ANT POLLINATION OF DACTYLORHIZA VIRIDIS

ANT POLLINATION OF DACTYLORHIZA VIRIDIS JEAN CLAESSENS AND BERNHARD SEIFERT is hardly any mention of ant pollination in orchids. In Europe, there is only one orchid known where ants play a significant role in pollination (Chamorchis alpina), although not exclusively. To our great surprise, in the Dolomites (Italy), we found ants pollinating another orchid, Dactylorhiza viridis. In this article, we will discuss the pollination strategy of the orchid and the behavior of the ants. This article is a reduced version of a manuscript originally published in Tuexenia (Claessens and Seifert 2017). Summary Dactylorhiza viridis is generally pollinated by Coleoptera (beetles) and Hymenoptera (sawflies, wasps, bees, and ants). This orchid offers nectar at the lip base as well as in the spur. In the Dolomites, a calcareous mountain region in Italy, we found reasonable numbers of Dact. viridis, all growing in the vicinity of nests of the ant Formica (Coptoformica) exsecta. The ants had discovered the nectar of the orchid as a supplemental food source and acted as pollinators of Dact. viridis, carrying, on average, three pollinaria on their head. Their repeated visits led to a high degree of geitonogamy. This is the first mention of ant pollination of Dact. viridis. Dactylorhiza viridis Introduction ©Jean Claessens Ant pollination is a rare event; ants are considered to be nectar robbers and not suitable pollinators. There Dact. viridis habitus. 154 ©Jean Claessens Dactylorhiza viridis, the Frog Orchid, formerly called Coeloglossum viride, can be found in a great variety of habitats and has a circumboreal distribution. It is a quite inconspicuous, an entirely green orchid with two to six alternate leaves. The lower ones are ovate; the upper ones are smaller and lanceolate. The flower spike can Dact. viridis flower spike. Orchid Digest, July, Aug., Sept. 2018 the most important group of Hymenopteran pollinators, pollinating Dact. viridis. The nectar produced at the lip base is accessible and is situated right under the two protruding viscidia. When feeding, the visitor bumps its head against the viscidia, and the two pollinaria (viscidia with the attached caudicle and pollinium) are attached to the forehead. In order to reach the stigma, the caudicles have to make a 90-degree forward bending. This takes quite some time, up to half an hour (Claessens and Kleynen 2011, Darwin 1877). This is thought to be an adaptation to the very slow pollination behavior of the main pollinators, the beetles. Hymenopteran pollinators can reach the nectar supply in the spur. Fruit set ranges from 7 to 84%. be entirely green, but often has a tinge of olive, brown, or reddish-brown. Montane plants tend to have more strongly colored flowers. The perianth segments form a closed hood, protecting the underlying column. The lip can vary in color, from yellow-green to reddish-brown. The basal central part is generally lighter colored. The lip is arching downwards and is three-lobed with a very short median lobe. The spur is short, sack-shaped and much shorter than the ovary. Nectar is produced in the spur and at the spur base, left and right of the spur entrance. Pollination Dactylorhiza viridis is an exception in pollination biology in that it is mainly pollinated by Coleoptera and Hymenoptera (Claessens and Kleynen 2011, Claessens, Kleynen, and Gravendeel 2014). Coleoptera are generally known for their destructive behavior, feeding on floral tissue and eating pollen. They were the first pollinators of primitive plants, but their role has been largely taken over by Hymenoptera. During many observation hours, we found that ichneumonid wasps are Ant Pollination ©Jean Claessens Pollination of flowers by ants is a rare event; there are less than 20 cases known of ant pollination. Ants are frequent flower visitors, but they do not function as the only or main pollinator. In orchids, it is even rarer: only in two tropical orchids, Leporella fimbriata and Mi- ©Jean Claessens ©Jean Claessens The Sailor beetle, Cantharis rustica, with pollinaria of Dact. viridis attached to its forehead. A honeybee, Apis mellifera, with a pollinarium on its forehead searching for nectar. Orchid Digest, July, Aug., Sept. 2018 An ichneumonid wasp with several pollinaria attached to its forehead. 155 Formica (Coptoformica) exsecta inspecting a flower of Dact. viridis. 