The role of insect water balance in pollination ecology: Xylocopa and Calotropis

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Abstract
Two carpenter bees (Xylocopa spp.) in southern Israel both use the asclepiad Calotropis procera as a primary nectar source. This plant genus is coevolved with carpenter bees, and aspects of the insect-flower interaction in Israel suggest that the smaller bee, X. sulcatipes, is the natural co-adapted pollinator, a view borne out by the geographical distributions of the species concerned. There are significant mismatches between the plant and the larger X. pubescens, involving physical fit and behaviour. These mismatches are particularly evident when the physiologies of the bees and the plant are considered. The different sizes and colours of the two bees lead to different daily activity patterns, only X. sulcatipes being thermally suited to, and thus abundant at, times of maximum nectar production by Calotropis. Similarly the water requirements of X. sulcatipes are finely balanced with the water production in the floral nectar; this bee gains just enough water when foraging to restore its blood concentration and production in the floral nectar; for deposition in the nest. X. pubescens does not incur net water loss in flight and gains too much water from Calotropis flowers, necessitating copious urination and ‘tonguelashing’. Hence physiological information can be of use in deciphering insect-plant coevolutionary patterns, and the water component of nectar is confirmed as a potentially major determinant of foraging activities. The circumstances where this will be particularly true, and cases where it may not apply, are discussed.