Mistletoes, Dendrophthoe gamblei, Viscum malurianum, Host range.

Mistletoes are the shoot or stem hemiparasites that depend on various host species to complete their lifecycle. Although they are often considered damaging the economically valuable trees (Rist et al., 2011), they throw interest among many plant taxonomists due to their attractive flowers and camouflaging habits. Additionally, they serve as a food source for many forest birds (Muche et al., 2022). Efforts by many taxonomists, starting from Miers in 1851, resolved the confusion in the delimitation of Viscaceae from Loranthaceae as a separate family. The major reasons for the separation include the type of embryosac development and the characteristics and sexuality of the flowers. The recent Angiosperm Phylogeny Group (APG IV 2016) classification merges Viscaceae under Santalaceae as the tribe Visceae, and the present study follows the same.

The initial works by Danser (1941); Rao (1957) enlighten the diversity of Viscum in British India and Indomalaya regions. The family Loranthaceae consist of 8 genera and 52 taxa in India (Sanjai and Balakrishnan 2012; Singh and Ranjan 2013; Singh, 2020; Sivaramakrishna, 2021), while the tribe Visceae includes four genera with 21 species (Sanjai and Balakrishnan 2006; Thriveni et al., 2010; Sanjai and Balakrishnan 2012; Sardesai, 2019). This indicates that the present-day mistletoes of India constitute only 73 taxa. Some of the studies on the diversity of stem hemiparasites in Karnataka of the 20th century include Gamble (1967); Saldanha (1996) documented only a limited number of species. A revisionary work on Loranthaceae and Viscaceae by Thriveni (2016) documented 24 taxa, providing details on their host ranges. Ravi et al. (2021), while compiling the seed plant diversity of Karnataka, listed 38 taxa of this group from the state.

Mistletoes parasitize a wide range of woody plants, particularly in forests and woodlands (Kuijt 1969) and also in cultivated fruit crops. They inhabit diverse biomes, including boreal forests, deserts, and also parasitize on monocotyledons and ferns. Both root and shoot parasites exhibit broad host ranges, with some species like Castilleja spp. and Cuscuta sp. parasitizing a greater number of hosts (Thriveni, 2016). The present study collectively documents the diversity of all stem hemiparasites, known as mistletoes, of Bandipur Tiger Reserve. The challenge of removal and conservation of both the parasite and host fascinates biodiversity conservationists, and the study of host preference and new host ranges plays a vital role in that. Thus, this study provides an updated list of mistletoes and their host ranges in Bandipur Tiger Reserve, Karnataka.

Bandipur Tiger Reserve (BTR), one of the oldest tiger reserves in India, is located between 75° 12′17" E - 76° 51′ 32" E and 11° 35′34" N - 11° 57′02" N, covering an area of 1036.05 km² in the central Western Ghats (Fig.  1). The reserve is situated at the trijunction of three southern states, extending from Mysore to Chamarajanagara districts. The average elevation ranges from 388 m to 1450 m, with Himavad Gopalaswamy Betta as the highest peak (1454 m). Extensive field surveys were conducted from November 2022 to June 2024 throughout Bandipur Tiger Reserve, covering all seasons. The collected specimens were identified using the available literature (Gamble 1925; Saldanha 1996; Saldanha and Nicolson 1976; Sanjai and Balakrishnan 2006; Balakrishnan et al., 2012 and Thriveni 2016). The specimens were processed using standard techniques (Jain and Rao 1977) and were deposited in the Madras Herbarium (MH), Southern Regional Centre, Botanical Survey of India.

Fig. 1. Map of Bandipur Tiger Reserve.

Taxonomic diversity of mistletoes: Around 12 genera and 73 taxa of mistletoes are distributed throughout India. Among these, 6 genera and 22 taxa are found in BTR, this constitutes 55% of mistletoes diversity of Karnataka (Table 1). Karnataka has the largest number of Viscum species among all Indian states (Sanjai and Balakrishnan 2006). The genus Viscum dominates with 10 species in the reserve, except Viscum monoicum, V. acacia, a recently collected species from Bangalore and Viscum mysorensis which is so far known only by its type collection, all the species are distributed in the reserve. This makes the reserve a hotspot for the genus Viscum in Karnataka. The recently discovered species Dendrophthoe gamblei, collected from the dry deciduous habitat of BTR at an altitude of 800-1000 m, is reported here as an additional distribution record for Karnataka state. Some of the photographs of mistletoes are given in (Fig.  2). 

Distribution of Indian Endemics: A total of 31 endemic mistletoe species are found in India (Balakrishnan et al., 2012). Among the 22 mistletoes documented in BTR, 10 are endemic to India, including 3 steno endemic species namely Viscum bandipurense, V. malurianum and V. subracemosum. These constitute 29% of the Indian endemic mistletoe species and 55% of the Karnataka endemic mistletoe species. Around four species show a restricted distribution to South India and Sri Lanka. Viscum malurianum, collected and described from Malur railway station, Kolar district, Eastern Karnataka (Sanjai and Balakrishnan 2001), was previously known only from the type locality (Sanjai and Balakrishnan 2006). The present study identifies an additional distribution location for this species in BTR, particularly on Ficus benghalensis from Gundlupet city to Melukamanahalli village along NH 766.

