Molecular identification of the EPF
Amplification of the isolated DNA sample using QRT-PCR results is presented in Fig 4. The statistical analysis of seven EPF isolates with qPCR amplification experiments showed no significant differences in the ANOVA that tested mean cycle threshold (CT) values from the positive (Fig 4). The Real-time amplification of the PCR product generated by specific primers confirmed that the three EPF isolates of B. bassiana, M. anisopliae and V. lecanii isolates under 20 cycles. Using this protocol, amplifying the control samples began around 20 cycles in a real-time experiment, whereas the target DNA (i.e., B. bassiana, M. anisopliae and V. lecanii isolates) was detected later to 21 cycles. Three isolated samples, B. bassiana (isolate BBK2), M. anisopliae (isolate MAA2) and V. lecanii (isolate VLJ2), were used in the next bio-assays.
@figure4
Identifying entomopathogenic bio-agents is one of the main challenges in using indigenous microbial pesticides (Samada and Tambunan, 2020). Morphological, developmental and physiological characteristics were the sole methods used for identification, but they required taxonomical experiences and took a long time
(Hetjens et al., 2021). Recently DNA and RNA-based molecular techniques have been used for taxonomical hierarchy and phonological classification (Goettel and Glare, 2005). The QRT-PCR protocols with specific primers have proven to be very sensitive in detecting and identifying the EPF isolates
(Sabbahi et al., 2009). They are also considered a standard approach for accurately and rapidly identifying microorganisms. The modified specific primers for PCR have been used effectively to detect and differentiate plant pathogenic fungi in the current study. The genomic DNA of the fungal isolates was isolated to obtain pure DNA from cadavers using CTAB.
LT50 assessment
BBA LT
50 and LT
90 with the corresponding 95% confidence limits after exposure to twelve isolates of
B. bassiana. M. anisopliae and
V. lecanii are presented in Table 1, along with the value of Pearson Chi-square, degree of freedom (df) and regression equations. Mortality in the control treatments was consistently below 10%. LT
50 values for
B. bassiana isolates varied from 110.39-113.74 h, while LT
50 values for
M. anisopliae varied from 71.88-75.27 h and LT
50 values for
V. lecanii isolates varied from 64.86-77.17 h.
LT
50 of BBA post-exposure to VLJ2 isolates of
V. lecanii, was the shortest 64.86 h, while the longest for VLK2 isolates 77.17 h. There were no significant differences between the four isolates of
V. lecanii based on the non-overlapping of the fiducial limits 95%. LT
50 for MAA2 isolate of
M. anisopliae, was the shortest 71.88 h, while LT
50 for MAJ2 isolate of
M. anisopliae, was the longest 75.27 h. No significant differences between the four isolates of
M. anisopliae. As for
B. bassiana BBK2 isolates LT
50 was the shortest 110.39 h, while BBA1 isolates LT
50 was the longest 113.74 h. No significant differences between the four isolates of
B. bassiana. Similar results were found with LT
90 of BBA post-exposure to isolates of
V. lecanii, M. anisopliae and
B. bassiana. Based on the discriminating dose bioassay,
B. bassiana isolate BBK2,
M. anisopliae isolate MAA2 and
V. lecanii isolates VLJ2 were selected for further experiments.
LC50 assessment
The probit and logit analysis graph for BBA (Fig 3) and LC
50 and LC
90 with the corresponding 95% confidence limits after exposure to isolates of B. bassiana. M. anisopliae and V. lecanii are presented in Table 2, along with the value of Pearson Chi-square, degree of freedom (df) and regression equations. Mortality in the control treatments was consistently below 10%. LC
50 of BBA post-exposure to isolates of V. lecanii was 46.47 ppm, M. anisopliae 269.53 ppm and B. bassiana was 251.48 ppm. LC
90 of BBA post-exposure to isolates of V. lecanii was 215 ppm, M. anisopliae 1311 ppm and B. bassiana was 1274 ppm.
