The average (± standard deviation) yield of IJs per G. mellonella larva was 49604 ± 20257 herein. The corresponding average of 15 populations (Shehata et al.,2019) related to Egyptian H. indica IJs emerged from the infected scarab grub, T. baal (Coleoptera: Scarabaeidae), in sandy soil, 46,960 IJs/grub, was not significantly (P ≤ 0.05) different from that of Hb-EG strain reported herein (t = 0.99; df = 33). Yet, the average (36,502 IJs/grub) of other 23 Egyptian H. indica populations (Shehata et al. 2019) in loamy sand soil was significantly different (t = 2.05; df = 41) from that of Hb-EG strain reported herein. However, when these 2 groups of populations were combined together (38 populations), the average (41,000 IJs/grub) of their released IJs was lower (P ≤ 0.05) than that of Hb-EG strain researched herein (t = 4.05; df = 56). This difference is possibly due to the used insect host, EPN species/strain, reproductive capacity, and/or genetic constitution, given the other aspects of the general identical procedure applied in the laboratory for IJs extraction and counting.
Insect mortality caused by Hb-EG strain was the greatest after the first baiting, but began to decrease over time (Fig. 1). Interestingly, some successive weeks had significant differences among numbers of the infected host such as weeks 5, 6, and 7, contrary to weeks 8, 9, 10, and 11. This might be due to the relatively low numbers of IJs over time. Overall averages (± standard deviations) of nematode-induced mortality in G. mellonella larvae were 4.63 ± 0.36 (for combined 1–4 weeks), 3.12 ± 0.47 for 5–8 weeks, and 1.92 ± 0.62 for 9–12 weeks after the start of baiting, respectively. Their corresponding percentages were 92.67, 62.33, and 38.33%, respectively. Nematode persistence, measured by the ability to infect G. mellonella larvae in soil over these baiting times, differed significantly (P ≤ 0.0001) among the 3 periods. Shehata et al. (2019) also found that the persistence of H. indica populations in the soil varied greatly concerning their total IJ numbers collected 10, 18, 26, and 30 days post-inoculation in sandy or loamy sand soil.
In contrast, under field condition, nematode capability for infection and reproduction ranged from 72 to 26%, 1–5 months post its inoculation via the infected wax moth larvae in the strawberry rhizosphere, respectively (Fig. 2). Obtained results showed that 92.7 and 62.3% of the exposed larvae were infected under laboratory conditions compared to only 72 and 50.4% in the field one and 2 months after inoculations, respectively. Generally, such an EPN efficacy gap is usually apparent between controlled and actual conditions due to harsh field events such as changes in soil temperature. Although EPN species/strains differ in their persistence in soil (e.g., Abd-Elgawad 2017a, b; Khan and Javed 2018 and Labaude and Griffin 2018), it is assumed that many biotic and physical factors can affect their persistence too. Biotic factors as availability of host insects (Hussaini, 2017), predation by other organisms such as mites and collembola (Wilson and Gaugler, 2004), and competition with other pathogens including even other EPNs (Labaude and Griffin, 2018) can play a significant role. Physical factors such as soil mulching (Leite et al. 2015), soil type and moisture (Hussaini, 2017), application pattern of the introduced EPNs (Wilson et al., 2003), ultraviolet radiation, and desiccation and temperature extremes (Baiocchi et al. 2017) can also affect the EPN persistence. So, the significant (P ≤ 0.05) differences found between beds (F = 4.41; df = 14) in the infected numbers of insects are likely attributed to one or more of these factors and should be considered for more effective biocontrol potential (Abd-Elgawad, 2019).
Non-significant (P ≤ 0.05) differences were found among the 5 beds in the number of infected insects at each of the tested time, i.e., 5 (F = 0.28, P = 0.89), 8 (F = 0.19, P = 0.94), and 16 (F = 1.38, P = 0.26) weeks post-inoculation (df = 4). When the data of the 5 beds were combined together for statistical analyses, non-significant (P ≤ 0.05) difference in the mean numbers of infected insects was found between 5th and 8th weeks (x̄ = 0.65 vs 0.9, n = 75) post-inoculation. Yet, each of these numbers differed (P ≤ 0.05) from the mean number (x̄ = 0.05, n = 75) of the infected insects, 16 weeks post-inoculation. Significant (P ≤ 0.05) differences were found among the cardinal directions of EPN movement (Fig. 3). The nematodes moved to the North had the least numbers. Their mean number over the 3 tested times was significantly (P ≤ 0.05) less than the corresponding number moved to the South or the East. Many factors may have a marked influence on EPN foraging behavior and movement direction. Kapranas et al. (2017) found that 2 levels of soil compaction and media with varying ratio of peat: sand could influence dispersal success of 3 EPN species, with different foraging strategies. Increasing peat content and compaction generally decreased IJ dispersal for Steinernema carpocapsae (ambusher), Heterorhabditis downesi (cruiser), and S. feltiae (intermediate). Of the 3 species, the least affected by peat content was H. downesi, whereas S. carpocapsae was the most adversely influenced by compaction. They found that dispersal of male and female IJs is also differentially affected by soil parameters and that this differentiation is species-specific. Other studies revealed that the EPNs respond directionally to insect host exudates (Dillman et al.,2012), plant volatiles (Jagodič et al.,2017), carbon dioxide (Ramos-Rodríguez et al. 2007), vibration (Torr et al.2004), and magnetic and electric fields (Ilan et al. 2013). Likewise, various biotic and abiotic factors can affect movement behavior (Perony and Townsend, 2013) such as group size (Bonnell et al. 2013), environmental spatial structure and resource availability (Reeve and Cronin 2010), avoidance of predators (Beauchamp, 2017) and genetic diversity (Johnson et al. 2016). Bal and Grewal (2015) found that the cruise foraging H. bacteriophora differ in its dispersal and foraging behavior from ambush foraging S. carpocapsae at the population level, and this behavior is affected by both the presence and absence of hosts and by their mobility. Ruan et al. (2018) reported that EPN dispersal continuously exhibited an aggregative pattern (independent movement was not observed). Given the narrow experimental range examined herein in addition to many unmeasured, hidden, physical, and biotic variables affecting EPN numbers, viability, and movement/dispersal, further studies on the effect of various edaphic and biotic factors on EPN efficacy and application for biological control are warranted with implications of using them for harnessing EPNs as biological pest control agents.