Specialized use of pollen vectors by Caesalpinia gilliesii, a legume species with brush-type flowers

The pollination of Caesalpinia gilliesii (Fabaceae) , a legume species with long, brush-type flowers, was studied in wild populations in Argentina. A total of 341 hawkmoths (Sphingidae), belonging to 10 species, with tongues ranging from 1.5 to 13.5 cm in length, were captured by means of light traps. Hawkmoths could be classified in four significantly different tongue-length classes or guilds: short (15.35 ± 1.11 mm), medium (30.23 ± 2.24 mm), long (55.85 ± 4.98 mm) and very long (95.61 ± 13.63 mm). A total of 133 of the hawkmoths, belonging to seven species, carried pollen of Cae. gilliesii , mainly on their venter. Successful contact with anthers while drinking nectar depended upon hawkmoth tongue length. The relative frequency of pollen-carriage and the mean tongue length of each species were positively and significantly correlated. Considering a null hypothesis of nonspecialization, in which hawkmoths carrying pollen are distributed at random among individuals of the four guilds, long-tongued hawkmoths carried Cae. gilliesii pollen more frequently than would be expected by chance. In contrast, short- and medium-tongued guilds carried pollen less frequently than is predicted by random expectation. Individuals with short tongues accessed nectar from below without touching the anthers, whereas very long-tongued hawkmoths, with tongues longer than the length range of the flower reproductive organs, can probably reach nectar without touching the fertile organs. Results show that a pterotribic pattern of pollen deposition on the body of long-tongued hawkmoths is operating in this species. This suggests that brush-type blossoms are not necessarily unspecialized, despite popular belief.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 579–592.     

The influence of nectar production on hawkmoth behavior,self pollination,and seed production in Mirabilis multiflora (Nyctaginaceae)

Nectar is the most common floral reward that plants produce to attract pollinators. To determine the effect of nectar production on hawkmoth behavior, pollen movement, and reproductive success in Mirabilis multiflora, I manipulated nectar volumes and observed the subsequent foraging behavior of the hawkmoth Hyles lineata and the resulting pollen movement patterns. Individual hawkmoths visited significantly more flowers on plants with more nectar. The increase in flower visits significantly increased pollen deposition on stigmas and pollen removal from anthers when nectar volume was raised to twice the highest level found in nature. As hawkmoths visited flowers consecutively on a plant, the proportion of self pollen deposited on stigmas increased significantly and rapidly. Based on simulated hawkmoth visits, seed set was significantly reduced for flowers later in a visit sequence. A simple model combining these results predicts that the form of selection on nectar production varies depending on pollinator abundance. Using a multiple regression analysis a nearly significant (P < 0.08) effect of stabilizing selection was detected during a single season as predicted by the model for the prevailing hawkmoth abundance. Although increased nectar production may indirectly affect plant fitness by reducing resources available for other plant functions, the direct effect of high nectar production on pollinator behavior and self pollination may generally limit floral nectar production.     

Floral scents of hawkmoth-pollinated flowers in Japan

Similarity among the floral scents of hawkmoth-pollinated plants was investigated with headspace samplings. Six of seven plant species belonging to different families were found to be rich in isoprenoids, among which linalool was the most common compound. Linalool showed rhythmicity with a nocturnal increase inLonicera japonica. These findings suggest that linalool is a common attractant for nocturnal hawkmoths. However, the composition of other isoprenoids, benzenoids and fatty acid derivatives varied markedly among the plant species examined. There was a significant correlation between species composition of flower-visiting hawkmoths and specific floral scents, suggesting that attractiveness to each hawkmoth species is dependent upon floral scent.     

Interactions between hawkmoths and flowering plants in East Africa: polyphagy and evolutionary specialization in an ecological context

