Dissertation Abstract:
Although bees are herbivorous insects, they are seldom studied
as such. My dissertation research characterizes the interactions between
a guild of solitary cactus-specialist bees and the cacti they visit, examining
these bees both as herbivores and as pollinators.
I first examined the reproductive biology of two species of Ferocactus
as background for the pollination study. Both F. cylindraceus
and F. wislizeni were obligate outcrossers, and neither species
was pollen-limited. Fruit set was high in both species, and reproductive
output was limited by architectural constraints.
Whereas most other herbivorous insects are antagonists of the plants
they feed on, bees are usually regarded as mutualists (pollinators) of
their host plants. However, not all floral visitors are pollinators, and
pollen-foraging specialists have been postulated by some to be particularly
antagonistic to the plants they visit. To test whether cactus-specialist
bees are actually mutualists of the cacti they visit, I examined both the
quantity and quality components of pollinator effectiveness of bees visiting
and F. wislizeni . Despite the generalized morphology of the flowers
of these plants, there were very few floral visitors other than three
species of cactus-specialist bees. Flowers of both species were pollinated
almost exclusively by cactus-specialist bees, primarily Diadasia rinconis.
I also tested the pollen preferences of four species of cactus bees.
Bees were offered flowers from their normal host cactus in which the stamens
had been removed and replaced with novel pollens. Novel cactus pollens were
accepted to some degree by all four species, but Diadasia rinconis
and D. opuntiae accepted more novel non-cactus pollens than did
either Lithurge apicalis or Idiomelissodes duplocincta. Both species of
Diadasia showed significant acceptance of Sphaeralcea (Malvaceae)
pollen, thought to be the ancestral host plant for the genus Diadasia.
Caged, naive D. rinconis bees did not initiate nesting when presented
with Sphaeralcea flowers alone; nesting began immediately when cactus flowers
were offered. Once nesting was underway, most D. rinconis bees switched
to Sphaeralcea if cactus flowers were removed. In both D. rinconis and
D. opuntiae, there were striking differences in pollen preferences among
individuals.
Summary of Research Experience and Interests
My research interests lie at the interface between herbivorous
insects and their host plants. I am currently studying the interactions between
pollen-specialist bees and the flowering plants they visit. This work combines
traditional pollination biology with the relatively unusual perspective of viewing
bees as herbivorous insects. In particular, I am studying the pollinator effectiveness,
foraging behavior, and host preferences of several taxa of cactus-specialist
bees. My research uses field-based ecological experiments to study the "how"
of pollen specialization (host recognition, degree of specialization), and to
determine whether these bees evince behavioral flexibility that would allow
them to switch to other plants if cacti were unavailable. My work also seeks
to locate these interactions along the mutualism-antagonism continuum, by characterizing
the costs and benefits of pollen-specialist bees to the plants they visit.
1. Pollen specificity in cactus-specialist bees
In the deserts of the southwest, the flowers of many cacti are
visited by a guild of relatively unrelated bees that all specialize on cactus
pollen. To test the host-specificity of these cactus bees, I placed novel pollen
into cactus flowers, and observed whether these bees would accept these novel
pollens. I used pollens from both native and non-native plants, including plants
that are used by close relatives of cactus bees. I addressed the following questions:
1. Do all cactus bees show the same strictness in specialization?
2. Is there behavioral variation within populations of each species of bee that
would allow
host shifts onto non-cactus hosts?
3. Do cactus bees recognize their host plant by recognizing the flowers (in
which case they
might collect any pollen found in a cactus flower), or do they recognize the
pollen itself?
(these two are not mutually exclusive).
I found that most non-cactus pollens were rejected by cactus
bees, suggesting that host acceptance takes place at the level of the pollen
itself, not the flower. Rejection often took place after landing, and hence
may involve chemosensory and/or tactile cues, in addition to the visual and
olfactory cues the bees can use during flight. In particular, I tested the pollen
specificity and preference of naive and experienced Diadasia rinconis
(Hymenoptera: Apidae), free-flying and in captivity, in choice and no-choice
experiments. I found that naive bees that had never seen cactus flowers, and
that were provided with other flowers, did not initiate nesting (and therefore
pollen-foraging) until cactus flowers were introduced. However, once they were
nesting with cactus pollen, they did switch to globe-mallow for pollen if cactus
flowers were denied them. Experienced individuals of both D. rinconis
and D. opuntiae accepted a few non-cactus pollens, including globe-mallow
pollen. Individuals of Lithurge apicalis (Hymenoptera: Megachilidae)
that were tested rejected all non-cactus cacti. Free-flying experienced individuals
of Idiomelissodes duplocincta (Hymenoptera: Apidae) accepted a few non-cactus
pollen, showing more behavioral flexibility than L. apicalis, but not
as much as D. rinconis and D. opuntiae .
