Variable responses of hawkmoths to nectar-depleted plants in two native Petunia axillaris (Solanaceae) populations

Abstract

Pollination success of deceptive orchids is affected by the density and distribution of nectar providing plant species and overall plant density. Here we extended the framework of how plant density can affect pollination to examine how it may promote the success of plant intraspecific cheaters. We compared hawkmoth behaviour in two native populations of Petunia axillaris, where we simultaneously offered rewarding and manually depleted P. axillaris. We asked whether pollinator foraging strategies change as a function of plant density and whether such changes may differentially affect nectarless plants. We observed the first choice and number of flowers visited by pollinators and found that in the dense population, pollinators visited more flowers on rewarding plants than on nectar-depleted plants. In the sparse population, such discrimination was absent. As we found no differences in nectar volume between plants of the two populations, the observed differences in plant density may be temporal. We reason that if differences were more permanent, an equivalent of the remote habitat hypothesis prevails: in a sparse population, cheating plants benefit from the absence of inter-and intraspecific competitors because pollinators tend to visit all potential resources. In a denser population, a pollinator's optimal foraging strategy involves more selectivity. This would cause between-plant competition for pollinators in a pollinator-limited context, which applies to most hawkmoth-pollinated systems. We propose that nectar-provisioning of plants can be density-dependant, with cheaters able to persist in low density areas.

Innate Adjustment of Visitation Behavior to Rewarding and Reward‐Minimized Petunia axillaris (Solanacea) Plants by Hawkmoth Manduca sexta (Sphingidae)

“…In accordance with field results (Brandenburg & Bshary ), we found no evidence for prior avoidance of ‘reward‐minimized’ plants. The inability to discriminate empty from otherwise identical rewarding flowers prior to probing seems to be a common pattern of foraging insects (Thakar et al.…”

“…This would certainly be the case of a cheater plant representing a mutation in an otherwise mutualistic lineage, as simulated in our experiment. The scent profile remained similar after nectar extraction (Brandenburg & Bshary ) and selection would favor cheaters that specifically reduce nectar production without changing anything else. Thus, we consider it unlikely that prior avoidance of intraspecific cheaters will be an important partner control mechanism.…”

“…To avoid replenishment, nectar was removed hourly. Such treatment yields clearly distinct nectar volumes between treatment groups (Brandenburg & Bshary ). To exclude that the tissue injury would elicit some kind of behavioral response in the pollinator, the control plants were also pierced in the floral tube, but above nectar levels.…”

“…To exclude that the tissue injury would elicit some kind of behavioral response in the pollinator, the control plants were also pierced in the floral tube, but above nectar levels. No scent difference could be detected between ‘with nectar’ and ‘reward‐minimized’ treatment in isolated flowers 30 min after cutting (Brandenburg & Bshary ).…”

Toward an Evolutionary Definition of Cheating

“…(3) Rhizobia bacteria that fix a low or negligible amount of nitrogen for their legume host (Kiers et al ). (4) Plants, such as those of the Orchidacea or honey mesquites ( Prosopis glandulosa ), that produce flowers without nectar, but which mimic the scent, morphology, and spectral color reflectance of flowers that do provide nectar to pollinators (Lopez‐portillo et al ; Thakar et al ; Anderson et al ; Anand et al ; Burkle et al ; Brandenburg and Bshary ). (5) Commelina mascarenica flowers that have three false stamens that carry no pollen but are used to lure in pollinators.…”