Indian sandalwood, Santalum album L. (Santalaceae), popularly referred to as the “Royal tree” of India, is one of the most treasured aromatic tree species in the world because of its unique fragrance, medicinal properties, and highly valuable heartwood oil (Paudel, 2025). Across the Indian subcontinent, irrespective of region, culture, or religion, sandalwood has historically remained an integral component of spiritual traditions, rituals, medicine, and royal heritage. The species is native to India and naturally distributed across tropical and subtropical regions between 30°N and 40°S, encompassing India, Australia, Southeast Asia, Hawaii, and several Pacific Islands (Subasinghe, 2013; Sandeep and Manohara, 2019). Sandalwood is an evergreen medium-sized tree characterized by a hemi-parasitic root system (McLellan et al., 2021), which enables the plant to establish haustorial associations with a wide range of host species for absorption of water and nutrients. Historically, sandalwood played an important role not only in culture and religion but also in international trade and geopolitical relationships. Historical records indicate that sandalwood facilitated trade connections between Indian kingdoms and foreign dynasties such as the Khmer Empire of Cambodia, Champa of Vietnam, and the Song Dynasty of China (Villiers, 2001). During the British colonial period, global demand for sandalwood increased substantially (Wu, 2021), and this demand has continued into the modern era. India's annual sandalwood heartwood exports are projected to exceed 8,000 tons by 2040 (Thomson et al., 2020), with sandalwood oil valued between USD 1,750–2,500 per kilogram and heartwood valued at approximately USD 100 per kilogram. Owing to its immense economic value, sandalwood paradoxically became both one of the most exploited and one of the most protected tree species in India, governed under stringent regulations related to ownership, harvest, transport, and trade. Decades of overexploitation and restrictive policies reduced annual production from nearly 4,000 tons during the 1960s to approximately 350 tons by 2014 (Aparna, 2015), which subsequentlyled to policy reforms and liberalization of sandalwood cultivation during the early 2000s. The high economic returns associated with sandalwood attracted private investors and farmers toward plantation establishment (Divakara et al., 2018), resulting in expansion beyondherbaceous hosts such as pigeon pea (Cajanus cajan) (Rai, 1990), maize (Zea mays), groundnut (Arachis hypogaea), green gram (Phaseolus radiatus) (Thomas, 2017), touch-me-not (Mimosa pudica), horse gram (Macrotyloma uniflorum), and joyweeds its traditional southern Indian distribution into non(Alternanthera spp.) (Durai, 2021) are commonly traditional states including Madhya Pradesh, Chhattisgarh, Gujarat, Rajasthan, Uttar Pradesh, Haryana, Punjab, West Bengal, and Assam (Mishra et al., 2018). During liberalization, the natural distribution of sandalwood was estimated at approximately 9,000 km², of which nearly 90% occurred in southern India (Srinivasan et al., 1992; Radomiljac, 1998; Jain et al., 2003). However, by 2024, sandalwood plantations had expanded to more than 23,100 hectares across India (Madhuvanthi et al., 2024), emerging as one of the major agroforestry- based commercial plantation systems in the country. Simultaneously, increasing global demand and declining availability of naturally grown sandalwood accelerated the transition toward plantation-grown sandalwood (Subasinghe, 2013; Thomson et al., 2020), prompting several countries including Australia, China, Fiji, Tonga, New Caledonia, Vanuatu, Malaysia, Thailand, Vietnam, and Sri Lanka to initiate commercial sandalwood plantation programmes (Subasinghe, 2013; Thomson et al., 2020). Although India remains the native home of S. album, Australia has emerged as a global leader in commercial sandalwood plantations, with nearly 12,000 hectares established in tropical Australia (Clarke, 2006; Thomson et al., 2020). Similarly, Vanuatu (Gillieson et al., 2008; Page et al., 2021), Loyalty Islands, and New Caledonia (Butaud, 2015) have also initiated commercial cultivation. Despite its high commercial value, sandalwood cultivation remains a long-term investment requiring nearly 15–25 years to attain harvestable maturity (Bush et al., 2020; Ota et al., 2022). The delayed economic returns, combined with substantial investment required for plantation establishment and theft prevention (Viswanath et al., 2020; Ota et al., 2022), have become major constraints limiting widespread adoption.

Furthermore, the emergence of serious pest and disease problems has intensified investor concerns by increasing the risk of plantation failure and economic losses.