Cumulative mortality
BBA cumulative post-treatments mortality with the isolates of the EPF B. bassiana, M. anisopliae and V. lecanii is presented in Fig 2. A significant difference in cumulative mortality was recorded between the three EPF. M. anisopliae showed a higher significant cumulative mortality during the first and second days post-application. Then V. lecanii recorded higher significant cumulative mortality starting from the third unit the seventh-day post-application. The 24 h monitoring intervals showed that V. lecanii, M. anisopliae and B. bassiana caused 50% cumulative death by the 3
rd, 4
th and 5
th day, respectively. V. lecanii, M. anisopliae and B. bassiana cumulative mortality of BBA on the 7th day were 92, 84.8 and 73.8%, respectively.
The current study evaluates the efficacy of EPF in controlling back bean aphids under laboratory environmental conditions. In most soil systems,
B. bassiana, M. anisopliae and
V. lecanii are naturally occurring EPF. These entomopathogens infect insects when their spores penetrate insect cuticles, produce toxins and cause the death of their host insect
(Islam et al., 2021). Identifying the virulence between the different entomopathogen species is one of the critical tools before further genetic, biochemical and environmental risk assessment investigation is carried out
(Plantey et al., 2019). On the other hand, these EPF were described to affect their host insects by starving them
(Mannino et al., 2019), deteriorating insect tissue
(Altinok et al., 2019) and discharging toxic substances
(Bamisile et al., 2021). The EPF fungi produce chitinase, protease and lipase enzymes that degrade the insect cuticle (Singh and Joshi, 2020). Once the fungal germ tube penetrates the insect cuticle, they start releasing more mycotoxins in the hemocoel that destroy insect cells and cause their death (Mahankuda and Bhatt, 2019).
Screening bioassays of EPF isolates under laboratory conditions is a crucial step toward identifying the most virulence strains prior to field assessments. Due to the constraint status of pesticide registration legislation and regulations in West Bank, the limitations on agrochemical importation and the high market prices of most pesticides. Many local growers showed an increased interest in developing an indigenous EPF over exotic isolates due to political and ecological boundaries.
The current study investigated the toxicity and virulence of three EPF isolates.
B. bassiana. M. anisopliae and
V. lecanii have been used to control many insects of economic importance,
Cylas formicarius (Reddy et al., 2014); Aphis craccivora (Maketon
et al., 2013);
Leptinotarsa decemlineata (Anderson and Roberts, 1983);
Rhynchophorus ferrugineus (Gindin et al., 2006); Agrotis ipsilon (Gabarty et al., 2014); and
Spodoptera littoralis (Amer
et al., 2008). Similar results were found in several studies;
Saranya et al., (2010) reported a 100% mortality of
A.craccivora post-application of
V. lecanii followed by
B.bassiana, M. anisopliae. LC50 value was the highest virulence for
V. lecanii compared to
B. bassiana,
M. anisopliae. Similar to our results, LT
50 was the highest for
V. lecanii, then
M. anisopliae and
B. bassiana. These three EPFs have been used wildly against aphids and other insect pests worldwide. They are cheap for mass production, have a broad host range and can tolerate a wide range of temperatures and humid conditions (Milner, 1997).
B. bassiana and
M. anisopliae are one of the most abundantly and commercially available and used EPF
(Peng et al., 2021), but
V. lecanii is the only hyphomycete fungi that attack aphids in greenhouses because they need very high humidity (Goettel and Glare, 2005).
Javed et al., (2019) reported that
V. lecanii have a higher virulence and mortality rate due to their ability to germinate under a broad range of temperatures and humidity, increasing their virulence.
LT
50 value of
V. lecanii against
Macrosiphoniella sanborni aphid was three days
(Jackson et al., 1985) and they caused higher mortality to
Myzus persicae (Sulzer) aphid than
B. bassiana (Javed et al., 2019), while
M. anisopliae was more efficient than
B. bassiana against brown plant hopper
(Atta et al., 2020), which is similar to our findings.