Pollination and Pollinators (Insects)

Pollination in its most basic sense is the transfer of pollen from the male sex organ (anther) to the receptive portion of the female sex organ (stigma) in flowering plants (Fig. 1). If the transfer is successful, it leads to fertilization, production of seed, and reproduction of the plant. This process often involves some sort of external vector such as wind, water, or animals. Some flowering plants may reproduce without the aid of pollen vectors, using mechanisms such as vegetative reproduction, apomixis, or automatic selfing. But our concern in this article is with animal vectors of pollen. Many kinds of animals may perform the ecological service known as pollination, including birds, bats, and some nonflying mammals. However, the dominant group of pollinators is the insects, especially bees.
 Floral parts of an almond blossom. The petals are color signals, the male stamen (anther and filament) and the female pistil (stigma, style, and ovary) are the reproductive parts. Nectar from the nectary and pollen from the anthers are food rewards to pollinators.
FIGURE 1 Floral parts of an almond blossom. The petals are color signals, the male stamen (anther and filament) and the female pistil (stigma, style, and ovary) are the reproductive parts. Nectar from the nectary and pollen from the anthers are food rewards to pollinators.


ENTOMOPHILY

Many flowering plants are adapted to insects as pollinators and provide primary rewards that attract and keep the pollen vectors returning to flowers. Rewards include nectar, pollen, lipid secretions, food bodies, scents, resins, and material for nest building. In addition to primary rewards, most insect-pollinated flowers also produce a number of cues or signals that distinguish them from other species and promote the ease by which an insect can relocate a rewarding flower, thereby encouraging the insect to move pollen from flower to flower of the same species. These signals include odors, colors,shapes, textures, and tastes. These signals are often combined into patterns that have been recognized as syndromes related to the type of pollen vector. For example, a typical butterfly-pollinated flower that would be red, have little odor, possess a landing platform, have nectar hidden at the base of a deep tube that can be reached by the long coiled butterfly proboscis, have nectar that would be high in amino acids, and have flowers that would be open during the day. In contrast, a typical hawk moth-pollinated flower would be white, have a strong sweet odor, lack a landing platform, have long stamens with freely swinging anthers, and would bloom nocturnally. Nectar position and composition would be similar to those of a butterfly flower. All these characteristics are well suited to the hovering flight and extremely long proboscis of a hawk moth (Fig. 2).
Pollinator syndromes are perhaps most readily distinguished in the tropics. However, not all flowers are easily classified in a pollinator syndrome. Many flowers are visited by guilds of visitors that include diverse taxa of insects. For these reasons some pollination biologists see little value in the use of pollination syndromes even for teaching. However, even when using a pollination syndrome approach, it is important to distinguish which taxa among the visitor guilds are actually effective pollinators of a flowering plant.
Many insect taxa visit flowers and thus are potential pollinators; however, only a few taxa are of prime importance. Minor groups include those Orthoptera that feed on pollen and some Heteroptera that visit flowers for nectar or those that use flowers as sites that attract prey items (e.g., Phymatidae, Reduviidae). Thrips (Thysanoptera) are common on flowers and often feed on pollen and other flower tissues. They may do more damage than good, but their positive contribution as pollen vectors is understudied. The major groups of insects that pollinate plants belong to the four largest orders of insects: Coleoptera (beetles), Diptera (flies), Lepidoptera (butterflies and moths), and Hymenoptera (bees, ants, and wasps).

Coleoptera

Beetles are often considered “mess and soil” pollinators in that while rumaging around in flowers feeding on pollen and other flower parts they pick up pollen on their bodies that is transferred to other flowers on subsequent visits. This type includes many that destroy some flowers by feeding on them (e.g., Scarabaeidae, Meloidae), but in the process others get pollinated. Some beetles are associated with pollination of some “primitive” flowers and have been considered responsible for pollination and diversification of early flowering plants (Angiospermae). Beetles of the family Nitidulidae feed on specialized food bodies on anther tips of the spice bush Calycanthus (Fig. 3).