The hemi-parasitic nature of sandalwood necessitates cultivation under multi-tiered agroforestry systems involving primary, secondary, and permanent host species during different stages of plant growth. More than 300 plant species have been reported as compatible hosts for sandalwood cultivation. During the early establishment phase, annual crops and utilized as primary hosts. These are subsequently replaced by intermediate or secondary hosts including custard apple (Annona squamosa), papaya (Carica papaya), guava (Psidium guajava), pomegranate (Punica granatum), lemon (Citrus limon) (Das, 2021a), drumstick (Moringa oleifera), and agati (Sesbania grandiflora) (Thomas, 2017), which support sandalwood growth during the initial 3–4 years. Eventually, permanent host trees are established within plantations, predominantly consisting of nitrogen-fixing woody species such as babul (Acacia nilotica) (Viswanath and Chakraborty, 2022), raspberry jam tree (Acacia acuminata) (Doddabasawa et al., 2020; Das, 2021b), neem (Azadirachta indica), Australian pine (Casuarina equisetifolia) (Viswanath and Chakraborty, 2022), kassod tree (Cassia siamea) (Doddabasawa et al., 2020), Indian rosewood (Dalbergia latifolia), shisham (Dalbergia sissoo), jamun (Syzygium cumini), Persian lilac (Melia azedarach), Malabar neem (Melia dubia) (Verma et al., 2024; Arunkumar et al., 2022; Radomiljac, 1998), and vilaiti babool (Prosopis juliflora) (Srikantaprasad et al., 2022). Although such diversified agroforestry systems enhance ecological resilience and plantation productivity, they may also unintentionally create favourable environments for polyphagous insect pests. Among these, the red stem borer, P. coffeae Nietner (Lepidoptera: Cossidae), has recently emerged as a serious pest in sandalwood plantations, particularly in southern Indian states (Sundararaj et al., 2019). The infestation potential of P. coffeae in sandalwood plantations has reached alarming levels, with infestation incidences of nearly 17% and mortality of approximately 10% among young plantation stock reported in affected regions (Sundararaj et al., 2019). Infestation by stem borers not only weakens the tree structurally but also facilitates secondary infection by decay fungi and opportunistic pathogens, often resulting in more than one-third loss of economically valuable heartwood. Initially recognized as a pest of coffee, P. coffeae has demonstrated remarkable ecological adaptability through rapid expansion of its host range and is presently reported from more than 80 plant species belonging to over 35 botanical families (Table 1). Such highly polyphagous behaviour highlights its ability to persist and migrate across diversified agroforestry systems, raising serious concerns

Table. 1. Tree species infested by Polyphagozerra coffeae Nietner

regarding host selection and plantation design in sandalwood cultivation. Several commonly recommended sandalwood host species including Psidium guajava, Casuarina equisetifolia, Terminalia spp., Acacia spp., and Eucalyptus spp. are themselves susceptible to infestation and may serve as alternate hosts and pest reservoirs facilitating migration and persistence of the insect within plantation ecosystems. Consequently, the selection of resistant or less susceptible host combinations becomes critically important for reducing infestation risks within sandalwood plantations. The capacity of P. coffeae to persist and migrate across multiple host species within agroforestry environments highlights the urgent need for integrated and host-aware pest management approaches that consider host susceptibility, intercrop combinations, pest migration pathways, and ecological interactions within plantation ecosystems. Although several studies have independently reported the occurrence of P. coffeae on different agricultural, horticultural, and forestry hosts, a comprehensive synthesis linking its expanding host range, agroforestry-mediated spread, and emerging threat to sandalwood plantations remains l acking. Furthermore, limited information is available regarding the ecological drivers responsible for the recent pest status shift of P. coffeae within sandalwood-based systems. Therefore, the present review critically evaluates the biology, host associations, infestation dynamics, ecological implications, and management challenges of P. coffeae, with special emphasis on sandalwood agroforestry systems. This review is the first comprehensive synthesis linking the expanding host range, ecological adaptability, and agroforestry- mediated spread of P. coffeae with the emerging risk to sandalwood-based plantation systems.

The species P. coffeae was originally described under the genus Zeuzera by John Nietner in 1861 from Ceylon (present-day Sri Lanka) and was historically known as Zeuzera coffeae under the genus Zeuzera established by Latreille in 1804. Walker (1856) subsequently elaborated the defining morphological features of the genus, which were further refined and compared in later taxonomic works by Hampson (1892), Seitz (1912), Turner (1918), and Gaede (1933). One of the earliest Indian species placed under the genus Zeuzera was Z. viridicans, described from Ganjam, Odisha, by Eschscholtz in 1821. Although the genus Zeuzera was once represented by nearly 52 species, only seven species are currently recognized following subsequent taxonomic revisions. Daniel (1962) included Z. coffeae in his systematic studies of the genus; however, the most significant modern revision was carried out by Yakovlev (2011), who established the genus Polyphagozerra to accommodate species previously grouped under Zeuzera based on distinct morphological and phylogenetic characteristics. Consequently, Z. coffeae was transferred to the genus Polyphagozerra and is now validly recognized as Polyphagozerra coffeae. At present, the genus includes only two recognized species, namely P. coffeae and P. reticulata.

P. coffeae is a highly polyphagous species exhibiting a broad geographical distribution throughout the tropical and subtropical regions of the Oriental biogeographical zone. The species has been reported from Bangladesh, Myanmar, Cambodia, China, India, Indonesia, Laos, Malaysia, the Philippines, Sri Lanka, Taiwan, Thailand, and northern Vietnam. Its distribution further extends into Australia, Papua New Guinea, and Pacific Island regions including Irian Jaya, indicating remarkable adaptability across diverse climatic conditions ranging from humid tropical lowlands to subtropical plantation ecosystems. The pest is particularly abundant in regions characterized by perennial plantation crops, forest plantations, and multi-storied agroforestry systems where continuous availability of woody hosts facilitates survival and population build-up. Owing to its wide host range and occurrence on economically important plantation crops, the insect is known by several common names depending on the principal crop affected. These include “carpenter moth” (Yakovlev, 2015), “coffee borer” (Beeson, 1941), “cocoa pod and stem borer,” “red borer” (Arora, 1976), “red branch borer” (Kalshoven, 1940; Waller et al., 2007), “red stem borer” (Sundararaj et al., 2019), and “walnut borer” (Gul and Wali-ur-Rehman, 1999; Ahmad, 2017).