156 ©Jean Claessens ©Jean Claessens ©Jean Claessens A sawfly, Dolerus gonager, with a bunch of pollinaria of Dact. viridis attached after visiting Dact. viridis. Formica (Coptoformica) exsecta withdrawing from a flower with a pollinarium attached to its forehead. crotis unifolia, are ants the exclusive pollinators (Jones 1975, Peakall and Beattie 1989, Peakall, Beattie, and James 1987, Peakall, Angus, and Beattie 1990, Peakall and Beattie 1991, Siegel 2014). In some species, ants act as additional pollinators, e.g., in Epipactis palustris (Brantjes 1981) or Epcts. thunbergii (Sugiura, Miyazaki, and Nagaishi 2006). Although ants frequently exploit the floral nectaries, they do not play an important role in pollination. There are several reasons for the rarity of ant pollination. Ants generally do not cover large distances, and their size limits the contact with the anthers and stigmas, meaning they can often reach the nectar without touching the anther or stigma. Moreover, their bodies are covered with an antibiotic secretion from the metapleural glands which reduces the pollen viability (Beattie et al. 1984, Dutton and Frederickson 2012, Sanderson and Wright 1989). The presence of dangerous predatory ants such as the swiftly moving and aggressive Formica species may deter effective pollinators such as Apidae (bees) or Diptera (flies) from visiting flowers. As a consequence, and seen from the perspective of a majority of plants, ants are unwanted molesters and nectar thieves. In order to achieve pollinator specification and to prevent visits by ants, many plant families develop very narrow bases of corolla tubes or tenuous flower spurs containing nectar. Some plant genera form, inside the flower, special nectary leaves strongly narrowing towards the base. Plants may also keep away the ants by blocking the approach to the flower. Examples of this method are the pools of dew or rain around the stem of Dipsacus or the glandular hairs along the stems. Plants can even produce extra-floral nectaries, leading the ants away from the reproductive organs of the plant. Ant plants share mutual characteristics: they are small plants with small, inconspicuous flowers placed near the stem (Beattie 2006, Hickman 1974). There Orchid Digest, July, Aug., Sept. 2018 ©Jean Claessens Formica (Coptoformica) exsecta, close-up showing a pollinarium attached to the clypeus. are photographs of ants visiting European orchids and carrying pollinia or pollinaria, but here ants always are incidental pollinators who accidentally touched the viscidium when searching for nectar. Up to now, the only orchid that is frequently pollinated by ants is Chamorchis alpina. It has the characteristic features of an antplant. Observations showed that the main pollinator, Formica lemani, visited and revisited the same inflorescence repeatedly (Baumann and Baumann 2010, Claessens and Kleynen 2011, 2016, Schiestl and Glaser 2012). Because the pollinia are borne on the caudicles, the pollinia do not come in direct contact with the metapleural gland secretions covering the ant’s body. In the harsher climate of the subalpine and alpine zone of the Alps, where increased wind velocity and lower temperatures reduce the activity of flying insects, ants became significant pollinators of Chm. alpina. Ants keep on visiting plants in bad weather conditions and, until late in the afternoon, when it is too cold for other insects. Pollination of Dactylorhiza viridis by Formica (Coptoformica) exsecta In July 2016, we visited the Italian Dolomites, a calcareous mountain region renowned for its rich flora. This region is also rich in orchids like Traunsteinera globosa, Dactylorhiza incarnata, Pseudorchis albida, Gymnadenia odoratissima, Malaxis monophyllos, and many others. Near Cortina d’Ampezzo is Passo di Falzarego, a high mountain pass. At 2,100 meters (6,890 feet) altitude, rich mountain meadows with gradients from calcareous to more acid substrate are found. The seepage from the slopes creates humid spots with among others: Dactylorhiza cruenta (accepted name Dactylorhiza incarnata subsp. cruenta), Pseudorchis albida, Gymnadenia rubra (accepted name Gymnadenia miniata), and Arnica montana. On the dryer parts grew Aster bellidiastrum, Alchemilla vulgaris, Erica carnea, Helianthemum nummularium, Antennaria dioica, Homogyne alpina, Polygonum vivipare, Horminum pyrenaicum, and the orchid Dact. viridis. When inspecting Dact. viridis from nearby, we saw Orchid Digest, July, Aug., Sept. 2018 to our great surprise the ant Formica (Coptoformica) exsecta Nylander carrying a pollinarium on the central region of its forehead. In further observations, we found more ants carrying pollinaria, one with no less than eight pollinaria attached (shown in the video on YouTube: Orchid pollination 22 - Ant pollination of Dactylorhiza viridis by the ant Formica exsecta). Three days later, the ants had lost much of their interest in the orchid; we only caught one ant. Inspection of the flowers showed that many flowers were pollinated; their main flowering period was over. However, the next two days we observed a nearby site where the orchids were still in full bloom. Those orchids were regularly visited by the