Diptera

Diverse flies, including male mosquitoes, various midges, carrion flies, pollen-feeding Syrphidae, and long-tongued nectar feeders (e.g., Bombyliidae, Acroceridae, Nemastrinidae), pollinate flowering plants. Pollination by flies is greatly understudied and underrated. Although flowers of pipevines are considered classical “trap” flowers that imprison flies with inward directed hairs until the flower has released pollen on them, flowers of the California pipevine, Aristolochia californica, exhibit a different mechanism involving a reward to retain flies until pollen is released. Midges of the family Mycetophilidae are the primary visitors and pollinators of A. californica. When flowers of A. californica first open, the stigma is receptive and a dark ring of glandular trichomes encircles the outer wall of the flower at the level of the stigma. Flies descend through the hooded entrance to the
(A) Uncoiled proboscis (tongue) of a hawk moth, Costa Rica. (B) Tubular flower of Lindenia rivalis (Rubiaceae), pollinated at night by hawk moths, Costa Rica.
FIGURE 2 (A) Uncoiled proboscis (tongue) of a hawk moth, Costa Rica. (B) Tubular flower of Lindenia rivalis (Rubiaceae), pollinated at night by hawk moths, Costa Rica.
White-tipped "food bodies" on anthers of Calycanthus (Calycanthaceae) that attract beetles (Nitidulidae) that pollinate the flower.
FIGURE 3 White-tipped “food bodies” on anthers of Calycanthus (Calycanthaceae) that attract beetles (Nitidulidae) that pollinate the flower.
lower bowl and are attracted to the area of the stigma and trichomes by a light window. Flies feed on the trichome surface, contacting the stigma and depositing pollen from previous flower visits. The stigma closes, the trichomes wilt, and the anthers shed pollen into the bowl and onto the flies. Because there is no more food available, flies exit the flower and seek another, thereby pollinating the next flower.
Most adult butterflies and hawk moths are well-known flower visitors. Other day-flying moths (e.g., Schinia of the Noctuidae, Adela, Incurvariidae) and nocturnal moths pollinate while settling or perching on flowers during feeding or oviposition. A very specialized relationship exists between yucca moths (Tegeticula, Prodoxidae) and their host yucca flowers (Yucca. Liliaceae). Female Tegeticula enter Yucca flowers, collect pollen into a ball in specialized maxillary palps, move to the apex of the pistil where pollen is deposited on the stigma, and oviposit into the base of the pistil where seeds will develop from the pollination behavior. Larvae of the moth develop in the fruit pod,feeding on a portion of the seeds. Thus, both insect and plant benefit from this highly mutualistic association; the plant gets pollinated and produces seed, some of which goes to producing new moths.

Hymenoptera

Some sawflies and parasitoid wasps feed on pollen, especially on open shallow flowers. Classical mutualism occurs with the pollination association of fig wasps (Agaonidae) and their floral hosts, figs (Ficus, Moraceae). Similar to the situation with yucca moths and yucca, a portion of seeds in fig host flowers provides nourishment for development of the pollinating wasps. The story is often more complex and involves more than one generation of wasp per year and more than one host.
Other aculeate wasps, both social and solitary, augment insect prey diets with nectar. One genus of solitary wasps, Pseudomasaris (Masaridini, Vespidae), is completely dependent on flowers for nectar and pollen as food for their young. Ants may visit flowers but because their metathoracic glands secrete mold inhibitors they inhibit pollen germination and are unlikely pollen vectors. Bees (superfamily Apoidea) are the single most important taxon of pollinating insects with 20,00030,000 species worldwide. Bees are derived from wasps and highly adapted to gathering pollen as brood food and nectar for flight fuel.

EXPLOITATION RELATIONSHIPS

Not all pollination relationships are mutualistic, that is, beneficial for both partners. Some are based on deceit or robbery, in which only one partner benefits and the other may even be injured.