The extensive geographical distribution of P. coffeae is closely associated with its exceptionally broad host range. The insect is presently known to infest more than 80 plant species belonging to over 35 botanical families, including economically important forestry, horticultural, fruit, plantation, and shade tree species. Historically, the pest was primarily recognized as a serious stem borer of coffee (Coffea arabica and C. robusta) plantations, particularly in India and Sri Lanka (Beeson, 1941; Mathur and Balwant Singh, 1956; Roychoudhury and Mishra, 2021). However, subsequent studies revealed infestation on numerous alternate hosts including tea (Camellia sinensis), cacao (Theobroma cacao), eucalyptus (Eucalyptus spp.), guava (Psidium guajava), walnut (Juglans regia), teak (Tectona grandis), casuarina (Casuarina equisetifolia), and several species of Acacia, Terminalia, and Melia. Such extensive host diversity significantly enhances the survival, persistence, and colonization potential of the pest within plantation ecosystems by enabling continuous pest carryover between seasons and across cropping systems.

In recent years, the distributional significance of P. coffeae has increased considerably due to the rapid expansion of sandalwood plantations across India and other tropical countries. Several species commonly utilized as primary, secondary, and permanent hosts in sandalwood agroforestry systems are highly susceptible to infestation by P. coffeae. Host plants such as Casuarina equisetifolia, Psidium guajava, Acacia spp., Terminalia spp., Melia dubia, Eucalyptus spp., and Grevillea robusta frequently act as alternate hosts and pest reservoirs within sandalwood plantations. The coexistence of multiple susceptible hosts within a single agroforestry landscape creates favourable conditions for pest survival and migration. Such host continuity further facilitates year-round population buildup within plantation ecosystems. Consequently, the spread and establishment of P. coffeae are strongly associated with diversified plantation systems supporting mixed woody hosts. The occurrence of the pest across geographically distant regions and taxonomically unrelated host plants demonstrates its high ecological plasticity and invasive potential within diversified agroforestry ecosystems. This wide host adaptability has transformed P. coffeae from a localized plantation pest into a major emerging threat to sustainable sandalwood cultivation, particularly in southern India where sandalwood-based agroforestry systems are rapidly expanding.

Adult moths of P. coffeae are white with distinct black speckles on the thorax and dark smudges on the forewings, possessing a wingspan of approximately 35–45 mm (Fig. 1a). Adult longevity ranges from 2 to 14 days, with an average lifespan of nearly seven days (Sathiamma and Bhat, 1974; Chang, 1984; Ahmad, 2017; Tavares et al., 2020). Females generally commence oviposition within two days after emergence and may lay between 500 and 1,000 eggs during their lifetime. Eggs are deposited in strings or clusters within bark crevices of twigs and young stems (Stebbing, 1903; Roychoudhury and Mishra, 2021) and hatch within approximately 10 days. Oviposition within bark crevices likely provides protection against desiccation, environmental stress, and natural enemies, thereby enhancing egg survival under tropical field conditions. Newly emerged larvae produce protective silk webbing that facilitates dispersal, minimizes injury during wind movement, and prevents desiccation (Beeson, 1941). Such silk production likely represents an adaptive dispersal strategy that enhances colonization of nearby host plants within densely planted agroforestry systems. After settling on suitable hosts, larvae initiate feeding by boring into petioles, leaf bases, or branch–stem junctions (Tonini et al., 1986; Monteys, 2015).

The larval stage represents the most destructive phase drying, and breakage of branches (Fig. 1c). A of P. coffeae and is primarily responsible for severe damage in sandalwood plantations. Larvae infest stems and branches ranging from 1.5–20 cm in diameter and enter the host through a single circular entry hole generally corresponding to the width of the larval tunnel. Once inside, larvae excavate irregular longitudinal galleries through the cambium and bast tissues, often extending up to 30 cm in length; under severe infestations, tunnelling may further extend into bark tissues and roots. Young larvae initially feed near the cambial region, enabling rapid access to nutrient- rich t issues while minimizing exposure to environmental stress and natural enemies. As development progresses, later instars penetrate deeper into the pith, sapwood, and eventually heartwood, particularly in young soft-wooded trees. This progressive shift from superficial cambial feeding to deeper internal tunnelling substantially increases physiological damage and structural weakening in sandalwood trees.