ants. In four days of observation, we caught 13 ants carrying pollinaria, varying from one to eight pollinaria. The mean was three pollinaria, indicating that the ants had frequently visited the orchid. On Passo Giau, near Passo di Falzarego, we were able to film pollination of Chamorchis alpina by the ant Formica lemani (a video is on YouTube: Orchid pollination 20: Pollination of Chamorchis alpina by the ant Formica lemani). The ants observed in pollinating Dact. viridis showed exactly the same behavior as observed in Chm. alpina (Claessens et al. 2016): fast inspections of the plants with many visits and revisits of the flowers but also periods of more than a minute in which the ant sat motionless with its head in the hood of the orchid. The ants had obviously learned where the nectar-secreting zone of the orchid was, for they purposefully went to the lip base to suck nectar. Their repeated visits resulted in a high degree of geitonogamy. Dactylorhiza viridis is mainly green, just like Chm. alpina, which attracts the ants mainly by means of floral scent. It could be that scent is also the main attractant for the ants in Dact. viridis, but we were unable to test this. In the vicinity of the orchid, were various ant nests, partly hidden in the grass. Formica (Coptoformica) exsecta, the narrow-headed ant, is a relatively common ant species, which can be found in diverse, open habitats with a high percentage of grasses. Its nests are often found in sites with a high cover percentage of grasses (Seifert 2007). Their main food sources are various invertebrate prey organisms and the honey dew of Homoptera, but nectarivory is regularly observed. Territories are effectively defended against other territorial ant species. The maximum foraging distance is usually lower than 13 meters (43 feet), and the ants do not prepare any roads. In both sites where we observed pollination, were several nests; all orchids stood within several meters of a nest. One day we also observed ichneumonid wasps visiting and pollinating the orchids. In the literature, there are no mentions of ants as pollinators of Dact. viridis. It seems that the ants at Passo di Falzarego have learned to exploit the nectar resources of the orchid. They can act as supplemental pollinators, a good strategy for an orchid in mountainous regions with low pollinator densities (Gómez et al. 1996). It is clear that ant pollination is a supplementary pollina157 tion event. Up to now the main pollinators observed were Coleoptera. More observations are needed to ascertain if the pollination by ants as observed at Passo di Falzarego is a local event or if it is typical but remains unobserved. Ants are omnipresent on plants and therefore attract little attention as possible pollinators. Another reason might be that Dact. viridis is not a “flashy” orchid and is quite common in the Dolomites, so this orchid is not actively looked for by orchid lovers. It is clear that we need more evidence about the occurrence of the observed pollination events.^ References Baumann, B. , and H. Baumann. 2010. “Pollination of Chamorchis alpina (L.) Rich. in the alps by worker ants of Formica lemani Bondroit: first record of ant pollination in Europe.” J. Eur. 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Schiestl, Florian P, and Florian Glaser. 2012. “Specific ant-pollination in an alpine orchid and the role of floral scent in attracting pollinating ants.” Alpine Botany 122 (1):1-9. Seifert, Bernhard. 2007. Die Ameisen Mittel-und Nordeuropas: Lutra Verlags-und Vertriebsgesellschaft Tauer. Siegel, Carol. 2014. “Orchids and Formicidae: Ants in Your Plants.” Orchid Digest 78 (3):150-161. Sugiura, N., S. Miyazaki, and S. Nagaishi. 2006. “A supplementary contribution of ants in the pollination of an orchid, Epipactis thunbergii, usually pollinated by hover flies.” Plant Systematics and Evolution 258 (1):17-26. About the Authors Jean Claessens is Honorary research associate of Naturalis Biodiversity Center. He has written many articles about European orchids. His special interest is macrophotography and the relationship between the form of an orchid flower and its pollinator. Many years of research resulted in the publication of a book: “The Flower of the European Orchid – Form and Function,” published in 2011. Website: www.europeanorchids.com Jean Claessens E-mail: jean.claessens@naturalis.nl Bernhard Seifert is a professional myrmecologist employed as curator of entomology at the Senckenberg Museum of Natural History Görlitz, Germany since 1981. He has written 166 papers in scientific journals on species delimitation, taxonomy, evolution, and ecology of ants. He is also interested in ant-bird and ant-plant relationships. Bernhard Seifert E-mail: bernhard.seifert@senckenberg.de Orchid Digest, July, Aug., Sept. 2018