Why doesn't nectar robbing decrease seed set in some bee-pollinated plants?

Abstract

Herbal impact on nectar robbing is species specific by pollinator species (Maloof and Inouye 2014). Unlike hummingbird pollinated plants, some bee-pollinated species does not suffer reproductive loss by nectar robbing (Maloof 2001, Mayer et al. 2014, Maloof and Inouye 2014). This observational study is investigating the probability that Bombus.sp forage either Mertensia ciliata or Corydalis caseana legitimately at least once before the flower is robbed. I also measured population-label robbing rate. I bagged the flower two days before, and on the days of experiment I waited in front of the uncovered flowers in order to voice- record Bumble bees’ visit type (legitimate vs. robbing). Furthermore, I conducted transects on the flower patch, and every time I saw the bee I recorded their visit type. Bombus sp. (B.flavifrons, B. mixtus. B. bifarius, B.appositus) was very likely visit legitimately first before it robbed (94.12 %, n=32), than robbing first (5.88%, n=2). Populational robbing rate was 8.98% (n=167), 35.29% (n=34) for C.caseana and 6.77%(n=133) for M. ciliata. Though this is observational study, I speculate that Bombus sp. is able to recognize flowers with low nectar volume. Unlike hummingbird that avoid low nectar flowers(Irwin 2000), I am guessing bumble bee may rob the flower to extract last bit of nectar. Therefore, nectar robbing may be more of a “Plan B” then “Contract Breaking” to both Bombus sp. and studied plant species, for flowers may already receive pollination, and Bombus sp. could extract last bit of energy.