Hawkmoths (Lepidoptera, Sphingidae) are considered important pollinators in tropical regions, but the frequency and degree of reciprocal specialization of interactions between hawkmoths and flowers remain poorly understood. Detailed observations at two sites in Kenya over a two‐year period indicate that adult hawkmoths are routinely polyphagous and opportunistic, regardless of their proboscis length. About 700 individuals of 13 hawkmoth species were observed visiting a wide range of plant species at the study sites, including 25 taxa that appear to be specifically adapted for pollination by hawkmoths. We estimate that 277 plant species in Kenya (c. 4.61% of the total angiosperm flora) are adapted for pollination by hawkmoths. Floral tube lengths of these plants have a bimodal distribution, reflecting the existence of two hawkmoth guilds differing in tongue length. Hawkmoths exhibited strongly crepuscular foraging patterns with activity confined to a 20‐min period at dusk and, in some cases, a similar period just before dawn. Corolla tube length appears to act as a mechanical filter as the longest‐tubed plants were visited by the fewest hawkmoth species and these were exclusively from the long‐tongued guild. Tube length showed a strong positive relationship with nectar volume, even after phylogenetic correction, which implies that plants with long corolla tubes are under selection to offer relatively large amounts of nectar to entice visits by polyphagous long‐tongued hawkmoths. Our study shows that diffusely co‐evolved pollination systems involving long‐tongued hawkmoths are clearly asymmetrical, with plants exhibiting a high degree of floral specialization, while hawkmoths exhibit polyphagous behaviour. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 199–213.     

Variation in pollinator preference between two Ipomopsis contact sites that differ in hybridization rate

Pollinator-mediated reproductive isolation is often a principal factor in determining the rate of hybridization between plant species. Pollinator preference and constancy can reduce interspecific pollen transfer between otherwise interfertile, coflowering species. The importance of this ethological isolation can be assessed by comparing the strength of preference and constancy of pollinators in contact sites that differ in the frequency of hybrid individuals. We observed visitation by hummingbirds and hawkmoths in natural single-species patches and artificial mixed-species arrays in two Ipomopsis aggregata/I. tenuituba contact sites-one with few hybrids, and one in which hybrids are abundant. Pollinator preference and constancy were stronger at the low-frequency hybrid site, especially for hawkmoths (Hyles lineata). Hawkmoths at the low-frequency hybrid site showed significant preference and constancy for I. tenuituba, while at the high-frequency site hawkmoths visited both species equally. One hypothesis that might explain these differences in hawkmoth foraging is that warmer nights at the low-frequency hybrid site allow for nocturnal foraging where the light-colored corollas of I. tenuituba have a visibility advantage. These differences in hawkmoth behavior might in turn affect hummingbirds differently at the two sites, through changes in nectar resources, leading to greater pollinator-mediated isolation at the low-frequency hybrid site. Our results suggest that differences in pollinator behaviors between sites can have both direct and indirect effects on hybridization rates between plant species.     

Generalized pollination system: Are floral traits adapted to different pollinators?

Flowers that are pollinated both during the day and at night could exhibit two different groups of pollinators and produce two different sets of attractants and rewards. We explored the patterns of emission of flower scents and production of nectar in the cactus Echinopsis chiloensis ssp. chiloensis, in relation to the patterns of activity of its diurnal and nocturnal pollinators. We measured frequency of flower visitors, analyzed floral scents, measured nectar production and sugar concentration, and performed pollination exclusion experiments. Bees were the main visitors at daytime and hawkmoths at nighttime. Diurnal scents were dominated by several compounds that can attract a wide range of pollinators, whereas nocturnal scents were less diverse and were dominated by (E)-nerolidol, a compound eliciting antennal responses in hawkmoths. Nectar volume and sugar concentration at night were similar to those recorded in hawkmoth-pollinated flowers. Daytime nectar volume was higher than those commonly found in bee-pollinated flowers, but similar to those found in flowers pollinated by several pollinators. Daytime sugar concentration was similar to those recorded in bee-pollinated flowers. Flowers of E. chiloensis ssp. chiloensis seem morphologically adapted to hawkmoth pollination, but diurnal and nocturnal pollinators contribute to similar extents to reproductive success. Additionally, diurnal and nocturnal pollinators showed a synergic effect on the product of fruit set and seed set. The results are discussed in terms of the linkage between floral traits and perception abilities and requirements of pollinators.     