Mature larvae (Fig. 1b) attain a length of nearly 40 mm and are characterized by a brown head capsule, hardened pronotal shield, pinkish to purplish dorsal abdominal segments, yellowish ventral surface, and glossy terminal plates. Internal feeding activity disrupts vascular tissues, interferes with translocation of water and nutrients, and ultimately causes wilting, characteristic symptom of infestation is the exudation of pinkish sticky sap from entry holes, which subsequently stains the bark dark brown to black. Active infestations are further indicated by extrusion of reddish-yellow, rice grain-shaped frass pellets (Fig. 1d) through secondary ejection holes, serving as important diagnostic indicators of larval activity. As larval development advances, galleries progressively enlarge and may eventually hollow out the stem, leaving only a thin outer shell of bark and sapwood. Such extensive internal tunnelling severely compromises structural integrity and predisposes infested tissues to secondary fungal infections, thereby accelerating branch failure, decline, and mortality in economically valuable sandalwood trees. The concealed feeding behaviour of larvae also delays symptom expression, causing infestations to remain undetected until substantial internal damage has already occurred.

Young larvae generally prefer tender shoots and young branches, whereas older trees with hardened heartwood are comparatively less preferred (Stebbing, 1903). Entry into the host commonly occurs through leaf axils or bud unions. Early instars produce straight cylindrical tunnels, whereas mature larvae create broader and more irregular galleries within stem tissues. The duration of the larval stage is highly variable and largely influenced by host species and environmental conditions. On Eucalyptus pellita in Indonesia, larval development exceeds 60 days (Tavares et al., 2020), whereas on Vitis spp. in Taiwan, the larval period ranges from 73 to 205 days (Chang, 1984). In Pakistan, larvae feeding on walnut (Juglans regia) complete development within approximately

150 days (Ahmad, 2017). Under southern Indian conditions, where P. coffeae infests sandalwood, coffee, and tea plantations, larval development generally extends for 4–5 months (Beeson, 1941). The considerable variation in larval duration across different host species indicates strong host-mediated developmental plasticity in P. coffeae, which likely enhances its ecological adaptability and persistence within diversified agroforestry systems. Such developmental variability may be influenced by differences in tissue moisture, nutritional quality, wood density, and defensive secondary metabolites among host plants. The prolonged larval period further increases the duration of concealed feeding within woody tissues, thereby enhancing cumulative internal damage and complicating timely detection and management in sandalwood plantations.

Prior to pupation, mature larvae prepare a circular exit hole in the infested stem or branch and seal it with a thin flap of bark. Within the tunnel, the larva constructs a loose silk mesh forming a pupal chamber reinforced with compacted frass. Pupation occurs inside this chamber, and the pupa is distinguished by backward-pointing serrations on abdominal segments and a blunt cephalic projection above the head. The pupal duration varies according to climatic conditions and host plant species. In E. pellita plantations in Indonesia, the pupal stage lasts 18–20 days (Tavares et al., 2020), while on Vitis spp. in Taiwan it ranges between 19–36 days (Chang, 1984). On walnut in Pakistan, pupation lasts 17–21 days (Ahmad, 2017), whereas in India, where the pest commonly infests coffee, tea, and sandalwood plantations, the pupal period generally extends for 3–4 weeks (Beeson, 1941). However, on cacao (Theobroma cacao), pupation is considerably shorter, lasting only 9–13 days (Sathiamma and Bhat, 1974). At adult emergence, the pupa wriggles partially out through the exit hole, facilitating successful moth emergence. Newly emerged adults expand and dry their wings within nearly 20 minutes before flight (Beeson, 1941; Tavares et al., 2020; Roychoudhury and Mishra, 2021).

The complete life cycle of P. coffeae generally spans 5–6 months under favourable environmental conditions. In India, Beeson (1941) reported adult emergence mainly during February, whereas pheromone trap-based studies from Indonesia demonstrated emergence beginning in March, peaking during May–June, and showing a second major flight peak during October (Suheri et al., 2020; Suheri et al., 2022). Similar bivoltine emergence patterns have been documented in several other regions, with major adult flights occurring during April–July and again in October (Chang, 1984; Almanoufi et al., 2012; Rohani and Samih, 2012; Hegazi et al., 2015). In cooler regions, larval hibernation has been observed from November to January (Ahmad, 2017), indicating adaptive diapause behaviour under unfavourable environmental conditions. The duration of the life cycle is strongly influenced by regional climatic conditions. In southern India and other low-elevation tropical regions, development is completed within 4–5 months, whereas under cooler northern Indian conditions and higher elevations, the life cycle may extend up to one year. In Myanmar, the species reportedly completes two generations annually (Beeson, 1941), while under continuously warm and humid conditions, P. coffeae may breed throughout the year, resulting in overlapping generations. Such climatic adaptability and continuous breeding behaviour may facilitate persistent field populations, rapid population build-up, and increased reinfestation pressure in sandalwood plantations, thereby complicating effective seasonal suppression and long- term pest management.

The concealed larval feeding behaviour of P. coffeae significantly complicates detection and management under field conditions. Since larvae remain protected within woody tissues for prolonged periods, visible symptoms generally appear only after extensive internal damage has occurred. This cryptic feeding habit contributes substantially to the increasing economic importance of the pest in sandalwood plantations. The deep tunnelling behaviour of mature larvae not only disrupts vascular transport but also reduces the effectiveness of contact insecticides and limits the success of conventional chemical management approaches. Consequently, sustainable management of P. coffeae requires integrated strategies involving early diagnosis, pheromone- based monitoring, plantation sanitation, biological control agents, and continuous field surveillance.

