Isolation, Behavioral Identification, and Pathogenicity Assessment of Entomopathogenic Fungi from a Forest Wood Borer

Summary

Here we present a protocol for obtaining entomopathogenic fungi from a forest wood borer and a substitutive way to evaluate their entomopathogenic activities using a Coleopteran model insect. This method is efficient and convenient for exploring entomopathogenic fungal resources from wood-boring insect pests in natural forests.

Abstract

Forest wood borers (FWB) cause severe tree damage and economic losses worldwide. The release of entomopathogenic fungi (EPF) during the FWB emergence period is considered an acceptable alternative to chemical control. However, EPF resources have been significantly less explored for FWBs, in contrast to agricultural insect pests. This paper presents a protocol for exploring EPF resources from FWBs using wild Monochamus alternatus populations as an example. In this protocol, the assignment of traps baited with M. alternatus attractants to different populations guaranteed the collection of adequate samples with natural infection symptoms, during the emergence periods of the beetle. Following finely dissecting integuments and placing them onto a selective medium, fungal species were isolated from each part of beetle bodies and identified based on both molecular and morphological traits.

Several fungal species were certified as parasitic EPFs via re-infection of healthy M. alternatus with spore suspensions. Their behavioral phenotypes on M. alternatus were observed using scanning electron microscopy and further compared with those on the Coleopteran model insect Tribolium castaneum. For EPFs that present consistent parasitism phenotypes on both beetle species, evaluation of their activities on T. castaneum provided valuable information on lethality for future study on M. alternatus. This protocol helped the discovery of EPF newly reported on M. alternatus populations in China, which could be applied as an efficient approach to explore more EPF resources from other FWBs.

Introduction

The devastation caused by insect pests has led to great ecological and economic losses in both forest and agricultural ecosystems. Most agricultural pests expose themselves to natural enemies or artificial control agents while damaging host plants. Instead, forest wood borers (FWB) nearly complete their whole developmental cycles inside host tree trunks¹, which raises large challenges to explore efficient biocontrol organisms from FWB in the wild field. What is even worse is that FWBs carry a great number of phytopathogens² or have an intimate relationship with these pathogens as their potential vectors^3,4, dramatically amplifying the negative effects of FWB on forest health. Excessive use of chemical insecticides can alleviate FWB severity, but the emergence of insecticidal resistance^5,6 limits their environmental application. In certain cases, insect parasitoids, predatory arthropods as well as entomopathogenic microbes were released as biocontrol agents to the distribution areas of FWB⁷ and were proven to be efficient and economically acceptable alternatives to chemical control^8,9,10.

Entomopathogenic fungi (EPF) are regarded to have the advantage in controlling FWB over most other microbial groups. Their spores can be carried by insect hosts and stably fixed on body surfaces via penetration into the cuticle or integument^8,11. EPF also present excellent adaptability to environmental stresses and some species colonize well in the tissue of trees as endophytes^12,13, facilitating their growth, survival, and transmission. However, compared to that in agricultural industries, the species diversity of EPF used in natural forest ecosystems is remarkably restricted^14,15,16. Beauveria bassiana (strain PPRI 5339) was evidenced as the most promising strain to promote an IPM program to Eucalyptus weevils in South Africa¹⁷ and the combination of two promising isolates of B. bassiana provided an opportunity for the practical microbial control of red palm weevil, Rhynchophorus ferrugineus, at different life stages in palm tree fields¹⁸. In addition to Beauveria and the well-known Metarhizium, other EPF genera of the order Hypocreales, especially species of Lecanicillium (many of which are now classified into the genus Akanthomyces^19,20), showed strong pathogenicity and high potential in management of forest pests, such as the Cypress aphid in Chile²¹.

The pine sawyer beetle Monochamus alternatus is a notorious pine forest pest in China and neighboring countries, which burrows into branches and trunks of pine trees to impede the transportation of nutrients and water^22,23,24. Moreover, M. alternatus also promotes the invasion of the plant-parasitic pine wood nematode (Bursaphelenchus xylophilus, PWN) as its main vector beetle. Another congeneric species of the beetle, M. galloprovincialis, has spread PWN in several countries in Europe in recent years²⁵. Previous research reported several genera of natural EPFs from Monochamus spp., such as Beauveria, Metarhizium, and Lecanicillium (Verticillium, an even former name of Lecanicillium), in Spain, Japan, and the Anhui/Zhejiang Provinces of China^26,27,28,29. Nevertheless, these collections of EPFs seem to be commonly restricted in a certain location, compared to the wide occurrence of Monochamus beetles in natural fields. As the M. alternatus beetle has a wide geographical distribution in China, it could be regarded as a representative wood borer to explore more potential EPFs across different populations.