Pollinator-mediated selection on flower-tube length in a hawkmoth-pollinated Gladiolus (Iridaceae)

Darwin's mechanistic model whereby selection favours plants with flower tubes that exceed the tongue length of the primary pollinator, was tested using unmanipulated plants of the hawkmoth-pollinated South African iris, Gladiolus longicollis. The study population was characterized by exceptionally large phenotypic variation in flower-tube length (range 56-129 mm). Directional selection on tube length was revealed by a significant positive relationship between this trait and both fruit and seed set. Selection was attributed to the effect of tube length on pollen receipt, as supplemental hand pollinations showed fruit and seed set in the population to be pollen limited. Indirect selection on tube length may also occur through the correlation of this trait with inflorescence height, although direct selection on the latter trait was significant only for seed set. The main pollinators at the study site were individuals of the large hawkmoth Agrius convolvuli that had tongue lengths of 85-135 mm. Other hawkmoths had tongues that were much too short to reach the nectar in G. longicollis flowers and seldom carried pollen of G. longicollis. Flowers with tubes shorter than the tongues of A. convolvuli are apparently not effectively pollinated because stigmas do not contact the moth's head effectively. This study demonstrates that selection may occur among plants with natural phenotypic variation in flower-tube length, and supports Darwin's model of pollinator-mediated selection.     

Context-dependent reproductive isolation mediated by floral scent and color

Reproductive isolation due to pollinator behavior is considered a key mode of speciation in flowering plants. Although floral scent is thought to mediate pollinator behavior, little is known about its effects on pollinator attraction and floral visitation in the wild. We used field experiments with wild hawkmoths and laboratory experiments with naïve hawkmoths to investigate attraction to and probing of flowers in response to indole, a volatile emitted by Ipomopsis tenuituba but not its close relative I. aggregata, both alone and in combination with floral color differences. We demonstrated that indole attracts wild hawkmoths to flowers, but has little effect on the rate at which those attracted moths probe flowers. In contrast, white flower color did not influence hawkmoth attraction in the field, but caused more attracted moths to probe flowers. Thus, the moths require both scent and high visual contrast, in that order, to feed at flowers at dusk. Their preference for indole-scented flowers is innate, but species-specific preference is mitigated by previous experience and plant spatial patterning. This context-dependent behavior helps explain why these Ipomopsis species show geographical variation in the extent of hybridization and may potentially explain formation of hybrid bridges in other systems of hawkmoth-pollinated plants.     

Specialization for hawkmoth and long-proboscid fly pollination in Zaluzianskya section Nycterinia (Scrophulariaceae)

The evolution of pollination systems has been investigated in Zaluzianskya section Nycterinia (Scrophulariaceae), a group characterized by very long-tubed flowers (up to 60 mm) that open either only at night (19 species) or during the day (one species). Field observations of three species: Z. natalensis , Z. elongata and Z. pulvinata , revealed that their flowers are pollinated shortly after dusk by hawkmoths. Further observations showed that Z. microsiphon , the sole species to have flowers that open only during the day, is pollinated exclusively by long-proboscid flies belonging to the family Nemestrinidae. A phylogenetic analysis of Z. sect. Nycterinia based on morphological characters indicates that Z. microsiphon evolved within a clade of night-flowering species. The shift from hawkmoth to long-proboscid fly pollination in Zaluzianskya was probably facilitated because long corolla tubes, white petals and dilute nectar in hawkmoth-pollinated flowers are pre-adaptations for pollination by long-proboscid flies. However, the autapomorphic features of Z. microsiphon , such as zygomorphy, diurnal flower anthesis and lack of scent, can be regarded as adaptations for long-proboscid fly pollination. Breeding system experiments on Z. natalensis and Z. microsiphon revealed the existence of genetic self-incompatibility. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 138 , 17–27.     

Pollination in Lilium sargentiae (Liliaceae) and the first confirmation of long‐tongued hawkmoths as a pollinator niche in Asia

The geographical distribution of pollinators is an important factor determining the biogeographical pattern of floral evolution and diversification. In Africa, a guild of plant species has converged in a floral syndrome for pollination by long‐tongued hawkmoths (predominantly Agrius convolvuli). It is hypothesized that such floral convergence could track the geographical distribution of long‐tongued hawkmoths, so it may not be confined to Africa. We investigated the pollination biology of Lilium sargentiae E. H. Wilson, which is endemic to China and exhibits traits suggestive of long‐tongued hawkmoth pollination. Lilium sargentiae was visited by A. convolvuli as well as small beetles (Nitidulidae) but pollination was mostly or totally effected by the moth. It was consistent with other hawkmoth‐pollinated plants in terms of floral tube length, nectar traits, tepal reflectance, and scent composition. We present the first experimental evidence for the hypothesis proposed above and for hawkmoth pollination in the widespread and ornamentally and economically important genus Lilium L. in a natural habitat. Our findings imply that long‐tongued hawkmoths (especially the extremely widespread A. convolvuli) as a distinctive pollinator niche may have underlain the evolution of long‐tubed (>8 cm) flowers across different continents.     