Thievery

Insect visitors to flowers may obtain the food items they seek without transferring pollen in the process. Some insects with mouthparts too short to reach nectar sequestered in the bottom of long tubes or spurs are able to penetrate the nectar-bearing structures with strong mandibles or maxillae. Such behavior is well documented for bumble bees, such as Bombus occidentalis in western North America or the related Bombus terrestris of Europe, when they encounter long-tubed flowers. This behavior is commonly exhibited by carpenter bees, especially in the tropics (Fig. 4). Insects that are mismatched in size with the flowers they visit may be effective gleaners of pollen from
Female carpenter bee (Xylocopa tabaniformis orpifex, Apidae) robbing nectar from base of a California fuchsia (Epilobium canum, Onagraceae) flower.
FIGURE 4 Female carpenter bee (Xylocopa tabaniformis orpifex, Apidae) robbing nectar from base of a California fuchsia (Epilobium canum, Onagraceae) flower.
the anthers, but rarely if ever contact the stigmas in the flowers they visit. These thieves often scavenge pollen from flowers adapted to other types of pollinators. For example, the evening primrose of the southwestern deserts of North America are typically adapted for pollination by night-flying hawk moths, but they are visited early in the morning after they have opened by solitary ground-nesting bees of the genus Andrena for pollen. In fact, these bees have become so completely adapted to collecting this source of pollen, and their seasonal synchrony and the morphology of their pollen transport structures are so specialized, that they visit no other plants for pollen. So although the bees specialize on these flowers, they are not effective pollinators because they are small enough that they rarely contact the stigmas with the pollen they are collecting.

Floral Deceit

Many floral deceit mechanisms in flowers take advantage ofbasic behaviors and instincts in insects, especially feeding, mating, and oviposition. Some flowers are green or brownish rather than being colorful and have putrid or rotten meat aromas rather than sweet odors. These are highly attractive to carrion flies seeking source foods rich in amino acids and suitable as sites for oviposition and rearing of their young. Other flowers mimic females of bees or wasps in such fine details of form, color, odors, and texture that male insects actually attempt to mate with these models; in the process they pick up and distribute pollen from one flower to another. This process is called pseudocopulation because it relies on mating attempts by male wasps or bees. The flowers bloom during the brief time when male insects are on the wing before females of the species emerge. During this period, flowers are the only potential sources of “mates” for the male insects. Once females of the hymenopteran species emerge, the floral mimics are forsaken by males for the real female.


CROP POLLINATION

Economic Value

The most recognized benefit of pollinators to humanity is their value as pollinators of many of the crop plants that we use for food and fiber. The principal pollinator managed for crop pollination has been and currently is the honey bee, Apis mellifera. Although calculating the value of pollination by honey bees is far from exact, the most recent estimate of the annual value of increased production of crops contributed by honey bee colonies rented for pollination in the United States is over $14.5 billion, on average, over 1996-1998. The “free” contribution from native pollinators, especially other bee species, is even less measurable, but attempts are under way to estimate their value to sustainable agriculture farms in central California.

Exemplar Crops

California is perhaps the leading state in rentals of honey bee colonies for crop pollination. In large measure this is the result of the continuing increase in the acreage of almond, which has increased from about 36,000 hectares in the mid-1960s to over 200,000 hectares in 2001. At the recommended five to eight honey bee colonies per hectare, more than twice as many commercial colonies as exist in the state are required to accommodate the demand. Thus, there is a mass movement of colonies into California each year from as far as the Dakotas and Texas and beyond to pollinate the crop.
Alfalfa is another crop traditionally pollinated by honey bees that was widespread in California in the mid-1960s. However, the honey bee is not an effective pollinator of alfalfa over much of the crop’s range. When two more efficient alternative pollinators (the introduced alfalfa leafcutting bee, Megachile rotundata, and the native alkali bee, Nomia melanderi) came under management for pollination of the alfalfa seed crop, much of the production shifted to the Pacific Northwest. Only the more southern areas of California continued to produce alfalfa seed solely with honey bees. Currently even some of these areas are augmented with alfalfa leafcutting bees.

