Pollination Deficits Research Articles

Open-pollination Provides Sufficient Levels of Cross-pollen in Spanish Monovarietal Olive Orchards

Most Spanish olive orchards are monovarietal as a result of the farmer's belief that the species does not require cross-pollination. Paradoxically, accumulated evidence from controlled experiments demonstrates that olive is partially self-incompatible and that cross-pollination increases yield and fruit quality in this wind-pollinated crop. With the aim of assessing cross-pollination deficit in large plots of the most widespread olive oil cultivar in Spain, fruit set was compared in two solid orchards of ‘Picual’ in response to self-, open-, and artificial cross-pollination. In both orchards, ‘Picual’ behaved as a self-incompatible cultivar with reduced fruit set under self-pollination (index of self-incompatibility = 0.21). However, cross-pollination rarely increased fruit set in comparison with open-pollination. Bagging experiments demonstrated that open-pollination provided enough cross-pollen to induce high levels of fruit set. The increase in fruit set in response to mechanical application of cross-pollen was limited to the trees directly receiving the pollen flow and only in one orchard. Consistently, airborne pollen concentration after a single terrestrial application significantly fits a decay curve with a short dispersal. In contrast with the limited dispersal of single mechanical applications, open-pollination results suggest that trees from plantations at least 250 to 500 m away are acting as unsuspected pollenizers. This is probably taking place to some extent in most traditional olive districts in Spain, and it explains why farmers have never demanded pollination designs in this crop. Modern homogeneous plantations can, however, change this situation dramatically and give rise to pollination deficits.

WILD BEE ABUNDANCE AND SEED PRODUCTION IN CONVENTIONAL, ORGANIC, AND GENETICALLY MODIFIED CANOLA

The ecological impacts of agriculture are of concern, especially with genetically modified and other intensive, modern cropping systems, yet little is known about effects on wild bee populations and subsequent implications for pollination. Pollination deficit (the difference between potential and actual pollination) and bee abundance were measured in organic, conventional, and herbicide‐resistant, genetically modified (GM) canola fields (Brassica napus and B. rapa) in northern Alberta, Canada, in the summer of 2002. Bee abundance data were collected using pan traps and standardized sweep netting, and pollination deficit was assessed by comparing the number of seeds per fruit from open‐pollinated and supplementally pollinated flowers. There was no pollination deficit in organic fields, a moderate pollination deficit in conventional fields, and the greatest pollination deficit in GM fields. Bee abundance was greatest in organic fields, followed by conventional fields, and lowest in GM fields. Overall, there was a strong, positive relationship between bee abundance at sampling locations and reduced pollination deficits. Seed set in B. napus increased with greater bee abundance. Because B. rapa is an obligate outcrossing species, the lack of pollination deficit in the organic (B. rapa) fields likely was due to the high bee abundance rather than a lower dependence of B. rapa on pollinators than B. napus canola. Our study illustrates the importance of wild bees to agricultural production and suggests that some agroecosystems may better sustain wild bee abundance, resulting in greater seed production. Further research on why some cropping systems, such as genetically modified, herbicide‐resistant canola, have low wild bee abundance would be useful for management of agroecosystems to promote sustainability of food production.

Pollen limitation and demographic structure in small fragmented populations of Brunsvigia radulosa (Amaryllidaceae)

The ecological consequences of disruptions in plant‐pollinator mutualisms are poorly understood. We examined how seed production and recruitment of juveniles in populations of the spectacular grassland geophyte Brunsvigia radulosa (Amaryllidaceae) correlate with various indices of habitat fragmentation, including habitat fragment area, population size and population isolation. The species was found to be self‐incompatible and adapted for pollination primarily by the long‐proboscid fly Philoliche aethiopica (Tabanidae). In places where this fly is locally extinct, carpenter bees appear to act as substitute, though less effective, pollinators. Seed production in B. radulosa showed a significant positive relationship with population size, but not with habitat fragment area or spatial isolation of populations when all three indices of habitat fragmentation were included as predictor variables in multiple regression models. Reduced seed production in small populations was attributable to pollen limitation, as supplemental hand pollinations resulted in proportionally greater increases in seed production in these populations. Pollen limitation appears to have demographic consequences; specifically, the proportion of juvenile plants in populations showed significant positive relationships with current levels of seed production per plant and size of populations. Thus the long term persistence of small B. radulosa populations in habitat fragments may be threatened by a pollination deficit.