The exceptionally broad host range of P. coffeae reflects remarkable ecological adaptability and largely explains its successful establishment across diverse plantation and agroforestry ecosystems. A substantial proportion of its reported hosts comprise economically important forestry, horticultural, fruit, and plantation species that are commonly cultivated alongside sandalwood, thereby facilitating continuous pest survival, inter-host movement, and year-round population persistence within plantation landscapes. Presently, P. coffeae has been reported from more than 80 host plant species belonging to over 35 botanical families, confirming its highly polyphagous nature and ecological plasticity.

Within the family Achariaceae, the pest has been recorded on Hydnocarpus kurzi, H. wightiana, and Taraktogenus kurzii (Beeson, 1941; Mathur and Balwant Singh, 1960; Roychoudhury and Mishra, 2021). In Annonaceae, infestation has been documented on Annona muricata and A. squamosa (Bhasin and Roonwal, 1954; Sridhar et al., 2002). Among Casuarinaceae, Casuarina equisetifolia is infortunatum and Vitex pubescens are also susceptible (Beeson, 1941; Mathur and Balwant Singh, 1956, 1961). Lauraceae hosts include Cinnamomum camphora, C. zeylanicum, Persea americana, and P. gratissima (Beeson, 1941; Browne, 1968; Roychoudhury and Mishra, 2021; Suheri et al., 2022). Additional hosts include Lagerstroemia speciosa (Lythraceae) (Beeson, 1941) and Manglietia conifera (Magnoliaceae) (CABI, 2005).

Within Malvaceae, several economically important hosts including Ceiba pentandra, Gossypium spp., Hibiscus rosa-sinensis, Ochroma lagopus, and cacao (Theobroma cacao) are attacked by P. coffeae (Beeson, 1941; Mathur and Balwant Singh, 1960–1961; Nair, 2007; Suheri et al., 2022). In Meliaceae, infestation has been reported on Cedrela febrifuga, Chickrassia tabularis, Melia azedarach, recognized as an important host frequently associated Swietenia macrophylla, S. mahagoni, Toona ciliata, T. with plantation ecosystems (Beeson, 1941; Nair, 2007; Roychoudhury and Mishra, 2021). Similarly, members of Combretaceae including Terminalia brassii, T. ivorensis, and T. bellirica are susceptible to infestation (Beeson, 1941; Browne, 1968; Senthilkumar and Murugesan, 2015; Sundararaj et al., 2019; Roychoudhury and Mishra, 2021), while Cryptomeria spp. of Cupressaceae have also been reported as hosts (Tavares et al., 2020).

Among Erythroxylaceae, Erythroxylon coca has been reported as a host (Mathur and Balwant Singh, 1959). In Euphorbiaceae, infestation occurs on Acalypha spp.

sinensis, Cedrela sinensis, and Cedrela toona (Beeson, 1941; Bhasin, Roonwal and Balwant Singh, 1956; Mathur and Balwant Singh, 1960–1961; Roychoudhury and Mishra, 2021). Members of Moraceae such as Artocarpus spp. are also susceptible (Suheri et al., 2022), whereas within Myrtaceae, infestation has been documented on Eucalyptus grandis, E. deglupta, E. pellita, and guava (Psidium guajava) (Beeson, 1941; Tavares et al., 2020; Roychoudhury and Mishra, 2021). In Phyllanthaceae, Phyllanthus emblica is a reported host (Beeson, 1941), while Grevillea robusta (Proteaceae) is also and cassava (Manihot esculenta) (Beeson, 1941; Bhasin and Roonwal, 1954; Suheri et al., 2022). Fabaceae represents one of the most important host families supporting P. coffeae, with infestation reported on Acacia auriculiformis, A. mangium, Albizia falcata, Amherstia nobilis, Bauhinia susceptible to infestation (Beeson, 1941; Mathur and Balwant Singh, 1960). Rosaceae hosts include Crataegus spp., Eriobotrya japonica, and Malus domestica (Mathur and Balwant Singh, 1959; Roychoudhury and Mishra, 2021; Tavares et al., 2020). malabarica, Cassia auriculata, C. fistula, C. grandis,The family Rubiaceae is particularly important C. siamea, Leucaena leucocephala, Pericopsis because it includes the earliest and principal host mooniana, Pterocarpus dalbergioides, Robinia pseudoacacia, Xylia dolabriformis, X. xylocarpa, and Castanea spp. (Beeson, 1941; Bhasin and Roonwal, 1954; Mathur and Balwant Singh, 1960–1961; Nair et al. , 1996 ; Wylie et al. , 1998 ; Nair, 2007 ; Roychoudhury and Mishra, 2021; Suheri et al., 2022; Tavares et al., 2020). The large number of susceptible Fabaceae hosts is particularly important in sandalwood agroforestry systems because many nitrogen-fixing tree species belonging to this family are routinely utilized as permanent hosts in plantation establishment.

