The pathogenicity test revealed that experimental novel strain of
Metarhizium anisopliae (
Metarhizium anisopliae strain DULS TTRA, Accession no. KT 119358) is highly pathogenic and virulent to both castes of
Odontotermes obesus causing hundred percent mortality by 3rd and 5
th day respectively in worker and soldier castes of
Odontotermes obesus under
in vitro condition (Fig 1 and 2)
.
The relative virulence of the identified entomopathogenic fungi of 14 days old pure culture on worker and soldier castes of
Odontotermes obesus treated with spore suspension by filter paper method were measured in terms of median lethal concentration fifty (LC50) presented in Table 1. LC50 of
M. anisopliae strain DULS TTRA against both the worker and soldier was lower than the Tericon M.A.
Differences also observed in fungus-induced mortality to termites with three equivalent conidial doses. The results revealed that mortality (%) of both castes of termite at different experimental concentrations of test fungi were significantly different from each other (p<0.001) after 5
th day exposure (Table 2). However, in response to contact with
Metarhizium anisopliae strain DULS TTRA in petri dishes,
Odontotermes obesus exhibited significant higher mortality of 100% in both worker and soldier caste compared to Tericon M.A. (84% in worker and 82.14% in soldier) at higher concentration of 1x10
9 conidia/ml. The mortality percentage, at the concentration of 1 x 10
8 and 1 x 10
7 conidia/ml of the experimental fungi were also in higher in comparison to that of the recommended control at the same concentration
in vitro condition (Table 2).
The median lethal time (LT 50)
i.e., the time taken for the death of 50% of worker and soldier caste of
Odontotermes obesus in response to the action of the experimental fungi at different concentrations of 1x10
7 to 1x10
9 conidia ml
-1 are presented in Table 2. The results revealed that the LT50 values of
Metarhizium anisopliae strain DULS TTRA was significantly lower than that of the recommended control at all the conidial concentrations when applied against both worker and soldier castes.
Potential use of
M. anisopliae as biocontrol agent against termite has been a challenge
(Chouvenc et al., 2011) and identification of new virulent fungal strain, decision making as regard the manner of use of the isolate and termite grooming behaviour are certain important issues that require concern. The fungal virulence is reported to have a strong correlation with the fungal removal by grooming behaviour
(Yanagawa et al., 2010) and hence the challenge posed by termite’s efficiency to avoid fungal infection rendering them fungal resistant, is possible to be averted by finding out of the highly virulent new strain. The pathogenecity study under
in vitro condition indicated
Metarhizium anisopliae strain DULS TTRA (Accession no KT 119358) is virulent against both soldier and worker caste of
Odontotermes obesus. The study revealed that isolated
M. anisopliae strain DULS TTRA caused 100% mortality of worker and soldier caste of
Odontotermes obesus by the 3
rd and 5
th day respectively (Fig 1 and 2). Earlier,
Singha et al., (2011) tried to control the tea termite
Microtermes obesi predominantly present in Barak valley, Assam by using recommended biocontrol agents namely
Metarhizium anisopliae (IARI) and
Metarhizium anisopliae (PDBC). They reported that both strain caused 100% mortality of the worker caste of termite by the 8
th day only of pathogenicity.
For a fungal pathogen to be successful, the fungus has to establish itself before killing the insect. However once established, saprophytic fungi may again grow and develop on the cadaver. In case of
M. anisopliae var
anisopliae, the toxin level produced
in vitro is reported to influence the killing in selected insects
(Kershaw et al., 1999). The mortality caused by the indigenous fungal strain in our study, might be due to secretion of destuxin and observed enhanced mortality after the second day of exposure (Fig 1 and 2) might be due to rapid penetration of the cyclic peptide toxin, destuxin into the termite’s hemocoel (Krutmuanga and Mekechay, 2005). After four to five days of infection, white mycelia of
M. anisopliae strain DULS TTRA developed and after six to seven days, green conidia appeared around the insect cadavers (Plate 1).
Susceptibility of termites to fungal infection is dose dependent (
Rosengaus and Traniello, 1997). However, the mortality occurring due to entomopathogenic fungi depends not only on the concentration of the conidial suspension but also on the time and manner of exposure
(Ansari et al., 2004; Ahmed et al., 2009). In our study,
Metarhizium anisopliae strain DULS TTRA caused 100% mortality of worker caste at the minimum concentration of 1x10
7 conidia ml
-1 by the 5
th day (Table 2). As mycelia development also took place by 4
th to 5
th day, therefore the strain could successfully establish before the death of the termites. In contrast,
Dong et al., (2009) isolated a new virulent
Metarhizium anisopliae variety
anisopliae from
Odontotermes formosanus in China and reported it to cause 100% mortality by the third day post-inoculation at a much higher concentration of 3x10
8 conidia/ml against the subterranean termite,
Odontotermes formosanus.
Pik-Kheng et al., (2009) however reported pathogenicity of isolates of
M. anisopliae causing 100% mortality at 1x10
7 conidia ml
-1 within 3 days post-inoculation but against on a different genus,
Coptotermes curvignathus.
Sileshi et al., (2013) reported that at the highest concentration of 1x10
9 conidial ml
-1, both
B. bassiana and
M. anisopliae isolates caused 100% mortality of
Macrotermes worker termite population after sixth days of exposure.
