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C. J. Potgieter et al.: Pollination in Plectranthus 101
iensis in Madagascar (Nilsson and Rabakondrianina
1988).
Stirton (1977) listed the following South African
insect visitors to cultivated plants of /3. neochilus
Schltr.: Hymenoptera - five species of Megaehile, three
species of Xylocopa, one species of Anthophora, Apis
mellifera (Apidae); Diptera - unidentified bombyliids,
Asarkina (Syrphidae); Lepidoptera -Macroglossum
trochilus (Sphingidae). Two species of Xylocopa
and Macroglossum trochilus also visited Plectranthus
barbatus Andr.. Only the bees were seen to work the
pollination mechanism effectively every time.
Huck (1992) reviewed pollination in the Lamia-
ceae and added Bombus diversus (Apidae, Hymenop-
tera) and Gurelca himachala (Sphingidae,
Lepidoptera) as pollinators of Plectranthus inflexus
Vahl ex Benth..
In summary the documented insect visitors to
Plectranthus belong to the families Anthophoridae,
Apidae and Megachilidae (Hymenoptera); Syrphidae,
Bombyliidae and Conopidae (Diptera) and Sphingidae
and other Lepidoptera.
This study considers variation in corolla-tube
length within seven species of Plectranthus and
correlates these data to various insect pollinators that
are recorded for the first time.
Materials and methods
Field observations and collections were made during the
flowering seasons (December-May) of 1995, 1996 and
1997. Voucher specimens of insects are lodged at the Natal
Museum Pietermaritzburg and names are listed in Appendix
1. Plant vouchers are lodged at the University of Natal
herbarium (NU) and are listed in Appendix 2.
Study sites. Field work was conducted at Umtamvuna
and Oribi Gorge Nature Reserves in southern Natal, South
Africa (Fig. 1). These sandstone gorges are separated by
about 35 kin and four of the endemic species occur in the
two reserves. Umtamvuna is closest to the coast with study
sites ranging from 3 to 8 kin inland, while Oribi Gorge is
situated about 15 km inland. Additional observations were
done at World's View and Ferncliffe Nature Reserve in
Pietermaritzburg, 75 kin inland and separated by 125 kin and
160kin from Oribi Gorge and Umtamvuna respectively.
Limited observations were made on P reflexus at Port St.
Johns (Fig. lb).
Species studied. The following species were studied:
P ambiguus and P. hilliardiae at Umtamvuna, P oribiensis
and P. zuluensis at Oribi Gorge, and P ecklonii, P ciliatus
and P maclagascariensis at all three study sites. Plec-
tranthus oribiensis and P hilliardiae are endemic to
southern Natal. Plectranthus reflexus is endemic to forest
along the Bulolwe River at Port St. Johns.
Observations. Populations of flowering Plectranthus
species were observed during the daytime and notes were
made of the types of insect visitors, type of floral reward
utilised and insect behaviour on the flowers. Voucher insects
were netted and killed in separate ethyl acetate-containing
vials to prevent pollen contamination. Each specimen was
set on a pin with its proboscis extended forward.
Length measurements. Measurements of proboscis
length were done from the tip up to the point of attachment
of the proboscis to the face of the insect. Corolla-tube
lengths of the relevant Plectranthus species were measured
from the base (at the junction to the calyx) to the mouth of
the corolla (at the point where the upper and lower lips
diverge). Style and filament lengths were also recorded;
where the filaments are partially attached to the corolla the
measurement included the length of the corolla. Both
posterior (shorter) and anterior (longer) filaments were
measured. These values were averaged and compared to
proboscis length of insect visitors.
Visitation frequency. To give an indication of the
importance of various insect visitors, an estimate of
visitation frequency was made by calculating the proportion
of observed visits made by each insect species. This
estimation was only done for species with proboscis lengths
that fall within a range that may promote outcrossing, i.e.
nectar and pollen robbing species were excluded.
Pollen loads. Pollen loads of insects were examined
under a Hitachi $570 scanning electron microscope to
establish whether insects carried mixed pollen loads. Insects
were examined under a dessecting microscope to establish
where pollen grains were deposited on the insect body.
Small pieces of double-sided tape were used to pick pollen
off various parts of the insect body and these were placed on
a stub, coated with gold-palladium and examined under the
scanning electron microscope (SEM). The percentages of
Plectranthus and foreign pollen were estimated.
Results
Chapter 2/ 17
Distribution. Figures 1 and 2 show the distribution of
the studied species, with longer-tubed species in Fig. 1
and shorter-tubed species in Fig. 2.
Phenology. Flowering times are indicated in Table
1. December to April comprises the main flowering
season, with intermittent flowering during the rest of
the year for some species. Plectranthus reflexus
flowers from January to March. The flowering of
Plectranthus within the gorges is strongly seasonal,
with marked overlaps occurring between species.
Time of visit. Flowering populations are visited by
insects between 8.00 and 17.00 with continuous visits
throughout the day.
Insect behaviour during visits. All the studied
Plectranthus species are herkogamous and dichoga-
tutus. Pollen is presented upon elongate filaments, and
after a few days these curl downward and the style
elongates and becomes receptive. While autogamy is
avoided in this manner, geitonogamy can occur.
Foraging behaviour of both dipteran and hymenop-