In Juglandaceae, hosts include Carya spp. and walnut (Juglans regia) (Ahmad, 2017; Suheri et al., 2022), while members of Lamiaceae such as Clerodendrum plants from which the species derived its name, namely coffee species Coffea arabica and C. robusta (Beeson, 1941; Mathur and Balwant Singh, 1956; Roychoudhury and Mishra, 2021). In Rutaceae, hosts include Citrus aurantium, C. medica, Citrus spp., Clausena lansium, and Flindersia brayleana (Beeson, 1941; Bhasin, Roonwal and Balwant Singh, 1956; Suheri et al., 2022). Salicaceae hosts include Aberia gardneri, Dovyalis spp., and Populus spp. (Bhasin and Roonwal, 1954; Suheri et al., 2022; Tavares et al., 2020). Importantly, within Santalaceae, the economically valuable sandalwood species S. is itself a confirmed host (Beeson, 1941; Mathur and Balwant Singh, 1961). In Sapindaceae, infestation has been reported on Filicium decipiens, Litchi chinensis, Nephelium litchi, Schleichera oleosa, and S. trijuga (Beeson, 1941; Mathur and Balwant Singh, 1959–1961). Additional hosts include Cestrum nocturnum (Solanaceae), tea plants Camellia sinensis and Thea sinensis (Theaceae), and Lantana indica and teak (Tectona grandis) belonging to Verbenaceae (Beeson, 1941; Bhasin, Roonwal and Balwant Singh, 1956; Mathur and Balwant Singh, 1960–1961; Nair, 2007).

The occurrence of P. coffeae on numerous host species commonly integrated within sandalwood agroforestry systems highlights the ecological importance of host- aware plantation planning and pest management. Intercrop combinations involving highly susceptible hosts may unintentionally facilitate pest survival, migration, and year-round persistence, thereby increasing infestation pressure on sandalwood plantations. The ability of P. coffeae to survive and reproduce on multiple associated host plants enables continuous population carryover and persistent reservoir formation within plantation ecosystems. Such ecological flexibility substantially increases reinfestation risk and complicates long-term management in sandalwood-based agroforestry systems.

Effective management of P. coffeae remains a major challenge in sandalwood agroforestry systems. Although chemical insecticides have traditionally been employed for borer management, their effectiveness against P. coffeae is often inconsistent because the larvae remain concealed within woody tissues for prolonged periods. Furthermore, concerns regarding environmental contamination, non-target toxicity, and possible pesticide residue accumulation in sandalwood heartwood and essential oil considerably limit the suitability of chemical control measures in sandalwood plantations (Gahukar, 2018; Pan et al., 2023). These limitations have necessitated increasing emphasis on sustainable and ecologically compatible management approaches, particularly biological control-based strategies.

Recent efforts to develop Integrated Pest Management (IPM) strategies for P. coffeae on different plantation and forestry hosts have shown promising results. Monitoring tools such as light traps, pheromone traps, and yellow sticky traps have been successfully utilized to detect adult emergence and seasonal flight peaks (Siregar and Tulus, 2023; Suheri et al., 2022), thereby enabling timely management interventions. Monitoring assumes particular importance in the management of cryptic borers such as P. coffeae, where larval stages remain concealed within stems and branches, making early detection difficult (Choi and Park, 2019). Field surveillance for symptoms including wilting shoots, extrusion of gummy frass, sap exudation, and branch brittleness is therefore essential for early diagnosis and localized intervention before severe internal damage develops. Pheromone- based monitoring may further improve prediction of adult emergence periods and facilitate synchronization of management practices with vulnerable stages of the pest.

In response to the limitations of chemical management, increasing attention has been directed toward biological and botanical alternatives. Entomopathogenic fungi such as Beauveria bassiana and Metarhizium anisopliae have demonstrated comparatively higher efficacy against P. coffeae than neem-based botanical formulations containing Azadirachtin (0.03–1% EC). Similar observations were reported by Suheri et al. (2024), who documented superior control efficacy of B. bassiana under both laboratory and field conditions. Likewise, Chandel et al. (2024) identified Heterorhabditis bacteriophora as an effective biological control agent against related stem-boring pests in apple orchards, indicating the broader potential of entomopathogenic organisms for management of concealed internal borers. Although neem-based insecticides remain widely preferred because of their broad-spectrum activity and comparatively low mammalian toxicity (Schmutterer and Singh, 2002; Abd El-Salam et al., 2018), their reduced efficacy against P. coffeae may largely result from inadequate penetration into deeply tunneled woody tissues where larvae remain protected. The comparatively superior performance of entomopathogens suggests that biological agents capable of infecting concealed larval stages may offer greater long- term potential for sustainable management of the pest.

Despite encouraging progress, a standardized IPM strategy specifically tailored for P. coffeae management in sandalwood plantations is still lacking. The high economic value and sensitive end-use of sandalwood products further restrict management options and necessitate the development of residue-safe and environmentally sustainable control measures. Current evidence suggests that an integrated strategy combining regular monitoring, plantation sanitation, biological control agents, selective botanical applications, and timely destruction of infested plant material may provide the most effective and sustainable approach for long-term suppression of P. coffeae in sandalwood agroforestry systems.

