Effectiveness of different Heterorhabditis isolates from Southern Benin for biocontrol of the subterranean termite, Macrotermes bellicosus (Isoptera: Macrotermitinae), in laboratory trials

Abstract

The host-finding ability of 28 Heterorhabditis sonorensis isolates and one H. indica isolate from southern Benin and one H. indica isolate from India was examined in vertical migration sand bioassays against workers of a target citrus termite pest, Macrotermes bellicosus . Thereafter, nine selected isolates were subjected to further investigations on virulence. Our results revealed that both H. sonorensis and H. indica isolates exhibit a cruiser type of search strategy and were capable, to various degrees, of migrating, infecting and killing workers of M. bellicosus in sand columns up to 20 cm long over a period of 3 days. However, only three isolates of H. sonorensis caused 100% mortality to M. bellicosus at the greatest depth tested. The exotic H. indica isolate (LN2) did not show strong finding ability compared to the indigenous one. Concerning virulence, differences were observed among isolates for their ability to invade workers of M. bellicosus. After 12 h post exposure, H. sonorensis from Ze (Ze2) and H. sonorensis from Azohoue (Azohoue2) exhibited the lowest invasion time with IT 50 = 3.35 and 3.67 h, respectively, and a higher penetration rate (11.4% and 10%, respectively) compared with the other isolates. In the concentration-mortality test, we found that, based on 95% confidence limits, all H. sonorensis and H. indica isolates appeared to be equal, with LC 50 values ranging from nine to 16 infected juveniles (IJ) termite −1 . Interestingly, 40 IJ termite −1 were enough to cause 80% mortality to M. bellicosus . Contrary to the LC 50 , the results of our studies clearly demonstrate that M. bellicosus exhibits a time-dependent susceptibility to the tested nematode isolates. So, the lowest LT 50 was observed for H. sonorensis Ze2 (LT 50 = 23.30 h), the highest for H. sonorensis Yokon (34.76 h). The LT 50 of the indigenous H. indica isolate was estimated to 24.07 h. In addition, all selected isolates were able to reproduce in M. bellicosus workers. The highest reproduction potential in M. bellicosus was observed with H. sonorensis Yokon with 20 213 IJ/termite followed by H. sonorensis Ze2 with 19 368 IJ/termites. All tested Beninese EPN isolates were pathogenic to the citrus termite pest M. bellicosus , with H. sonorensis Ze2 being the most virulent.