The incidence of E. furcellata started during the 36th meteorological standard week. The mean adult population and egg mass of E. furcellata were 1 and 0.6/m2, respectively, during the 37th meteorological standard week (MSW) and reached a peak of 3.4 (adult) and 2.4 (egg mass)/m2 during the 40th MSW. Thereafter, the population started declining and attained lowest of 0.6 (adult) and 0.2 (egg mass/m2) during the 47th MSW and disappeared during the 48th MSW, which coincided with the harvesting of maize crop in the field. Similar population dynamics of the predatory bug were reported by Snehel et al. (2017).
As illustrated in Fig. 2, adults showed more consumption rate to the nymphs under laboratory conditions (df = 2, 27; P < 0.0001; F value: male = 733.31; female = 432.40; 2nd nymph = 263.2; 3rd nymph = 261.56; 4th nymph = 496.14). The predation rate of the adult female was higher than the male. It consumed 126 ± 4.76, 88 ± 1.37, and 69 ± 1.32 of 2nd, 4th, and 6th instars of S. frugiperda, respectively. Among the nymphs, the trend of predation gradually increased from the 2nd nymphal instar to the 5th in all the exposed larval instars. The 5th nymph has more potential than the rest of instars. It consumed 68 ± 5.41, 45 ± 0.71, and 38 ± 1.03 larvae of 2nd, 4th, and 6th instars of FAW. Obtained results are similar to those of Tuan et al. (2016) who reported that the female (N2–N3, N4, and N5) nymphs of E. furcellata consumed 7.6, 19.3, and 57.1 larvae of S. litura, respectively. The higher predation rate in female might be required to prepare for oogenesis and longevity in the adult stage. There were many reports of E. furcellata feeding on other lepidopteran caterpillars; however, literature on FAW is lacking hence the larvae of different taxonomic group used for comparisons.
A total of 6 egg masses of E. furcellata were collected from each location (Jhansi and Bengaluru), where number of eggs varied from 23 to 61. Among the egg masses collected from Jhansi, 4 egg masses were found parasitizing with Telenomus sp. (Hymenoptera: Platygastridae) (Fig. 3a); the remaining 2 were parasitized by Gryon sp. (Hymenoptera: Platygastridae) (Fig. 3b). The percent parasitism of Telenomus sp. varied from 78.26 to 91.80%, whereas percent parasitism by Gryon sp. was 65.11 and 71.69, respectively, from 2 egg masses. On the other hand, among the egg masses collected from Bengaluru, one egg mass was parasitized by Trissolcus sp. (Hymenoptera: Scelionidae) (Fig. 3c) and 2 egg masses were parasitized by Gryon sp. The percent parasitism of Trissolcus sp. was 77.78% and 85.18 and 100% by Gryon sp., respectively, from 2 egg masses. Gryon sp. was the common egg parasitoid observed in both locations. Interestingly, all the adult parasitoids (Gryon and Trissolcus) emerged from egg masses were males, except for Telenomus sp. where females also were obtained.
The role of insects in any ecosystem has always varied depending on human choice, because Gryon sp., Trissolcus sp., and Telenomus sp. were important egg parasitoids commonly used around the world for the management of hemipteran bugs (Cornelius et al. 2016; Buffington et al. 2018); however, in the present study, they were found parasitizing eggs of the predatory bug, E. furcellata, which plays a role of predator in maize ecosystem. The result of the current study indicates the role of E. furcellata, as a biocontrol agent; however, under field conditions, it is assumed that its potential might be limited by existing secondary parasitoids. In the current study, E. furcellata is not only effective in reducing FAW but also in managing other lepidopteran pests of maize like Helicoverpa armigera (Hübner), Spodoptera litura, and Ostrinia furnacalis (Guene) (Semillano and Corey 1992; Nebapure and Meena 2011). However, the efficiency of the predator is not fully expressed in nature due to existing native secondary parasitoids like Gryon sp., Trissolcus sp., and Telenomus sp. The presence of such secondary parasitoids in nature may limit the potential of the predator. Therefore, further field studies are required to confirm the hypothesis and for better understanding the functioning of the food web, as multi-trophic level systems where secondary parasitoids are common, even in simple agricultural ecosystems such as greenhouses.