Managed sandalwood plantations have emerged as a major commercial investment sector in India, encouraging both farming communities and private entrepreneurs to cultivate S. album under agroforestry, farm forestry, and mixed plantation systems.” (Bunney et al., 2023; Chavan et al., 2024). Although sandalwood cultivation offers considerable economic opportunities because of the high market demand for heartwood and essential oil, the transition of sandalwood from natural forest ecosystems into intensive plantation systems has also introduced complex ecological and phytosanitary challenges (Ota et al., 2022; Viswanath et al., 2020). Presently, large- scale sandalwood plantations covering approximately 23,100 hectares are being actively established across India by farmers and private stakeholders (Madhuvanthi et al., 2024). “However, many new growers and commercial investors remain inadequately informed about the ecological challenges and emerging pest and disease threats associated with sandalwood cultivation in agroforestry systems.” (Sundararaj et al., 2019; Das, 2021a). “Although more than 150 insect species have been reported feeding on S. album, most historically remained minor pests and rarely attained serious economic importance.” (Remadevi et al., 2005). However, during recent years, stem borer complexes have increasingly emerged as major limiting factors affecting plantation productivity and survival. Sundararaj et al. (2019) documented the occurrence of stem and bark borers such as Indarbela quadrinotata Walker and Aristobia octofasciculata Aurivillius in sandalwood plantations along with the red stem borer P. coffeae. At the time of those observations, P. coffeae was regarded as a comparatively minor pest; however, over the past six years, it has evolved into one of the most serious threats to sandalwood plantation establishments, with infestation levels reaching 11–22% in certain plantation regions (Shanbhag et al., 2026). This transition from a minor pest to a major economic pest represents a significant ecological concern for the expanding sandalwood industry.

The emerging pest status of P. coffeae is likely driven by multiple interacting ecological and anthropogenic factors. The rapid expansion of large-scale sandalwood plantations into non-traditional cultivation regions has increased the density, continuity, and spatial connectivity of susceptible host plants, thereby facilitating pest establishment, survival, and dispersal. Unlike naturally distributed sandalwood populations occurring within heterogeneous forest ecosystems, plantation-based agroforestry systems frequently consist of repeated combinations of susceptible host species planted in close proximity, creating highly favourable conditions for pest colonization and persistence. The highly polyphagous nature of P. coffeae, with its ability to infest more than 80 host plant species across 35 botanical families, further enhances its ecological adaptability within diversified agroforestry systems. Several species commonly integrated as primary, secondary, or permanent hosts in sandalwood plantations—including guava (Psidium guajava), casuarina (Casuarina equisetifolia), eucalyptus (Eucalyptus spp.), acacia (Acacia spp.), pomegranate (Punica granatum), and teak (Tectona grandis)—are themselves susceptible to infestation by P. coffeae (Beeson, 1941; Nair, 2007; Sundararaj et al., 2019; Roychoudhury and Mishra, 2021; Suheri et al., 2022). These associated host plants may therefore function as persistent pest reservoirs, facilitating continuous population carryover and inter-host migration within plantation landscapes. The extensive host range of P. coffeae reflects considerable ecological plasticity and suggests adaptive evolutionary mechanisms enabling successful colonization of taxonomically diverse woody hosts.

The hemi-parasitic nature of sandalwood cultivation further contributes to the ecological complexity of pest management. Successful establishment of sandalwood requires the integration of primary, secondary, and permanent host plants at different growth stages (Radomiljac, 1998; Thomas, 2017; Doddabasawa et al., 2020; Verma et al., 2024). While agroforestry diversification generally improves biodiversity and ecological resilience (Amare et al., 2019), the presence of multiple susceptible host plants within the same system may unintentionally favour highly polyphagous pests such as P. coffeae. The simultaneous occurrence of suitable annual crops, horticultural species, and perennial woody hosts provides a continuous ecological network that supports pest survival throughout the year. Such agroforestry complexity may explain the increasing incidence of P. coffeae in managed plantations compared with naturally dispersed sandalwood populations.

The concealed feeding behaviour of P. coffeae significantly increases its destructive potential and complicates management efforts. Larvae remain hidden within stems and branches for prolonged periods, feeding internally on cambial tissues, sapwood, and eventually heartwood, before visible symptoms become apparent (Beeson, 1941; Chang, 1984; Ahmad, 2017; Tavares et al., 2020). Consequently, infestations are often recognized only after wilting, stem breakage, drying of branches, or complete mortality of young plants occurs. The cryptic internal feeding habit significantly limits the efficacy of conventional pesticide applications and necessitates monitoring-based integrated pest management approaches.

Internal tunnelling also predisposes affected stems to secondary invasion by decay fungi and opportunistic pathogens, further accelerating deterioration of economically valuable heartwood. Since sandalwood is primarily cultivated for its medicinal and aromatic heartwood, stem damage caused by P. coffeae may result in severe economic losses extending beyond simple plant mortality.

Climate variability may further intensify the future threat posed by P. coffeae in sandalwood agroforestry systems. Several studies on stem borers and plantation pests have demonstrated that increasing temperatures and altered rainfall patterns can influence insect developmental rates, voltinism, survival, and geographical distribution (Hegazi et al., 2014; Suheri et al., 2022). Warmer environmental conditions may accelerate larval development and increase the number of generations per year, thereby facilitating faster population buildup. Additionally, drought stress and environmental fluctuations may physiologically weaken sandalwood plants, increasing their susceptibility to infestation. Considering that sandalwood plantations are increasingly being established in semi-arid and climatically variable regions of India, understanding the interaction between climate stress and pest incidence has become increasingly important.