Crop Pollinators Other Than Honey Bees

Although honey bees are readily available, easily transportable in large quantities, and generalist pollinators, they are not universal pollinators. There are some crop flowers, such as figs, that require insect pollination (specialized wasps) and cannot be pollinated by honey bees. There are other crop flowers that can be pollinated by honey bees but for which honey bees are not the most effective pollinators; these include crops such as alfalfa, squash, and greenhouse tomatoes. Another concern about excess reliance on a single pollinator for a wide variety of crops has been the widespread decimation of feral honey bees by the “vampire” mite, Varroa; increased cost of treating colonies to maintain healthy pollinating units; and reduction in numbers of beekeepers and colonies available for pollination. Warnings about this overdependence on honey bees and the general decline of pollinators because of factors such as loss of habitat and pesticides were issued in 1996 in a landmark publication by Buchmann and Nabhan.
One of the first insects introduced into North America specifically to pollinate a crop was the fig wasp, Blastophaga psenes, for production of edible Smyrna figs in southern California in 1899. Attempts to produce edible figs in California in the late 1800s failed until it was recognized that wasps from the wild ancestral caprifig, Ficus carica, were required. Edible figs contain predominantly pistilate flowers; pollen from male flowers of the caprifig is vectored by fig wasps. The growing of caprifigs containing introduced fig wasps, harvesting the fruits with a new generation of fig wasps, and then hanging them in baskets in trees of edible figs became a common practice in southern California, a process called “caprification.”
Other insects that have been used for commercial pollination on a small scale include various flies, especially for breeding hybrid seed crops in cages by seed companies. Most of these have been mus-coid flies, the pupae of which are readily available from insectaries. Results of large-scale open-field trials using carrion or other baits to attract flies have been equivocal for crop pollination.
Various species of non-Apis bees have been and are being studied for their management potential for pollination of crops. In the late 1950s, studies were begun to manage two bee species (M. rotundata and N. melanderi) that are more effective than honey bees as pollinators of alfalfa for seed production.
More recently mason bees in the genus Osmia have been studied for pollination of crops in North America and Europe. Osmia lignaria propinqua, referred to as the “blue orchard bee,” has been successfully managed to pollinate tree fruits in western North America. It is a cavity nester, like the alfalfa leafcutting bee, but uses mud partitions to create brood cells. Many of the management techniques were adapted from those used for Megachile, but modified to accommodate specific life history and behavioral attributes, including early spring activity and a single generation per year. Successes in managing other species of Osmia include O. cornifrons in Japan, O. cornuta in Spain, and O. rufa in Britain and France. In North America, Osmia are being studied for pollination of blueberries (O. ribifloris) and clovers (O. sanrafaelae).
In the late 1980s, major breakthroughs in year-round production of bumble bee colonies completely altered and expanded hothouse production of tomatoes. Tomato flowers require “buzz” pollination (i.e., vibration of flowers to release pollen from the apical pores of their specialized anthers) and bumble bees are much more effective at this than are honey bees or humans who hand pollinate with vibrating tools. This led to extremely large-scale movements of bumble bee colonies and queens, primarily B. terrestris, from central Europe, New Zealand, and Israel to many nations throughout the world. Some of this trafficking was unnecessary because closely related species were available for use at some locales. In Japan, environmental concerns have been expressed over the thousands of imported B. terres-tris colonies and the subsequent establishment of this species outside the greenhouse environment. Males of this species will mate with queens of local species and produce viable offspring. Similar environmental concerns are being raised in other countries. In Canada and the United States, importation of B. terrestris was not sanctioned, but local bumble bees have been successfully reared and used in hothouse tomato production. East of the 100th meridian (a line that runs from central North Dakota to central Texas), B. impatiens was the species of choice and west of this line, B. occidentalis. These bees were used in their respective areas of distribution until 1998, when a disease outbreak was reported in the western species. Since then, the eastern B. impatiens has been imported into all western states, again causing concern in some areas that establishment outside its normal range may produce environmental damage.
Although pollinators are generally considered beneficial insects, importations to new areas should be done with considerable care to avoid environmental risks, such as introduction of disease organisms, nectar thieving, decreased pollination of nontarget native plants, enhanced pollination of introduced weeds, and genetic contamination of and competition for food and/or nest sites with native pollinators. The local fauna should be studied and searched for candidate species that could be suitably managed before any exotic species are introduced. Rearing technology is already available for several species of cavity-nesting bees, for bumble bees, and for some soil-nesting species. These methods may be modified and applied to local species that show promise for solving difficult pollination problems.
If it is deemed necessary to introduce a new pollinator, these should be thoroughly screened for biotic enemies (e.g., parasites, disease organisms) before being introduced. They should be monitored after release in the new environment to determine their efficiency in pollinating the target crop and to detect any adverse environmental effects.

