Abstract
The foraging behaviour of pollinators can influence their efficiency in pollinating certain plant species. Improving our understanding of this behaviour can contribute to an improvement of management techniques to avoid pollination deficits. We investigated the relationship between the number of visits of bumble bees (Bombus impatiens) to tomato flowers (Lycopersicon esculentum) and two variables related to the quality of the resulting fruits (weight, number of seeds), as well as the relationship between foragers’ thoracic weights, physical characteristics of thoracic vibrations (main frequency, velocity amplitude), amount of pollen removed from flowers, and the quality-related variables. In addition, we studied the capability of foragers to assess the availability of pollen in flowers. Tomato weight and seed number did not increase with the number of bee visits, neither were they correlated with the foragers’ thorax weight. Thorax weight also did not correlate with the amount of pollen removed from the flowers nor with the physical characteristics of vibration. Vibration characteristics did not change in response to the amount of pollen available on tomato flowers. Instead, foragers adjusted the time spent visiting the flowers, spending fewer time on flowers from which some pollen had already been removed on previous visits. The quantity and the production-related variables of tomatoes are not dependent on the number of bee visits (usually one visit suffices for full pollination); bigger foragers are not more efficient in pollinating tomato flowers than smaller ones; and B. impatiens foragers are capable of evaluating the amount of pollen on a flower while foraging and during pollination.
Highlights
Given the importance of bumble bees in tomato pollination and considering the putative relationship between the bees' behaviour and their efficiency as pollinators, we investigated the following questions: (1) Do fruit set and production-related parameters depend on the number of bumble bee (Bombus impatiens) visits? (2) Are big foragers more efficient in pollinating tomato flowers than small individuals? (3) Are B. impatiens foragers capable of evaluating the amount of pollen available in a flower during pollination?
In a second set of experiments, we evaluated whether the amount of pollen removed from tomato flowers is related to the physical aspects of the thoracic vibrations generated by the bumble bee foragers during flower visits
Are fruit set and weight and seed number of tomatoes related to the number of bee visits?
Summary
Bumble bees (Bombus spp.) are highly efficient pollinators of tomato (Lycopersicon esculentum Miller) flowers and, for commercial purposes, yield far better results than honeybees, manual vibration, or self-pollination (Banda & Paxton 1991; Kevan et al 1991; Dogterom et al 1998; Morandin et al 2001a, 2001b; Palma et al 2008; Choi et al 2009; Torres-Ruiz & Jones 2012). Among them the bumble bees, generate thoracic vibrations when visiting tomato flowers therewith facilitating the release pollen from the anthers ("buzz-pollination"; Buchmann & Hurley 1978, Buchmann 1983). To a certain extent, on the amount of pollen transferred to the stigma (Morandin et al 2001a). It has been suggested that the quality of tomatoes (weight, size, seed number) does not increase any further at flower visitation rates above one or two bumble bee visits (Bombus impatiens; Morandin et al 2001a). Pollen is transferred inadequately to the stigma, seed production is impaired, therewith resulting in sub-optimal crop yields ("pollination deficit"; Vaissière et al 2011)
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