The global expansion of sandalwood cultivation also raises important concerns regarding the future distribution and economic significance of P. coffeae. Countries such as Australia, Indonesia, Vietnam, Thailand, Cambodia, Fiji, Vanuatu, and Malaysia are increasingly promoting commercial sandalwood cultivation (Subasinghe, 2013; Thomson et al., 2020; Bush et al., 2020; Page et al., 2021). Interestingly, many of these countries have already reported the occurrence of P. coffeae or related stem borers on forestry and horticultural hosts (Varma et al., 2007; Ahmad, 2017; Tavares et al., 2020; Suheri et al., 2022). This overlap between expanding sandalwood cultivation zones and the known geographical distribution of P. coffeae suggests a high likelihood of future pest outbreaks in emerging sandalwood industries outside India. Therefore, proactive monitoring and host-aware plantation planning will be essential to minimize future transboundary pest risks.

Management of P. coffeae in sandalwood plantations remains particularly challenging because of the medicinal and commercial importance of sandalwood products. Extensive application of synthetic chemical insecticides is generally undesirable, as pesticide residues in sandalwood heartwood and oil may compromise product quality and medicinal acceptability (Gahukar, 2018; Pan et al., 2023). Consequently, pest management in sandalwood plantations should primarily rely on integrated pest management (IPM) approaches emphasizing biological control, regular monitoring, plantation sanitation, and careful host selection. Monitoring tools such as pheromone traps, light traps, and sticky traps have shown promise in detecting adult emergence and monitoring population dynamics in plantation systems (Suheri et al., 2020; Suheri et al., 2022; Siregar and Tulus, 2023). Early diagnosis based on visual symptoms such as wilting shoots, gummy exudation, frass extrusion, and drying branches is also essential for timely intervention.

Recent studies suggest that biological control agents may offer safer and more sustainable management alternatives against P. coffeae. Entomopathogenic fungi such as Beauveria bassiana and Metarhizium anisopliae have demonstrated promising efficacy against stem borers under laboratory and field conditions (Suheri et al., 2024). Similarly, e n t o m o p a t h o g e n i c n e m a t o d e s s u c h a s Heterorhabditis bacteriophora have shown effectiveness against related internal borers in orchard systems (Chandel et al., 2024). However, because larvae remain protected within woody tissues, many botanical and contact insecticides exhibit limited effectiveness against the pest. Therefore, future management research should focus on improving delivery systems for biological agents, developing pheromone-based mass trapping technologies, and formulating eco- compatible IPM protocols specifically tailored for sandalwood agroforestry systems.

Intercrop and host plant selection also play a critical role in the successful management of P. coffeae. In plantation regions where the pest is already prevalent, inclusion of highly susceptible host species may substantially increase infestation pressure and management costs. In the present observations, host plants such as guava, casuarina (Fig. 1e), and pomegranate (Fig. 1f), were found to support P. coffeae infestation and potentially contribute to pest persistence within sandalwood plantations. Therefore, in addition to selecting high-quality sandalwood planting material, identification and selection of comparatively resistant host combinations is equally important for long-term plantation sustainability. Since sandalwood was introduced into intensive agroforestry systems only during the last two decades, scientific understanding regarding optimal host combinations, pest ecology, and host-mediated infestation dynamics remains limited. Consequently, every emerging pest issue associated with sandalwood plantations requires immediate scientific attention and region-specific investigation.

Overall, the emergence of P. coffeae as a serious pest in sandalwood agroforestry systems represents an important ecological and economic challenge for the rapidly expanding sandalwood industry. Its broad host range, concealed feeding habit, adaptability to multi- host agroforestry environments, and increasing geographical spread collectively complicate management efforts. Sustainable mitigation of this pest will require integrated approaches involving early diagnosis, ecological understanding, region-specific host selection, biological management strategies, and continuous monitoring. Future research should prioritize evaluation of resistant host combinations, pest- host chemical ecology, climate- driven distribution shifts, and development of standardized IPM protocols suitable for medicinally important tree crops such as sandalwood.

The emergence of P. coffeae as a serious pest in sandalwood agroforestry systems represents an important ecological and economic concern for the expanding sandalwood industry. Its broad host range, concealed feeding habit, and adaptability to multi-host agroforestry environments make management particularly difficult. The increasing use of susceptible intercrops and permanent hosts in sandalwood plantations may unintentionally facilitate pest persistence and spread. Sustainable management therefore requires integrated approaches involving early diagnosis, host-aware plantation design, biological control, and continuous monitoring. Considering the rapid global expansion of sandalwood cultivation, proactive research and region-specific integrated pest management strategies are essential to minimize future economic losses.

CAMPA provided financial support to carry out particular research - Sanction Order No.75/2019/ ICFRE (R)/RP/SFRESPE (CAMPA)/ AICRP-3/Main

File/28 dated:10/01/2020

Author declares there is no conflict of interest