Infectivity of H. bacteriophora and S. aciari against O. obesus workers
The mortality observed in O. obesus was subjective to the dose as well time of exposure and varies accordingly. Perusal of data showed a considerable difference between the mortality induced by both EPNs species. In both cases, the lowest mortality rates were registered at 24 h, which tended to increase with the increase in concentration, as well as exposure time. At 24 h, H. bacteriophora reported mortality rates of 10, 30 and 40% at 200, 250 and 300 IJs/termite, respectively, while S. aciari showed 10 and 30% mortality rates at 250 and 300 IJs/termite, respectively. Both H. bacteriophora and S. aciari were able to achieve a mortality of more than 50% within 48 h., followed by a gradual increase. As expected, both nematode species registered highest mortality rates after prolonged periods of exposure (72 and 96 h) by H. bacteriophora andS. aciari, respectively, at the highest IJ dosage of 300 IJs/termite. In case of H. bacteriophora, 100% mortality rate was observed at a time interval of 72 h., while S. aciari lagged behind at a time interval of 96 h. In case of control, due to absence of any sustenance, except water, the mortality observed was 10 and 30% after 72 and 96 h., respectively.
The LD50 and LT50 values of both the EPNs species differed significantly. The LD50 values of H. bacteriophora were 693.194, 105.691, 23.237 and 13.054 IJs/termite at 24, 48, 72 and 96 h, respectively. The LD50 of S. aciari was considerably higher at 2997.000, 215.737, 84.431 and 42.040 IJs/termite at 24, 48, 72 and 96 h, respectively. In the case of LD50 values, the LT50 values of H. bacteriophora viz. 72.817, 66.431, 57.595, 52.708, 43.113, 33.541 and 26.639 h at inoculation rates 50, 100, 150, 200, 250 and 300 IJs/termite, respectively, were lower than those S. aciari (Fig. 1). The LT50 values being 99.616, 85.040, 72.817, 65.957, 55.271, 43.951 and 31.761 h at 10, 50, 100, 150, 200, 250 and 300 IJs/termite, respectively (Fig. 2). Emergence of IJs was observed after 5–6 days from the cuticle of the dead worker termites in case of both EPNs species (Fig. 5a, b). At the highest level of inoculation (300 IJs/ termite), the average amount of IJs of H. bacteriophora and S. aciari produced from O. obesus was 521 and 320 worker−1, respectively.
Infectivity of H. bacteriophora and S. aciari against A. ipsilon larvae
The results showed that none of the EPNs species were able to cause any mortality the first 2 days, but on the 3rd day, H. bacteriophora registered mortality rates of 10, 10, 20 and 30% at 150, 200, 250 and 300 IJs/larva, respectively. In case of S. aciari, mortality rates of 10, 20 and 20% were observed at 200, 250 and 300 IJs/larva, respectively. EPNs had a faster invasion rate and subsequent mortality with respect to smaller insects for all EPNs strains as noticed in the case of O. obesus. Hence, the longer time required by the EPNs to cause mortality in A. ipsilon. At 96 h, H. bacteriophora was able to cause at least 60 and 70% mortality rate at 250 and 300 IJs/larva, respectively, whereas S. aciari caused mortality rates of 50% at inoculation rates of 250 and 300 IJs/larva, respectively. The highest mortality rate (100%) was recorded by H. bacteriophora at 144 h at an inoculation rate of 300 IJs/larva, while S. aciari reported 100% mortality rate at 168 h at inoculation rate of 300 IJs/larva. In control, 10, 20 and 40% mortality rates were observed at 120, 144 and 168 h, respectively.
Perusal of the mortality data obtained by both the EPNs species indicated that at all concentrations and exposure periods attempted, H. bacteriophora caused a significantly greater mortality rate in the larvae of A. ipsilon than S. aciari. This trend is further established when the LD50 and LT50 values were taken into consideration. The LD50 values of H. bacteriophora were 1314.790, 200.752, 131.532, 56.107 and 35.711 IJs/larva at 72, 96, 120, 144 and 168 h, respectively, while those of S. aciari 2649.610, 308.319, 232.993, 168.329 and 71.192 IJs/larva at 72, 96, 120, 144 and 168 h, respectively. With respect to H. bacteriophora, the LT50 values were 156.655, 153.592, 127.233, 110.660, 104.691, 93.697 and 83.050 h at 10, 50, 100, 150, 200, 250 and 300 IJs/larva, respectively (Fig. 3), while the LT50 values of S. aciari were 173.144, 161.743, 150.077, 136.084, 115.928, 103.283 and 97.921 h, at inoculation rates of 10, 50, 100, 150, 200, 250 and 300 IJs/larva, respectively (Fig. 4). Emergence of IJs was observed after 10–12 days in the larval A. ipsilon (Fig. 5c, d). At the highest level of inoculation (300 IJs/larva), the average amount of IJs of H. bacteriophora and S. aciari produced from A. ipsilon was 18,450 and 15,320 larvae−1, respectively, 3 days after the first IJ emergence.