Synergistic interactions between two invertebrate pathogens: an endophytic fungus and an externally applied bacterium

Authors: WAKIL, WAQAS - Ayub Agricultural Research Institute; YASIN, MUHAMMAD - Bahauddin Zakariya University; Shapiro Ilan, David

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2020
Publication Date: 11/30/2020
Citation: Wakil, W., Yasin, M., Shapiro Ilan, D.I. 2020. Synergistic interactions between two invertebrate pathogens: an endophytic fungus and an externally applied bacterium. Frontiers in Microbiology. 11, 522368. https://doi.org/10.3389/fmicb.2020.522368.
DOI: https://doi.org/10.3389/fmicb.2020.522368

Interpretive Summary: Broad spectrum chemical insecticides can harm humans and other non-target organisms. Therefore, environmentally sound alternatives to broad spectrum chemicals must be sought. Certain fungi and bacteria are natural-occurring biological insecticides. These fungi and bacteria are safe to humans and other mammals. The fungi can sometimes colonize a plant and live inside of it (then the fungus is called an endophyte). The fungi and bacteria are sold commercially to control a variety of economically important insect pests. However, in some cases the level of efficacy (how many pests are killed) is not sufficient. Therefore, methods to improve insect control using fungal and bacteria insecticides are needed. In this study, we investigated the potential to combine bacteria bio-insecticides with fungal bio-insecticides living inside the plant. As a model insect, we used an important insect pest, the red palm weevil. We discovered that combining fungal endophytes and bacterial bio-pesticides is highly beneficial because the level of insect mortality observed was synergistic (greater than the sum of the two treatments when applied alone). Therefore, this research indicates potential to improve natural insect pest control by combining endophytic fungi and bacterial bio-insecticides for control of red palm weevil and other pests.

Technical Abstract: Certain invertebrate pathogenic fungi in the order Hypocreales are endophytes of plants. Interactions between these entophytic fungi and invertebrate pathogen groups that are applied in an inundative manner have not been explored. We investigated the combined insecticidal properties of an endophytically colonized entomopathogenic fungus, Beauveria bassiana, and an aqueous suspension of the bacterium, Bacillus thuringiensis (Bt). The treatments were applied the red palm weevil (RPW) Rhynchophorus ferrugineus (Oliver). Initially, five isolates of B. bassiana (WG-11, WG-40, WG-41, WG-42 and WG-43) were screened for their potential to establish endophytes by inoculating date palm leaf petioles. Although all isolates exhibited the ability to colonize, one of the isolates (WG-41) was deemed most effective and was used in subsequent experiments. Rhynchophorus ferrugineus instars were exposed to endophytically colonized B. bassiana petioles, or petioles dipped bacterial suspensions or both treatments exposed to larvae sequentially. Mortality was low in single pathogen treatments, while in combined treatments increased mortality, and decreased pupation, adult emergence and egg eclosion were observed. Synergistic levels of larval mortality, based on Co-Toxicity Factor (CTF=20), were observed when insects were exposed to the simultaneous application of WG-41 with 40 µg ml-1 of Bt (for all three larval stages tested). Treatment effects on larval duration, pupal duration, frass production, diet consumption and weight gain were also more pronounced in combined applications relative to single-pathogen applications. Our research indicates that the combined use of endophytic B. bassiana and Bt results in high levels of R. ferrugineus mortality; the approach should be tested under field conditions. Furthermore, other combinations of endophytic entomopathogenic fungi and bacterial biopesticides should be explored for synergy and insecticidal activity against other pests and in other cropping systems.