Specialized Pollination by Large Spider‐Hunting Wasps and Self‐Incompatibility in the African Milkweed Pachycarpus asperifolius

Specialized pollination systems in flowers with exposed nectar are difficult to explain because there are usually no morphological traits, such as long spurs, that could function to exclude particular flower visitors. Observations of the milkweed Pachycarpus asperifolius in KwaZulu-Natal, South Africa, showed that its flowers are visited mainly by large spider-hunting wasps belonging to the genus Hemipepsis (Hymenoptera: Pompilidae), despite producing copious amounts of nectar in an exposed position. Cage experiments showed that these wasps are effective in removing and depositing P. asperifolius pollinaria. Pollinaria become attached to the palps (and, to a lesser extent, the legs) of wasps. Palps are frequently broken, either when they become jammed in the guide rails or when pollinia are inserted. To try to understand why P. asperifolius flowers are visited almost exclusively by wasps, we presented droplets of nectar and control sugar solutions of the same concentration (ca. 70%) to honeybees (Apis mellifera scutellata). Honeybees readily consumed the sugar solutions but rejected P. asperifolius nectar, suggesting that secondary compounds in P. asperifolius nectar may deter insects other than pompilid wasps. Experimental hand pollinations conducted in the field showed that P. asperifolius is genetically self-incompatible and thus completely reliant on pollinators for seed production. We conclude that P. asperifolius is specialized for pollination by large pompilid wasps and that its nectar functions as the primary filter of flower visitors.