Observations of hawkmoth pollination in the South African orchid Disa cooperi

Johnson, S. D. 1995. Observations of hawkmoth pollination in the South African orchid Disa cooperi . - Nord. J. Bot. 15: 121–125. Copenhagen. ISSN 0107–055X.
Disa cooperi is a robust grassland orchid with long-spud white flowers which are strongly scented in the evening. Observations at a site in Natal province, South Africa showed that hawkmoths are frequent visitors to the orchid at dusk. The hawkmoth Basiothia schenki was an effective pollinator of D. cooperi; this hawk-moth has a medium length proboscis (x = 4.3 cm) which can be fully inserted into the nectar-containing spur of the orchid. Pollinaria are attached ventrally to the basal portion of the proboscis where it joins the head. Another hawkmoth, Agrius convolvuli , commonly foraged on nectar from D. cooperi , but did not carry pollinaria, probably because its proboscis is too long (x = 10 cm) to allow contact between the thick basal portion of the proboscis and the orchid column. Lips in Disa are typically linear and do not produce scent, thus the autapomorphic spathulate and scent-producing lip of D. cooperi indicates that hawkmoth pollination is derived in this species, probably from a long-proboscid fly pollinated ancestor.     

Geographic variation in floral scent of Echinopsis ancistrophora (Cactaceae); evidence for constraints on hawkmoth attraction

Variation in floral phenotype (color, depth, nectar) suggests incipient specialization for bee or hawkmoth pollination across the geographic distribution of Echinopsis ancistrophora , with flower depth ranging from 4.5 to 24 cm. We used chemical and behavioral analyses to test whether fragrance has evolved in concert with morphology in these Andean cacti. Floral scent (145 total compounds) was collected using dynamic headspace methods and analyzed with gas chromatography–mass spectrometry, revealing subspecies-specific odors dominated by sesquiterpenes in E. ssp. ancistrophora and arachnacantha and fatty acid derivatives or aromatics in E. ssp. cardenasiana and pojoensis . Compounds indicative of sphingophily were not consistently found in moth-pollinated plants, and total scent emissions were significantly lower in populations with nocturnal anthesis. In wind tunnel assays, Manduca sexta moths were attracted to scent of ssp. ancistrophora from both bee and hawkmoth-pollinated populations, but not to scent of ssp . cardenasiana . However, hawkmoths were most attracted to the methyl benzoate-dominated scent of a distant relative, Echinopsis mirabilis . Thus, hawkmoth-pollinated descendants of the E. ancistrophora lineage may be phylogenetically constrained to emit weak, sesquiterpene-dominated fragrances that are not optimally attractive to hawkmoths, or floral scent may be under stronger selection by destructive flower visitors.     

Pollen Transport in the Dark: Hawkmoths Prefer Non Crop Plants to Crop Plants in an Agricultural Landscape

In many previous studies hawkmoths have been found to be involved in nocturnal pollination of many plants. But their role in crop pollination is still unexplored. This study tried to evaluate their potential to carry pollens of crop. Hawkmoths were collected by light traps and slides were made from the pollen attached to their body. The hawkmoths and the pollens were identified. A pollen transport network was created using the data. The species level indices of each plant species in the network were estimated. Only a small fraction of pollens were made by the crops among the total types of pollens carried by the hawkmoths. Rest of the pollens belonged to that of the non crops. The species level indices suggest the crop pollens to be relatively less important in the network as reflected in the lower values of species degree and species strength. The higher species level specialisation values of crop pollens suggested they are restricted to be carried by few hawkmoth species only. It is clearly revealed that hawkmoths carry mostly non crop pollens, not crop pollens.     