POLLINATOR DECLINE

Pollination biologists have worked for years studying the behavioral, ecological, and evolutionary intricacies of animal-plant pollination relationships. With the environmental movement of the early 1970s came awareness that mistreatment of the environment could also negatively affect these unique relationships. However, very few biologists sounded the alarm at that time that trouble was brewing for pollination relationships and pollinators, especially at the ecosystem or landscape level.
In the early to mid-1990s, the issue of pollination/pollinator problems was again brought to the attention of the biological community and the informed public, through the Island Press publication of The Forgotten Pollinators in 1996 by Stephen Buchmann and Gary Nabhan. Although the topic contains anecdotal accounts of pollinator problems, the message was clear—everyone concerned with the pollination of plants needed to pay serious attention to what appeared to be an emerging picture of global pollinator decline. A subsequently important publication by Allen-Wardell et al. in 1998 pointed out the potential threat of pollinator decline to the human food supply. Adding to the general concern for declining pollinators was the fact that European honey bees in the New World tropics, and several western and southwestern U.S. states, were being systematically replaced with Africanized honey bees, and that all honey bees were being attacked and significantly reduced in numbers by two species of parasitic mites. Continued careless use of pesticides and bacterial infections were also cited as factors in the decline of honey bees.
The above trends have led to many scientific conferences worldwide to address the issue of pollinator decline and its potential consequences to crop plants and to native wildland plants. The first important global conference was held in 1998 in Sao Paulo, Brazil, where numerous relevant issues on pollinator decline were discussed by more than 60 pollinator/pollination professionals representing several New and Old World countries. The Sao Paulo meetings and subsequent meetings in other parts of the world have put into motion new research directions in the field, which are listed below.
1. Documenting pollinator decline. One of the main recommendations emerging from Sao Paulo was to seek quantification of pollinator declines through careful case history studies. Long-term monitoring of pollinators and comparative assessments at specific study areas were recommended approaches for gathering the needed data on decline. This has proved to be quite a challenge because of natural fluctuations in pollinator populations and the lack of long-term baseline data. Where decline has been detected, loss of habitat is suggested as the main cause.
2. Causes of decline and restoration of pollinators. Habitat loss is a convenient general explanation for the cause of decline, but detailed information is needed on the precise factors causing decline. Further, specific causes of decline are not always obvious, and this becomes an important issue when considering projects to restore or establish pollinators and their required resources (often diverse) to an area.
3. More research on non-honey-bee pollinators. Honey bees can be likened to a monoculture in agriculture; overdependence on a single organism in agriculture can lead to disastrous results when natural mortality factors get out of balance. Such was the case with blight on a large portion of the U.S. corn crop several years ago. Because honey bees are now showing great vulnerability to two species of parasitic mites, more research on these parasites and on other bees (especially native solitary species) is being conducted.
4. Conservation of pollinators. Much has been written on conserving pollinators and especially bees. Unfortunately, there is little evidence to suggest that conservation recommendations, which would result in measurable increases in pollinator numbers, have been put into action. Many possibilities exist for increasing pollinators through manipulations of preferred food plants, nonflo-ral plant products (e.g., resins), and planned efforts to increase nesting sites and other requisites such as alternate food plants for moths, beetles, wasps, flies, bats, etc.
5. Increasing awareness of pollinator services. The conservation of pollinators and calling attention to vital services provided by pollinators become issues of information transfer that biologists must address. They are the only professionals who know the needs and fragilities of small organisms such as bees, flies, beetles, and nocturnal organisms such as moths and bats. Pollinator/pollination professionals will need to form closer working relationships with policy-makers, land stewards, and a wide variety of government and nongovernmental organizations to realize future successes in the management of pollinators.
There are at least two courses of action that pollinator biologists could pursue now to assist declining pollinator populations. First, they can collaborate with other biologists who are also concerned about decline of their specific organisms (e.g., birds, mammals) and habitat. Building a coalition of concerned biologists with integrated management plans for habitat protection for several threatened species could be effective if land stewards, associated with the habitat, were receptive and willing to participate in some way as stakeholders in the project. Second, biologists could also work in a variety of ways toward conserving areas known to naturally harbor healthy populations of pollinators, preferably several types. Biologists are aware, through years of field experience, which areas have good diversity and abundance of, for example, bees and moths.

