First report of the fungus Actinomucor elegans Benjamin & Hesseltine belonging to Odontotermes obesus (Rambur) (Isoptera: Termitidae) in India

Highlights

• Actinomucor elegans was identified using morphological and molecular techniques.

• The pathogenicity, median lethal time, & median lethal concentration of A. elegans against Odontotermes obesus were assessed.

• A. elegans was highly pathogenic to O. obesus, recording 90% mortality with 5 × 10⁷ spores mL⁻¹ and 94% mortality at 168 h.

• The LC50 and LT50 of A. elegans were found to be 5.974 × 10⁶ spores mL⁻¹ and 108 h, respectively.

Abstract

Termites are pests notorious for causing grave damage to standing crops and buildings. Odontotermes obesus (Rambur) (Isoptera: Termitidae) is a commonly found species in the Indian subcontinent known to cause serious damage to crops. The fungus Actinomucor elegans Benjamin & Hesseltine is reported to have a ubiquitous distribution and been isolated from various hosts, as well as substrates. We have collected the isolates of A. elegans from the soil and dead termites present in the root zone of wheat plants in Haryana (India). The isolates were subjected to molecular analysis based on both morphological-and ITS (internally transcribed spacer) region and confirmed as A. elegans. After the identity verification, the pathogenicity of isolates against worker caste termites of O. obesus was tested by the topical application of an aliquot of the control or suitable suspension (0.5 mL) to the dorsum of each termite. The concentrations of the tested fungus A. elegans were 5 × 10² to 5 × 10⁷ conidia mL⁻¹, and their mortality against worker individuals of O. obesus was evaluated. Lethal time and concentrations were estimated. The assay was completely repeated three times. The results of the pathogenicity test revealed that A. elegans was highly pathogenic to worker caste termites of O. obesus, recording 90% mortality with 5 × 10⁷ spores mL⁻¹ and 94% mortality at 168 h. The median lethal concentration (LC50) and the median lethal time (LT50) of A. elegans were found to be 5.974 × 10⁶ spores mL⁻¹ and 108 h, respectively. Our results have provided new opportunities to control termites in agriculture, horticulture, and forest ecosystems. However, additional investigations both in controlled and field conditions are necessary, and when using A. elegans var. elegans, human safety also needs to be considered. Further investigations related to secondary compounds released by this fungus would add a new dimension to termite management strategies.

Introduction

Termites belong to the order Isoptera, which consists of 2500 species, of which 300 species are considered pests. These insects are social, and their behavioral roles and caste system support the defense in a termite colony for food gathering and brood care; they may also be impacted by differences in the tolerances of the soldier and the worker to pesticides (Rao et al., 2005). Workers have a desire to shift easily in search of new sites when accompanied by soldiers (Chen and Henderson, 1997), and therefore an increase in the soldiers’ susceptibility to insecticides may affect the survival of the colony. Out of 300 species of termites found in India, around 35 species have been observed to cause severe damage to crops and timbers in buildings (Rajagopal, 2002). Termites cause a yield reduction up to 50% and reduce the quality, which results in a reduction in the market price (Pardeshi et al., 2010). Termites could cause over $40 billion worth of damage to wooden materials annually across the world (Rust and Su, 2012). In India, the estimated yield loss of 15–25% in maize and monetary losses of $2.12 billion has been reported (Joshi et al., 2005). Odontotermes obesus (Rambur) (Isoptera: Termitidae), a widespread termite species in India, has been reported to cause significant damage to Triticum aestivum L. (6–40%), Arachis hypogaea L. (10–30%), Cicer arietinum L. (10–20%), Saccharum officinarum L. (24–76%), and Oryza sativa L. (5–20%) (Kushwaha, 1998; Pardeshi et al., 2010; Ranjith et al., 2018). Novel technologies recently developed for the control of termites include non-repellent termiticides, physical barriers, monitoring-baiting programs, and insecticide-impregnated polymer barriers (Su, 2002).

Farmers use a wide range of insecticides to control this termite pest effectively. Most pesticides have adverse effects on natural enemies such as parasitoids, predators, and entomopathogens and cause resistance, resurgence, and unwanted effects on non-target organisms and humans (Desneux et al., 2007; Boopathi et al., 2017). Biological control is a sustainable alternative to environmentally-harmful insecticides that can help protect people, the environment, and plants (Bellows, 2001; Boopathi et al., 2013, 2015a, b, 2016). A wide range of chemical insecticides is used against termites, and only a very few or no microorganisms are commercially available for the management of termites (Kushwaha, 1998). Entomopathogenic fungi have been verified as a viable option for the control of insects, with the advantage of integrating with other control methods or agents (Araujo et al., 2020; Baja et al., 2020; Khun et al., 2020). The entomopathogenic fungi (EPF) have shown a promising epizootic potential against insect pests in both the greenhouse and field conditions (Lacey et al., 1996; Boopathi et al., 2015a, b). In India, these fungi were discovered to be effective against insect pests since the 1990s. Many economically important insect pest species in the orders Isoptera, Coleoptera, Lepidoptera, and Hemiptera are susceptible to EPF isolates (Geetha, 2000; Aiswariaya et al., 2007; Boopathi et al., 2013, 2015a, b). The pathogenic potentials of Actinomucor elegans Benjamin & Hesseltine and Anisoplia austriaca (Herbst) to control the chafer beetle and in the wheat ecosystem, respectively, in the Province of Kurdistan (Iran, Karimi et al., 2015) have been reported. A. elegans is known to cause invasive mucormycosis in humans (Mahmud et al., 2012). It is widespread and isolated from various substrates and hosts like the rhizospheres of different crops such as oat (Avena sativa L.), corn (Zea mays L.), barley (Hordeum vulgare L.), T. aestivum, hop (Humulus lupulus L.), and alfalfa (Medicago sativa L.), as well as rabbit and mouse dung, feathers, and free-living birds’ nests (Phalip et al., 2006; Domsch et al., 2007).

Fungal species are usually classified according to their conidial shape and placement on the conidiogenesis apparatus (Feng et al., 1994). Rapid and uncomplicated approaches to identify the species composition in fungal communities using the sequences of specific regions in the fungal genome are found to be reliable (Horton and Bruns, 2001). Several studies have demonstrated the use of the amplification of polymerase chain reaction, restriction fragment length polymorphism (Hedegus and Khachatourians, 1993), random amplified polymorphic DNA, inter simple sequence repeats, and microsatellites (Jensen et al., 2001) for the identification of the fungal species. Interspecific and intraspecific variation of fungi can be analyzed using sequencing of specific regions such as internal transcribed spacer and intergeneric spacer of rDNA (Sanon et al., 2009). Considering the above facts and the quantum of damage inflicted by termites to crops in India, this study was carried out to explore the possibility of finding an effective biocontrol agent for termite management. The fungal isolate, obtained from the root zone of wheat and dead termites, was identified using morphological and molecular techniques, and its pathogenicity, median lethal time, and median lethal concentration against the worker caste of O. obesus termites were assessed under laboratory conditions.