Increased phenoloxidase activity constitutes the main defense strategy of Trichoplusia ni larvae against fungal entomopathogenic infections

Authors: Duffield, Kristin; ROSALES, ALAYNA - Bradley University; Muturi, Ephantus; Behle, Robert; Ramirez, Jose

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2023
Publication Date: 7/23/2023
Citation: Duffield, K.R., Rosales, A., Muturi, E.J., Behle, R.W., Ramirez, J.L. 2023. Increased phenoloxidase activity constitutes the main defense strategy of Trichoplusia ni larvae against fungal entomopathogenic infections. Insects. 14(8), article 667. https://doi.org/10.3390/insects14080667.
DOI: https://doi.org/10.3390/insects14080667

Interpretive Summary: The cabbage looper is an important agricultural pest worldwide and is frequently used as model organism for measuring the effectiveness of biopesticides, including those derived from pesticidal fungi. To better understand how cabbage loopers resist and survive infection of insect-killing fungi, we measured several insect defense parameters following exposure to two species of fungi. Our results indicate that the cabbage looper’s main defense mechanism is the melanization cascade, with activation of several melanization-related genes soon after infection. This differs from what has been reported in other insect-fungi studies and might represent a target for disruption in future cabbage looper control strategies that integrate fungal biopesticides. Our study also shows that one well-known insect antifungal immune response does not participate in the cabbage looper antifungal defense, differentiating this important pest from other insects. Overall, this study provide insights into the host response strategies employed by cabbage loopers for protection against pesticidal fungi and aids in the design of more effective biological control strategies of this pest.

Technical Abstract: The cabbage looper Trichoplusia ni is an important agricultural pest worldwide and is frequently used as a model organism for assessing entomopathogenic fungi virulence, though few studies have measured the host response repertoire to fungal biocontrol agents. Here, we quantified the immune response of T. ni larvae following exposure to two entomopathogenic fungal species: Beauveria bassiana and Cordyceps javanica. Results from our study demonstrate that T. ni larvae exposed to fungal entomopathogens had higher total phenoloxidase activity compared to controls, indicating that the melanization cascade is one of the main immune components driving defense against fungal infection and contrasting observations from other insect-fungi interaction studies. We also observed differences in host response depending on the species of entomopathogenic fungi, with significantly higher induction observed during infections with B. bassiana than with C. javanica. Larvae exposed to B. bassiana had increased expression of genes involved in prophenoloxidase response and the Imd, JNK, and Jak/STAT immune signaling pathways. Our results indicate a notable absence of Toll pathway-related responses, further contrasting results to other insect-fungi pathosystems. Important differences were also observed in the induction of antimicrobial effectors, with B. bassiana infections eliciting three antimicrobial effectors (lysozyme, gloverin, and cecropin), while C. javanica only inducing cecropin expression. These results provide insight into the host response strategies employed by T. ni for protection against entomopathogenic fungi and increases our understanding of insect-fungal entomopathogen interactions, aiding in the design of more effective microbial control strategies of this important agricultural pest.