In the present protocol, we introduce a specific procedure exploring EPFs from several geographical populations of M. alternatus in southern China. This protocol uses a model Coleopteran beetle as a substitute to perform entomopathogenicity assays, under the condition that the tested fungal species has a consistent behavioral phenotype on both beetle species. This protocol can also provide insights into EPF exploration for other forest wood borers, in which the diversity of their entomopathogenic fungal species is underestimated or less investigated.

Protocol

1. Isolation of fungi from M. alternatus (Figure 1) Collect the beetle sample Collect the pines sawyer beetles M. alternatus using commercial traps (see Table of Materials) baited with attractants before the predicted emergence periods of the beetle populations in naturally infected pine forests. NOTE: In this study, beetles were collected from five geographical regions of southern …

Representative Results

Isolation and identification of fungal isolates from M. alternatus With the aid of attractant traps, a large number (approximately 500 beetles in total) of M. alternatus were collected from five geographical regions. Beetle cadavers with typical symptoms of infection by entomopathogenic fungi were picked; then, body integuments of every beetle were dissected into several positions as described in protocol step 1.3. As a result, more than 600 fungal isolates were …

Discussion

Different geographical populations of FWB may develop varied interactions with the natural entomopathogenic fungi, due to long-term environmental adaptation of EPF species to local climate factors and the specific genotypic population of the host insect^44,45. Expansion of the sampling sites to multiple insect occurrence regions helps increase the possibility of acquiring diverse strains or species of EPF from their natural hosts, as described by previous studies …

Materials

1.5 mL, 2 mL centrifuge tubes	Biosharp	BS-15-M
10 µL pipet tips	Sangon Biotech	F601216
10 µL, 20 µL, 100 µL, 200 µL, 1,000 µL pipettes	Rainin
1,000 µL pipet tips	Sangon Biotech	F630102
2 mL cryogenic vials	Corning	430659
20x PBS buffer	Sangon Biotech	B548117-0500
200 µL pipet tips	Sangon Biotech	F601227
2,000 bp maker	TaKaRar	SD0531
50 mL tubes	Nest	602052
50% glutaraldehyde solution	Sangon Biotech	G916054
50x TAE buffer	Sangon Biotech	B548101
6x loading buffer	TaKaRar	SD0503
Agarose	Sangon Biotech	A610013
Anhydrous ethanol	Jkchemical	LB10V37
Biochemistry Cultivation Cabinet	Shanghaiyiheng	LRH-250F
Chloroform	Juhua	61553
Commercial beetle traps	FEIMENGDI	BF-8	www.yinyouji.com
Gel imager	Bio-Rad	GelDoc XR+
Glycerol	Sangon Biotech	A600232
High speed refrigerated centrifuge	Sigma	D-37520
High-Pressure Steam Sterilization Pot	Mettler Toledo	JA5003
Isopropyl alcohol	General-reagent	G75885B
Nucleic acid dye	Sangon Biotech	A616696
Optical Microscope, OM	Leica	DM2000
Parafilm	Parafilm	PM996
PCR meter	Heal Force	Trident960
Penicillin G	Marklin	GB15743
Potato dextrose agar, PDA	Oxoid	CM0139
Potato dextrose broth, PDB	Solarbio	P9240
Primers	Sangon Biotech	/
Primers Taq	TaKaRar	RR902A
Rapid Fungi Genomic DNA Isolation Kit	Sangon Biotech	B518229
Scanning Electron Microscope, SEM	Hitachi	S-3400N
Streptomycin	Marklin	S6153
Tetracycline	Marklin	T829835
Tween-80	Marklin	T6336
Vacuum freeze dryer	Yamato	DC801
Vortex Shaker	HLD	WH-861
β-Mercaptoethanol	Marklin	M6230