In the present study, the common fungal species obtained from both indigenous and invasive insects were A.niger, A. tenuissima, T. irritans, F. proliferatum, M.racemosus, S. brevicaulis, F. chlamydosporum, F. solani, and A. flavus (14.1-37.7% of total insects). Alternaria alternata occurred a low incidence on aphids and PFF (10.6%). These results are more or less similar to previous reports from different countries. In Greece, Christias et al. (2001) isolated Alternaria alternata from dying and dead aphids. The fungus caused the most damage to the hemocytes but the adipose tissue and the gonads were also affected.
Six Fusarium species were isolated more frequently from aphids than from other insects. In India, Jayasimha et al. (2012) demonstrated that F. semitectum caused 79.90 and 64.40% mortality in nymphs and adults of okra aphid (A. gossypii), respectively, under laboratory and greenhouse conditions.
In Egypt, Abd El-Ghany et al. (2012) reported that F. chlamydosporum caused 58% mortality in the larval stage of Galleria mellonella L., showing the possibility of applying this fungal species as a biocontrol agent. More recently, Abdel Galil et al. (2019) found that F. chlamydosporum, F. proliferatum, F. semitectum, F. solani, F. verticillioides, P. chlamydosporia var. catenulata, S. brevicaulis, and Verticillium sp. exhibited high virulences against wheat and bean aphids causing 60-100% mortality of the tested aphids.
Pathogenic activity of EPF depends on the ability of degrading insect’s cuticle components by active secretion of hydrolyzing enzymes including lipases, phospholipases, proteases, and chitinases (Abd El-Ghany et al. 2012). In the present work, most of the tested fungal strains were able to produce lipolytic enzyme with the most active fungi being B. atrogriseum, F. chlamydosporum, A. niger, C. spicifer, and F. proliferatum. Lipases are responsible for the hydrolysis of ester bonds of lipoproteins, fats, and waxes found at the interior parts of the insect integument (Ali et al. 2009). Saleem (2008) tested the lipolytic activities of several fungal strains and found that A. alternata, A. flaves, A. sydowi, A. terreus, and C. spicifer were good producers of this enzyme.
Phospholipases are enzymes responsible for degrading phospholipids of insect’s cuticle. Variation in phospholipases activity was observed among the 26 tested fungal strains. In the present study, the most active fungal strains were A. niger, M. racemosus, P. chlamydosporia var. catenulata, S. brevicaulis, and A. flavus. Fungal strains belonging to Epicoccum, Fusarium, and Nigrospora exhibited intermediate phospholipase activity. Ali et al. (2019) observed that clinical and environmental strains of A. flavus, A. niger, and A. terreus were active producers of esterase, phospholipase, and protease enzymes.
As shown from the obtained results, marked variations were observed in the proteolytic activities among the 26 tested fungal strains. The best producers were S. brevicaulis, A. flavus, A. niger, A. sydowii, F. semitectum, and C. spicifer. Shimizuy et al. (1993) emphasized that extracellular proteases were even found in insect hemolymphs. Working with A. niger, Milala et al. (2016) reported that the maximum proteolytic activity was achieved after 48 h of incubation at 40 °C, using casine as a substrate.
Chitinolytic abilities of the fungi tested in the present study were generally lower than other enzymatic activities. The prominent fungal strains included A. flavus, A. niger, B. atrogriseum, F. semitectum, F. solani, N. oryzae, P. chlamydosporia var. catenulata, and S. brevicaulis. These enzymes act synergistically with chitinases in the solubilization of the insect cuticle. Establishing a relationship between the production of hydrolytic enzymes and the virulence of EPF may be useful in developing of screening methods for identifying new isolates with increased virulence and also for the development of bioproducts based on them (Montesinos-Matias et al. 2011). Chitinase was also produced by a halophilic strain of A. flavus isolated from Gulf of Suez, Egypt, with activity 620.54 U/l (Beltagy et al. 2018). Therefore, they suggested that the insecticidal effects of EPF could be directly linked with the activity of cuticle hydrolytic enzymes. Enzyme secretion by EPF may be involved in the degradation of cuticular polymers during pathogenesis, assisting in the penetration of the insect exoskeleton and providing nutrients for fungal growth. These enzymes can act synergistically, helping fungi to control insect pests and pathogens that attack productive crops, and offer potential economic benefit to agribusiness.