Distribution in BTR:  The distribution of mistletoes depends on long-term habitat disturbance, climatic factors, and the availability of dispersers and host species (Devkota et al., 2011). The forests of BTR, with regard to mistletoe distribution, are broadly classified into dry deciduous, scrub forest, moist deciduous, and semi-evergreen forest types. The distribution pattern of mistletoes in BTR shows that the dry deciduous forest supports the highest diversity, followed by scrub forest. However, species such as Taxillus recurvus and Macrosolen parasiticus were found to occur on a greater number of hosts in all forest types. Marginal and fragmented forests promote invasion and provide habitat support to these groups (Bach et al., 2005). Habitat destruction, particularly road construction, is the major cause of mistletoe diversity. Along highways and jeep roads, the population is higher, around 80% of mistletoe diversity in BTR observed along the two major highways. The species Taxillus recurvus, found irrespective of altitude and forest type, should be considered a major concern for conservation efforts, as its continuous spread may cause damage to native trees.

The rapid spread and increasing host ranges in BTR: These perennial semi-parasitic plants are identified into two groups: those that can grow on diverse host species and those that specifically grow on a single or narrow range of hosts (Glatzel and Geils 2009; Okubamichael et al., 2016). The former type of plants can be observed to greater extents and have a higher chance of survival (Nickrent and Musselman 2004). Dendrophthoe falcata is an example from Karnataka known to grow on 411 species of woody plants (Thriveni and Shivamurthy 2010). Most of the species in BTR prefers grow in diverse host ranges, but the number of colonies per host varies greatly. Some species observed with higher number of colonies in specified host such as Scurrula cordifolia on Ficus bengalensis and Cordia dichotoma, Taxillus recurvus on Phyllanthus emblica, Macrosolen parasiticus on Terminalia tomentosa. While Viscum heyneanum and V. malurianum prefers to grow specifically on Atalantia racemosa and Ficus benghalensis respectively. The present study also documented the spread of endemic mistletoe on endemic trees, especially Taxillus heyneanus on Deccania pubescence and Dendrophthoe gamblei on Hardwickia binnata. Although the number of colonies observed was only 1–5 per individual host and it is not affecting the survival of the host species.

The characteristic berries of mistletoes, which contain mucilaginous substances, promote the adherence of seeds to their host species. Despite the problems caused by mistletoes, they are a major food source for birds. The low nutritional value of mistletoe makes the birds to consume large quantities of fruits to meet their caloric needs which are a vital strategy for the survival and spread of mistletoes (Szmidla et al., 2019). This dependency on birds for seed dispersal highlights the high degree of coevolution between mistletoes and birds (Medel et al., 2004) and plays a significant role in their pollination and dispersal (Devkota, 2005). Bird species that feeds on mistletoe berries in BTR include Purple sunbird, Bee-eaters, Asian Koel, Pale billed flower peckers and Leaf birds.

Mistletoe embryos are capable of germinating from the fruit, but they cannot break the exocarp without the assistance of birds (Ko et al., 2014). However, during the study, it was observed that mature fruits of Viscum articulatum in herbarium specimens exhibited good germination and haustoria formation even after repeated exposure to alcohol. The growth of mistletoes on a specific host depends on factors such as the host's nitrogen content, compatibility, chemical interactions between the parasite and host, and the role of birds in depositing mistletoe seeds on the aerial parts of plants (Muche et al., 2022). Despite this, mistletoes can grow on a wide range of taxonomically unrelated hosts (Docteres Van Leeuwen 1954; Kuijt, 1964). Healthy parasites on healthy hosts can spread from one colony to as many as 4-5 colonies within two years. The infected plants have more chance of further spread of mistletoes than the uninfected plants. During the study period, the infection of Taxillus recurvus on Phyllanthus emblica increased from 1 colony to 5 colonies in Bandipur Tiger Reserve. Taxillus species produce epicortical roots that allow them to infest quickly within the same host species. Thriveni and Shivamurthy (2010) listed the host ranges and host specificity of most mistletoe in Karnataka, showing that only a few species rely on a limited number of hosts. However, the host range and host preference of mistletoes still need further study, as host ranges tend to increase over time in India. The present study also recorded additional host species for Dendrophthoe gamblei, Helicanthes elastica, Taxillus cuneatus, and Taxillus heyneanus (Table 1).

Table 1: List of species documented in BTR.

G. Rekha, L. Prakash, P. Rameshkumar and K.A. Sujana  (2024). An Account on Diversity, Distribution and Host Range of Mistletoes in Bandipur Tiger Reserve, Karnataka, India. Biological Forum – An International Journal, 16(12): 148-153.