TROPICAL POLLINATION

The literature is filled with fascinating case histories of individual tropical plants and their pollinators. More recently, researchers have been investigating pollination systems involving groups of prominent pollinators (e.g., bees, bats, moths, hummingbirds) and their plants in major tropical life zones. A few larger, long-term studies from the New and Old World tropics have also provided the first community pollination patterns for a high percentage of the representative plant life forms. These latter studies are particularly helpful in elucidating diversity and frequency of pollination systems for conservation work, as well as for interesting comparisons with temperate environments.
In Table I, pollination systems of two lowland forest sites and one midelevation cloud forest (1200-1800 m) in Costa Rica are compared and contrasted with one lowland forest site in Malaysia. From about 40 to 70% of the surveyed plant species in each of the four sites were pollinated by bees. Although less frequent than bees, birds were important pollinators in Costa Rica’s cloud forest and wet forest and in the Malaysian forest. Beetles were important in the Costa Rican wet forest and in Malaysia; moths were important in all three Neotropical forests. The four sites had numerous plant species that were visited (and pollinated) by a variety of general insects.
The importance of animals, especially insects, as pollen vectors of tropical plants was clearly demonstrated in a classic paper by Bawa in 1974, in which he reported that most tree species in a lowland dry forest of Costa Rica were obliged to outcross. Through controlled pollinations, he demonstrated that a high percentage of the tree species tested were self-incompatible (incapable of self-pollination) or dioecious (having separate male and female plants of a species). Until that time, many biologists believed that self-pollination was probably the rule in tropical forests. Flower-visiting animals, and especially insects, were not viewed as capable travelers between widely distributed tropical plants. Subsequent studies on interplant movements and foraging patterns of these animals have substantiated their capacities to move among flowering plant species at levels required to produce abundant fruit crops.
Some general, but limited, comparisons of tropical vs. temperate pollination systems are possible with the information available in the literature. First, the flora of many low- to midelevation temperate habitats is mostly pollinated by bees. Estimates vary widely from 70 to more than 90%, depending on locality. Flies and Lepidoptera may also be important in the pollination of temperate plants. Second, the diversity of pollination systems is comparatively lower in temperate environments because most lack, for example, the more specialized bird, bat,beetle, and fig wasp systems. Third, when lowland tropical and temperate forests are compared, the high diversity of tropical trees and their dependence on animal pollination become immediately apparent. Temperate forests have relatively low tree species diversity and most, such as conifers, oaks, willows, elms, and maples, are wind pollinated. In contrast, wind pollination is rare in tropical forests (Table I).

SPECIALIZED VS. GENERALIZED POLLINATION

As mentioned previously, there is ongoing controversy about specialized vs. generalized pollination systems and the associated concept of pollination syndromes that propose to characterize a plant as to a particular pollinator type. There can be little doubt that some plants have highly specialized systems that can be easily characterized by floral morphology and behavior alone, such as fig flowers and fig wasps, certain orchids and their specific bee relationships, and long, white-tubed fragrant flowers that open at night and are pollinated by long-tongued hawk moths (Fig. 2). There are, however, many instances in which flowers attract a wide variety of visitor types, making characterization of pollination syndromes difficult. For example, there are examples of “large-bee flowers” that are regularly visited by small bees, butterflies, and wasps; hawk moths that visit “bat flowers”; bees that visit both “hawk moth flowers” and “bat flowers” the morning after. Further, there are flower types that attract a wide diversity of insect visitors (Table I). Who are the pollinators and who are the visitors to these flower types? Do many or most plants have the option of being pollinated by a variety of potential vectors?
The answers to these questions will be forthcoming through carefully planned experimental studies that include evaluations of all visitors and their capacity to transport pollen on their body parts, as well as between plants. Floral behavior must be studied simultaneously, especially with regard to breeding system and period of stigmatic receptivity. These case studies should take much of the speculation and guesswork out of pollinators, visitors, and pollination ecology.

