Using alfalfa grown for seed, we tested the hypothesis that seed set for a fixed number of pollinations is lower when the standing crop of open flowers is high than when it is low. This could occur if pollinators are more likely to move between flowers on the same plant, causing self-pollination, when flowers are abundant. The hypothesis has practical implications for agricultural production because self-pollinations produce fewer seeds per pod than do cross-pollinations in alfalfa. We simulated seed set in a model that included published movement patterns of the alfalfa leafcutting bee, Megachile rotundata (F.) (Hymenoptera: Megachilidae), and estimates of seed set when a series of flowers on the same plant are hand pollinated. Seed set from a fixed number of pollinations averaged only ≈6–7% higher when standing crop was low than when standing crop was high. This small increase in seed set would be difficult to detect experimentally because of high variability between plants. M. rotundata appears to move with a 56% probability of leaving a given raceme and a given plant; this foraging behavior results in few flowers visited per plant. A pollinator with a higher probability of leaving racemes and plants could set slightly more seed than M. rotundata, whereas a pollinator with a lower probability of leaving racemes and plants would be more affected by flower standing crop than is M. rotundata. The distribution of flowers visited per raceme by bees in alfalfa can be predicted by superimposing the movement probability on the distribution of open flowers per raceme. This conceptualization of bee movement on flowers differs from (but is not mutually exclusive of) resource-driven optimal foraging models, and is useful for predicting geitonogamy.
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