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development of plants and animals (Stirton 1981). Kevan (1975) discussed pollination
with respect to the use of insecticides and concluded that the elucidation of critical
pollination phenomena, which have far-reaching ramifications on whole ecosystems, is
clearly needed. Johnson & Steiner (2000) mentioned the urgent need for pollination
studies to assess the viability of plant populations i.t.o. conservation.
Baker & Hurd (1968) stressed that detailed studies of plant families should be
complemented by intensive studies of a single species so that the mechanics of the
pollination systems may be discovered, their operations placed on a meaningful
quantitative basis and their modes of evolution inferred with some measure of certainty.
For example, restrictions may be placed on the geographical distribution of plants by
the lack of suitable pollinators and, conversely, the opportunity for anthophilous
animals to live in an area may be limited by a lack of suitable flowers for them to visit
(Baker & Hurd 1968). In an overview of anthecology in the Labiatae, Meeuse (1992)
refers to the “limited number of case histories” [of pollination syndromes] in the
Labiatae, pointing out that more case histories are required.
Previous studies on Plectranthus pollination
Scott Elliot (1891) reported the honeybee Apis mellifera L., 1758, a bombyliid fly and
two lepidopterans as visitors to P. ecklonii Benth. in South Africa. Marloth (1932) did
not record any insect visitors to two Plectranthus species from the Cape (South Africa),
but noted that self-pollination would be unlikely as the stigma matures after the last
anther has withered. Van der Pijl (1972) further mentioned butterflies and species of
Bombus Latreille, 1802 and Apis L., 1758 bees as visitors to Plectranthus species in
Nepal, Australia and Java. Gupta et al. (1984) studied the foraging activity of two Apis
species on Plectranthus rugosus Wall. [=Isodon rugosus (Wall. ex Benth.) Codd] in India,
and found that bumble bees and lepidopterans also visit the flowers. In Madagascar
Pachymelus limbatus Saussure, 1890 bees (Anthophoridae) and a Stylogaster Macquart,
1835 fly species (Conopidae) are visitors to Plectranthus vestitus Benth., with the former
species being the principal pollinator (Nilsson et al. 1985). Pachymelus limbatus was also
shown to exhibit male patrolling and territoriality associated with plants of Plectranthus aff.
vestitus Benth. and P. madagascariensis (Pers.) Benth. in Madagascar (Nilsson &
Rabakondrianina 1988).
Stirton (1977) listed the following South African insect visitors to cultivated plants of
Plectranthus neochilus Schltr.: Hymenoptera - five species of Megachile Latreille, 1802,
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