pressure from invasive plant species, such as the related milkweed, the dog‐strangling vine (DSV), Vincetoxicum rossicum (Kleopow) Barbar. (Asclepiadaceae). During the A. syriaca flowering period in July 2016, we surveyed bees in open habitats along a DSV invasion gradient and inspected 433 individuals of 25 bee species in 12 genera for pollinia: these were affixed to bees that visited A. syriaca for nectar and contain pollen packets that are vectored (e.g., transferred) between flowers. Of all bees sampled, pollinia were found only on the nonindigenous honeybee, Apis mellifera (43% of all bees identified), as well as one individual bumblebee, Bombus impatiens Cresson. Pollinia were recorded from 45.2% of all honeybees collected. We found no relationship between biomass of DSV and biomass of A. syriaca per site. There was a significant positive correlation between A. syriaca biomass and the number of pollinia, and the proportion vectored. No relationship with DSV biomass was detected for the number of pollinia collected by bees but the proportion of vectored pollinia declined with increasing DSV biomass. Although we find no evidence of DSV flowers attracting potential pollinators away from A. syriaca and other flowering plants, the impacts on native plant–pollinator mutualisms relate to its ability to outcompete native plants. As wild bees do not appear to visit DSV flowers, it could be altering the landscape to one which honeybees are more tolerant than native wild bees.

Novel resources: opportunities for and risks to species conservation

“…Conversely, many native plants in altered landscapes have become dependent on non‐native honeybees and bumblebees as their sole or complementary pollinators (MacIvor et al . 2017). Several wildlife species use human‐built structures, often analogous to traditional resources, as novel breeding and roosting sites (Caballero et al .…”

Suitability of native milkweed (Asclepias) species versus cultivars for supporting monarch butterflies and bees in urban gardens

“…Native bee populations are declining ( Cameron et al, 2011 ; Koh et al, 2016 ) and millions of urban pollinator gardens are being planted to help their plight ( Phillips, 2019 ). Milkweed flowers produce abundant nectar ( Wyatt & Broyles, 1994 ), and are highly attractive to bees and other nectar-feeding insects ( Fishbein & Venable, 1996 ; MacIvor et al, 2017 ; Baker & Potter, 2018 ). Because milkweed pollen is enclosed within pollinia, nectar is the only reward that milkweeds offer their pollinators ( Wyatt & Broyles, 1994 ).…”

“…Because milkweed pollen is enclosed within pollinia, nectar is the only reward that milkweeds offer their pollinators ( Wyatt & Broyles, 1994 ). Large bees and wasps are the most effective milkweed pollinators, whereas most of the smaller visitors are unable to transfer pollina and do not provide pollination services to milkweed ( Kephart, 1983 ; Ivey, Martinez & Wyatt, 2003 ; MacIvor et al, 2017 ).…”

Peer Review #1 of "Suitability of native milkweed (Asclepias) species versus cultivars for supporting monarch butterflies and bees in urban gardens (v0.1)"

“…Native bee populations are declining (Cameron et al, 2011;Koh et al, 2016) and millions of urban pollinator gardens are being planted to help their plight (Phillips, 2019). Milkweed flowers produce abundant nectar (Wyatt & Broyles, 1994), and are highly attractive to bees and other nectar-feeding insects (Fishbein & Venable, 1996;MacIvor et al, 2017Baker & Potter, 2018. Because milkweed pollen is enclosed within pollinia, nectar is the only reward that milkweeds offer their pollinators (Wyatt & Broyles, 1994).…”