Innate Adjustment of Visitation Behavior to Rewarding and Reward‐Minimized Petunia axillaris (Solanacea) Plants by Hawkmoth Manduca sexta (Sphingidae)

Plant–pollinator interactions offer an excellent system to study the stability of mutualistic interactions. While nectar production requires resources and a reduction could in principle benefit plant fitness, only few angiosperms lack nectar, and thus cheat from a pollinator's perspective. Cheating behavior may be scarce because of pollinator foraging behaviors that select for nectariferous plants. Shorter inspection duration, interaction with fewer flowers, or even complete avoidance of plants with low/no nectar may reduce the fitness of cheating plants. The effectiveness of pollinator strategies may depend on how they are implemented. Innate strategies would invariably decrease the fitness of a cheating plant, while learned responses allow cheaters to exploit naïve pollinators. Here, we studied the foraging strategies of the hawkmoth Manduca sexta during interactions with nectariferous and reward‐minimized Petunia axillaris. We found that neither naïve nor experienced hawkmoths discriminated a priori between rewarding and nectar‐less plants. However, naïve hawkmoths displayed reduced probing time per flower and number of flowers visited on reward‐minimized plants during the first trial, without showing further improved discrimination with experience. In conclusion, the foraging decision rules of hawkmoths that may reduce the fitness of reward‐minimized plants appear to be innate, with little scope for additional learning.     

Pollen fates and the limits on male reproductive success in an orchid population

Male reproductive success in higher plants depends largely on the fates of pollen, but current methodologies have given only partial insights into this important aspect of plant mating. We present a detailed analysis of the proportions and absolute amounts of stained pollen involved in six key fates for the hawkmoth-pollinated African orchid Disa cooperi . Despite being packaged into pollinaria, high proportions (> 0.95) of the pollen removed from anthers were lost during transport by hawkmoths in both years. The proportion of pollen lost correlated positively with the number of pollinaria removed from a plant, so that pollen export did not vary with pollen removal. Most pollen was dispersed to neighbouring plants, with rare long-distance dispersal up to 65 m. Of the pollen that reached stigmas during both years, roughly equal amounts were involved in facilitated self-pollination vs. cross-pollination, but the relative proportions of these fates differed between years. Contrary to expectation, we found that self-pollination between flowers did not increase with the number of open flowers, even though moths probed significantly more flowers on larger plants. However, during both years the fraction of removed pollen exported to other plants declined significantly with increasing self-pollination on the source plant, indicating that once self-pollination occurred it reduced (discounted) subsequent pollen export opportunities. The packaging of pollen into pollinaria in orchids appears to increase overall transfer efficiency by at least an order of magnitude relative to plants with granular pollen. Nevertheless, considerable uncertainties remain in the male reproductive success of individual orchids. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 175–190.     

Hawkmoth pollination of aerangoid orchids in Kenya, with special reference to nectar sugar concentration gradients in the floral spurs

The African orchid flora has a high proportion of species with long-spurred white flowers. Few data exist to test the prediction that this floral syndrome pattern reflects an important role for hawkmoth pollination in the evolution and ecology of these orchids. The pollination biology of five aerangoid orchid species (Rangaeris amaniensis, Aerangis brachycarpa, A. confusa, A. thomsonii, and A. kotschyana) was investigated in Kenya. Four of these have long spurs (>10 cm) and were pollinated by Agrius convolvuli and Coelonia fulvinotata. Aerangis confusa, which has relatively short spurs (ca. 4 cm), was pollinated by the short-tongued hawkmoths Hippotion celerio and Daphnis nerii. Nectar frequently filled the entire spur in some of the study species, even at anthesis. Sugar concentration of the nectar of four species was found to vary from ca. 1% at the mouth of the spur to 20% at the tip. Gradients were expressed more strongly in species with long, straight spurs. Species with spirally twisted spurs showed both steep and shallow nectar gradients. These gradients, previously unknown in plants, may function as a "sugar trail," enticing long-tongued hawkmoths to probe deeply into spurs without incurring the cost of filling an entire spur with concentrated nectar. In addition, the most concentrated nectar is kept out of reach of short-tongued pollinators.     

Nectar replenishment and pollen receipt interact in their effects on seed production of Penstemon roseus