My results suggest that cactus-specialist bees in the genus Diadasia
may have the behavioral flexibility to switch to other plants if cacti are not
available, but that not all cactus bees may be so flexible. They also suggest
that the history of host-use in the clades to which cactus bees belong may play
a role: globe-mallow is thought to be the ancestral host plant for the genus
Diadasia , and globe-mallow pollen was accepted by Diadasia cactus bees
but not by other cactus bees.
2. Pollination biology of barrel cacti
Because host-plant specialization is widespread in both bees
and other herbivorous insects, it might suggest that specialization per se is
adaptive for herbivores. However, selective forces on bees may be quite different
than those that affect other herbivores. Bees are thought to have a beneficial
effect on the plants they visit through their dispersal of pollen, so plants
should not need to defend themselves against bees, the way they do against other
herbivorous insects. On the other hand, many flower visitors
are neutral or even harmful in their effect on the plant (e.g., removing pollen
or nectar without pollinating the flower). Using the technique of "single-pollinator
visits," I investigated the effect of cactus-specialist bees, and other flower
visitors, on two sister species of barrel cacti, Ferocactus cylindraceus
and F. wislizeni, over a 3-year period. Despite the fact that many insect-pollinated
plants are visited by a wide variety of visitors, both specialist and generalist,
I found that these two plants are visited primarily by three species of cactus-specialist
bees, and that these specialist bees are also the best pollinators. Visits by
specialists accounted for 100% of the seed production in F. cylindraceus
, and 90% in F. wislizeni. I also found that visits by female bees led
to greater seed production than visits by male bees, and that visits for nectar
alone resulted in the greatest seed production. My results suggest that the
relationship between cactus bees and cacti is primarily mutualistic (in contrast
to other herbivorous insects).
3. Plant reproduction, growth, and demography
As part of my investigation into the pollination biology of barrel
cacti, I conducted hand-pollinations to determine the breeding system of the
plants. I also measured plant size and reproductive output over a 5-year period,
to elucidate the role of plant size in reproduction, to determine whether there
is a tradeoff between growth and reproduction, and to identify those factors
that impact reproductive output. I found that, despite the fact that they are
sister-species, they differ significantly in their flowering phenology, seeds
per fruit and seed mass. In both species, as in many plants, plant size had
a positive effect on reproductive effort and on overall fecundity. Also in both
species, a minority of individuals had the ability to self; most were obligate
outcrossers. Even in selfers, insect visitation was required to set fruit. They
were interfertile and may be hybridizing in the wild. Fruit set (excluding flowers
and fruits damaged by herbivores) was extremely high in both species (90-95%).
Overall fecundity was also high, around 14,000 seeds/plant/year for F.cylindraceus
, and 22,000 for F.wislizeni . These data allowed me to place the results
from my single-pollinator visits into the overall context of the normal reproduction
of these plants.
I am also a member of a team conducting a long-term study of
the growth, reproduction and demography of a disjunct population of Echinocactus
horizonthalonius, which has been ongoing since 1995. We have found
that these cacti are extremely slow-growing, and despite regular reproduction,
are experiencing very low recruitment.
4. Solitary bee and wasp nesting and foraging behavior
The reproductive success of solitary bees and wasps depends in
part on their ability to locate suitable nesting sites. During my undergraduate
work at U. C. Berkeley, I conducted a study of the nest-substrate preferences
of twig-nesting bees and wasps. These insects burrow into dead, pithy stems,
and dig nesting cells and galleries for their offspring. I offered them twigs
from a range of plant species, and in different sizes, to determine what their
nesting requirement were. I found that Pemphredon wasps preferred large,
woody stems, possibly because they dig complex, branching burrows. Ceratina
acantha bees, which dig a simple linear tunnel, preferred woody twigs but
had no size preference.
I have also studied the phenology and activity of several nesting
aggregations of two cactus-specialist bees, Diadasia rinconis and D.
opuntiae , in and around Tucson in 1998-2000. My data provide supporting
evidence for the partial bivoltinism of D. rinconis that has been postulated
by previous workers.
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Last Updated: 30 October 2001
© copyright 2001 Margrit E. McIntosh