CHEMICAL ECOLOGY AND POLLINATION

Chemical communication between flowering plants and their pollinators and between conspecific pollinators in relation to floral resources is commonplace in many natural communities. Flowers release a variety of odors that attract pollinators and other visitors. Some of these are sweet and highly fragrant, as with many “moth flowers” and some “bee flowers.” Some are unpleasant to humans but highly attractive to flies and beetles. Other floral fragrances fall somewhere between fragrant and unpleasant such as musky odors associated with some flowers that attract a variety of visitor types.
In the New World tropics, chemicals are emitted from certain orchid species that attract only males of the Euglossini tribe of bees, better known as orchid bees. These orchids and “their” bees have evolved a unique relationship in which chemical substances are scratched from special regions of the orchid flowers by male bees. The compounds are collected in special leg glands that have unique apertures on the hind legs. According to theory, the scratched compounds are metabolized and transformed into chemical messages or pheromones that male bees use for attracting females for mating. Some male orchid bees form elaborate leks or mating rituals, which they use to lure females. During the process of scratching chemicals, orchids belonging to genera such as Catesetum, Cycnoches, and Stanhopea have cleverly evolved elaborate mechanisms for attaching or actually gluing a pollen packet (or pollinia) on the bee for transport to the next orchid, thereby effecting cross pollination. Each orchid species glues its pollinia on a characteristic location of the bee. Where orchid diversity is rich, it is common to see some bees with pollinia on their heads, other bees with pollinia on their thora-ces or abdomens, and still other bees with pollinia on more than one body location, indicating their visits to more than one orchid species.
There is also chemical communication among members of some social and solitary bee species. Some honey bee species scent mark flowers, and this informs others of the same species of a very recently visited flower. The mark also serves to alert a bee that it has just visited

TABLE I

Percentages of Pollination Systems Represented in Four Tropical Forests

Costa Rica Malaysia
Dry forest” Cloud forest1 Wet forest0 Dipterocarp forestd
Bee 69.5% 44.4% 38.4% 50.4%
Moth 7.3 6.2 8 1.1
Bat 2.4 3.1 3.6 1.5
Bird 2.2 9.6 14.9 7
Beetle 1.1 2.4 12.7 20.7
Wasp (large) 0.2 1.6 2.5 0
Butterfly 0.9 1.9 4.3 2.2
Fly 0 0.2 1.8 0
Fig wasp 0.4 0.5 ? 0
Mammal (arboreal) 0 0.1 0 0.4
General insect 15.3e 20.4e 11.2e 13.7
Wind 0.9 5.2 2.5 0
Miscellaneous 0 4.5 0 3

aZV = 465 plant species. Source: Frankie et al. (2003). bZV = 1100 plant species. Source: Frankie et al. (2003). cZV = 276 plant species. Source: Kress and Beach (1994). dZV = 270 plant species. Source: Momose et al. (1998).
eSeveral plant species listed in this category may prove to be primarily bee pollinated.
a particular flower that it marked, thereby conserving its energy and time. This behavior has also been observed frequently in large carpenter bees (Xylocopa); chemicals of the mandibular and mesosomal glands were found responsible for the scents. Stingless bees in the Neotropics regularly mark flowers and nearby vegetation in establishing a scent trail back to inform the nest where a good floral source can be located. Some bumble bees (Bombus) also scent mark flowers, making these flowers less attractive to other foraging bumble bees.

CONCLUDING REMARKS

Despite years of study and an enormous literature, we still have much to learn about pollination and pollinators. The field of study is particularly challenging today because of the many questions surrounding pollination/pollinator relationships in human-impacted environments. Many questions (and problems) will require new approaches and methods and will need to be better integrated with societal needs and structures.
Suggested studies for the future include: (a) more detailed work on chemical relationships between pollinators, visitors, and flowers;
(b) more attention paid to actively conserving, protecting, and restoring pollinators at local, regional, national, and international levels;
(c) improving methods for managing pollinators for production of human food crops; (d) development and transfer of information on pollination and pollinators to a wide variety of new audiences such as policy/decision-makers, government agencies, nongovernmental conservation organizations, and managers; and (e) developing monitoring methods to gauge effects of global warming on pollinators and the plants they pollinate.

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