Resource supply and pollen delivery are often thought to equally limit seed production in animal-pollinated plants. At equilibrium, plants should show no response to experimental pollen supplementation because resources limit seed set above the current level of pollen attraction, while experimental reduction in pollen deposition below the equilibrium level would reduce seed set. The predicted equilibrium may be disrupted, however, if plants expend additional energy to replenish removed nectar. We investigated the combined effects of nectar removal and pollen delivery on female reproductive success of Penstemon roseus (Plantaginaceae), a hummingbird-pollinated plant that replenishes removed nectar. We first documented that the frequency of experimental nectar removal was correlated with total nectar secretion; and increased frequency of nectar removal resulted in increased female reproductive costs to the plant. Trade-offs between investing resources in nectar and investing resources in seeds were then investigated in two contrasting natural populations by removing nectar from flowers at increasing frequencies while simultaneously hand-pollinating flowers with increasing amounts of pollen. Seed set was lowest at low levels of pollen deposition, highest at medium-sized pollen loads, and intermediate when pollen loads were highest. At both sites, the frequency of nectar removal and pollen deposition had an interactive effect on seed production, in that intermediate levels of nectar removal result in the absolute highest seed set, but only at intermediate pollen loads. At high pollen loads, seed set was higher following little to no nectar removal, and at low pollen loads, all rates of nectar removal affected fecundity equally. Seed mass responded to nectar removal and pollination differently than did seed set. High levels of nectar removal and pollen delivery both lowered seed mass, with little interaction between main effects. Our findings are among the first to demonstrate that nectar replenishment costs and pollination intensity jointly affect seed production. This conflict between nectar replenishment costs and pollen-limiting factors results in trade-offs between pollinator attraction and seed provisioning. Thus, resource allocation towards nectar production should more often be considered in future studies of pollen limitation.     

Colour learning when foraging for nectar and pollen: bees learn two colours at once

Bees are model organisms for the study of learning and memory, yet nearly all such research to date has used a single reward, nectar. Many bees collect both nectar (carbohydrates) and pollen (protein) on a single foraging bout, sometimes from different plant species. We tested whether individual bumblebees could learn colour associations with nectar and pollen rewards simultaneously in a foraging scenario where one floral type offered only nectar and the other only pollen. We found that bees readily learned multiple reward–colour associations, and when presented with novel floral targets generalized to colours similar to those trained for each reward type. These results expand the ecological significance of work on bee learning and raise new questions regarding the cognitive ecology of pollination.     

Carbon source for reproduction in a spring ephemeral herb, Corydalis ambigua (Papaveraceae)

1.  The carbon source for reproduction in plants may differ between flowering and fruiting stages. To clarify how spring ephemerals use current photosynthetic products for reproduction, the allocation patterns of photosynthate at flowering and fruiting and the effects of resource limitation on reproductive performance in Corydalis ambigua were assessed.
2.  A ¹³C tracing experiment revealed that about 20% of the current photosynthetic carbon was used for reproduction at both flowering and fruiting. The proportion of ¹³C allocated to fruits was constant irrespective of the light level. In contrast, ¹³C translocation to tubers increased at fruiting, and this trend was accelerated when plants were shaded.
3.  Defoliation treatment significantly reduced nectar production and tuber mass, while seed production was not affected. Therefore, when carbon assimilation was limited, carbon was preferentially allocated to current reproduction (seeds) rather than to pollinator attraction (nectar) or storage (tuber).
4.  If seed production is partly supported by carbohydrate reserved in the old tissue of tubers, nectar and seed production may not compete strongly for carbon sources. In contrast to the ability of high seed production, the susceptibility of nectar production to current photosynthesis indicates that seed production of this species is basically limited by pollen capture.
5.  Therefore, temporal separation of resource pool for reproduction may mitigate the joint limitation of seed production between pollinator attraction and resource availability. Temporal variation of the sink–source balance of storage organ is crucial to understand the cost of reproduction in perennial plants.     

Arbuscular mycorrhizal fungi influence visitation rates of pollinating insects

Abstract.  1. Arbuscular mycorrhizal (AM) fungi can increase a number of plant traits to which pollinating insects are known to respond. These include total plant size, flower number, flower size, and amount of pollen produced.
2. It was hypothesised that these effects would lead to a different visitation rate of pollinating insects on mycorrhizal and non-mycorrhizal plants. To test this idea, three species of annual plants ( Centaurea cyanus , Tagetes erecta , and Tagetes patula ) were grown with and without AM fungi and the visits by pollinating insects were recorded over a 2-month period.
3. In all three species, mycorrhizal plants experienced a greater number of pollinator visits per flower per unit time. Diptera and Hymenoptera were the predominant insects and the latter order showed the strongest response.
4. Here, it is suggested that mycorrhizal fungi increase floral visitation rates by insects, but that the mechanism varies from one plant species to another. In C. cyanus , it appears to be due to flower number per plant, in T. patula it is individual inflorescence size, and in T. patula it is nectar standing crop per inflorescence.