Efeito da polinização por abelhas e outros insetos na produção de sementes de cebola

A deficiência de polinização tem sido apontada como uma das causas da baixa produção de sementes na cultura da cebola. O objetivo deste trabalho foi avaliar a relação da presença de abelhas e outros insetos em flores de duas cultivares de cebola, Allium cepa L. (Alliaceae), com a produção de sementes. Foram registradas a diversidade e a freqüência de insetos nas flores de cebola e o efeito polinizador foi testado. O comportamento de Apis mellifera foi observado diretamente nas flores e a fidelidade verificada a partir do pólen nas corbículas. Representantes de Hymenoptera e Diptera foram os visitantes florais mais abundantes. Houve correlação entre a freqüência de A. mellifera com número de umbelas com flores, em ambas cultivares, e de outros insetos em Crioula Alto Vale. A produção de sementes com livre visitação de insetos apresentou acréscimo superior a 20% em relação às parcelas sem insetos e com visita de uma abelha. A. mellifera transportou mais de 70% de pólen de cebola. A presença de A. mellifera é indispensável para a produção comercial de sementes de cebola.

Effects of Habitat Fragmentation on Pollinator Diversity and Plant Reproductive Success in Renosterveld Shrublands of South Africa

Abstract: We examined the effects of habitat fragmentation on the pollinator diversity and reproductive success of seven perennial plant species in renosterveld shrublands in South Africa. We sampled pollinators in small (0.5–2 ha), medium (3–10 ha), and large (>30 ha) habitat fragments during the peak flowering period in spring and summer. We also compared fruit set and seed set in the seven plant species on different‐sized fragments. Hand‐pollinated controls were used to determine pollination deficits in three species. Seed‐germination studies were done on two species to determine the effect of reduced seed set on reproductive output. Overall, the species richness of bees, flies, and butterflies did not vary significantly among different‐sized fragments. However, the abundance of particular species of bees and monkey beetles (Scarabaeidae: Hopliini) was significantly affected by fragment size, together with other factors such as vegetation cover and the ratio of grass to shrubs. Fragment size and distance to large remnants of vegetation had a significant influence on seed or fruit set in four of the seven plant species examined. One species failed to set any seed in small‐ and medium‐sized fragments. Comparisons of fruit and seed set among hand‐pollinated and naturally pollinated plants confirmed that seed set was pollinator‐limited in the orchid Pterygodium catholicum. Our results suggest that perennial plant species respond in different ways to fragmentation, and that populations on small fragments do not always experience pollination deficits. Existing classifications of pollination systems, in relation to their vulnerability to fragmentation, do not adequately explain our results. We argue that there needs to be a greater focus on the habitat requirements of pollinators to predict the effects of habitat fragmentation on pollination systems and plant reproductive success.

Evaluation of pollination as the cause of erratic fruit set in apricot ‘Moniqui’

SummaryApricot (Prunus armeniaca L.) is a species particularly prone to erratic fruit set. However, the causes of this behaviour are poorly understood since the main limitation in the study of erratic fruit set lies in the difficulty of evaluating all its possible causes in orchard conditions. This results in the eradication of interesting apricot cultivars, such as ‘Moniqui’, in spite of their increasing commercial demand due to their good organoleptic qualities. In this work the influence of pollination deficit in erratic fruit set is ascertained in orchard conditions and the self-compatibility and intercompatibility relationships among four of the main Spanish apricot cultivars are also studied. While ‘Moniqui’ is self-incompatible, ‘Bulida’, ‘Canino’ and ‘Paviot’ proved to be self- compatible. Likewise, all the intercrosses between them behave as compatible. An evaluation of both fruit set and pollen tube behaviour in random and supplementary pollinated flowers with pollen from ‘Canino’ in a commercia...