Insect Pest and Disease Threats to Forest Plantations

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1		Pham Quang Thu	+ 1128 word(s)	1128	2021-09-30 08:01:22	\|
2	update layout and reference	Rita Xu	+ 705 word(s)	1833	2021-11-23 03:51:30	\|

This entry is adapted from the peer-reviewed paper 10.3390/f12101301

The planted forest area in Vietnam increased from 3.0 to 4.4 million hectares in the period 2010–2020, but the loss of productivity from pests and diseases continues to be a problem. During this period, frequent and systematic plantation forest health surveys were conducted on 12 native and 4 exotic genera of trees as well as bamboo across eight forest geographic regions of Vietnam. Damage caused by insects and pathogens was quantified in the field and laboratory in Hanoi.

damage incidence damage index defoliation forest health monitoring

1. Introduction

Vietnam has made great strides in regreening the country in recent decades ^[1]^[2]. Due to national policies and targets for afforestation and the rehabilitation of natural forests, the forest area expanded from 9.40 million ha in 1990 to 14.67 million ha in 2020 ^[3]. Vietnam’s 2006–2020 Forest Development Strategy expressed the goal of increasing the total forest cover from 37% in 2006 ^[4] to 42% by 2020 ^[3]. Forest development efforts of the past 10 years have greatly contributed to Vietnam’s sustainability strategy ^[5]. In particular, the plantation forestry sector has made a significant contribution to the growth in forest cover and now comprises 26.4% (ca. 4.4 million ha) of the total forest area in Vietnam ^[3]. About half of the forest plantations are managed by small stakeholders and the remainder by private companies ^[3]^[6]. Therefore, the plantation forest sector is vitally important to the livelihood of millions of rural households ^[7]^[8]. Exotic species, mainly acacias and eucalypts, have been widely planted in Vietnam ^[6]. It has been estimated that fast-growing Acacia hybrids can provide 33–56% of the total household income ^[9]. Furthermore, the export of wood and forest products earned US$9.4 billion in 2018 ^[6] and increased to US$12.3 billion in 2020 ^[10]. Even though the plantation area has continued to increase over time, it is insufficient to meet the demand of the local wood processing industry ^[11]. Reducing losses from damage caused by insect pests and pathogens can help to secure the future wood supply that Vietnam needs for its domestic and international markets.

Over time, the incidence of pest and disease problems in Acacia and Eucalyptus stands has increased globally ^[12]^[13]^[14]^[15], including in Vietnam. Old et al. described 13 fungal pathogens associated with Eucalyptus plantations in Vietnam ^[16]^[17]. The most common Eucalyptus diseases were caused by Cryptosporiopsis eucalypti, Cylindrocladium reteaudii, and Ralstonia solanacearum. Illustrations of the most common biotic problems (21 pests, 23 pathogens) in Acacia, Eucalyptus, and Pinus plantations were provided in a field guide for advisors and growers ^[18]. With the exception of native Pinus, most of the studies on forest health in Vietnam have focused on exotic species, mainly Acacia and Eucalyptus ^[19]^[20]. Currently, there are five major pest species (Ericeia sp., Helopeltis sp., Phalera grotei, Pteroma plagiophleps, and Xylosandrus crassiusculus) and four major pathogens (Ceratocystis sp., Corticium salmonicolor, Phytophthora cinnamomi, and Pythium vexans) threatening the productivity of Acacia plantations in Vietnam. Furthermore, six major pest species (Aristobia testudo, A. approximator, Biston suppressaria, Leptocybe invasa, Sarothrocera lowi, and Trabala vishnou) and five major pathogens (Cylindrocladium sp., Cryptosporiopsis eucalypti, Ralstonia solanacearum, Teratosphaeria destructans, and T. zuluensis) have been damaging Eucalyptus plantations. As the research effort on forest health in Vietnam has increased over the past two decades, many reports focusing on individual pests or pathogens of interest have been published ^[18]^[20]^[21]^[22]^[23]^[24]^[25]^[26]^[27].

2. Field Surveys—General Procedures

Forest health surveys were undertaken annually from 2011 to 2020, in the eight forest geographic regions of Vietnam (Figure 1), with support from the Ministry of Agriculture and Rural Development. Field observations were carried out on forests planted with 16 tree species (Table 1). We relied on information gathered by local foresters as well as our own observations to select the surveyed plantations in each studied forest region. Three fixed plots (40 × 25 m) were randomly set up in plantations comprising the tree species present in each region and they were assessed over ten years. Selected plots were at least 20 m from plantation edges, roads, or forest gaps. About 25% of trees (at least 30 trees) in each plot were randomly selected and assessed for damage from insect pests and/or pathogens. Surveys usually involved three repeat visits a year undertaken in spring (February–April), summer (May–July), and autumn (August–October). Where tree injury was observed (e.g., defoliation, leaf senescence, shoot dieback, tree death), we quantified the damage.

Figure 1. Map of Vietnam showing the eight forest geographic regions.

Table 1. Details of forest plantation species that were surveyed for pests and pathogens.

Host	Area in 2020 (ha)	Planting Region *	Native/Exotic	Commercial Use
Acacia spp. and hybrids	2,000,000	NE, NW, NP, NC, SC, HC, SE, SW	Exotic	Paper, plywood, timber for construction and furniture
Ailanthus triphysa	500	NE, NC, SC, SE	Native	Plywood
Chukrasia tabularis	35,000	NE, NW, NP, NC, SC, HC	Native	Timber for furniture
Cinnamomum cassia	210,000	NE, NW, NC, SC	Native	Bark for export and seasoning, oil for medicine
Dalbergia tonkinensis	2000	NE, NW, NP, NC, SC, HC, SE	Native	Timber for furniture and handcrafts
Dendrocalamus barbatus	120,000	NE, NW, NP, NC	Native	Culm for construction, activated carbon, handcrafts and paper, young shoots for food
Dendrocalamus latiflorus	15,000	NE, NW, NP, NC, SE	Exotic	Young shoots for food, culm for paper
Dipterocarpus alatus	20,000	NC, SC, HC, SE	Native	Timber for furniture, construction
Eucalyptus spp. and hybrids	400,000	NE, NW, NP, NC, SC, HC, SE, SW	Exotic	Paper, plywood, timber for construction
Fernandoa brilletii	6000	NE, NW, NC	Native	Timber for furniture
Hopea odorata	20,000	NC, SC, HC, SE	Native	Timber for furniture and boat
Illicium verum	42,000	NE, NW	Native	Fruit for export and seasoning, oil for medicine
Melaleuca cajuputi	36,000	NE, NP, NC, SC, SW	Native	Paper, activated carbon, poles for construction
Melaleuca leucadendra	32,000	SE, SW	Exotic	Paper, activated carbon, poles for construction
Nauclea orientalis	500	SE, SW	Native	Plywood
Neolamarckia cadamba	1000	NE, NC, SE, SW	Native	Plywood

* Forest zones (see Figure 1): NE, North East; NW, North West; NP, North Plain; NC, North Central; SC, South Central; HC, Highland Central; SE, South East; SW, South West.

3. Field Surveys—Insects

Insect survey methods included eye tracking, sweep netting, suction sampling, and lure traps. Adult folivores were captured with collecting nets on aluminum poles, and placed in killing jars. Plastic boxes (VietNhat Plastic Joint Stock Company, Hanoi, Vietnam) with nylon mesh covers were employed to transport the living larvae, pupae, and eggs to the laboratory. Fresh leaves were included for the larvae to feed.

For wood borers, samples were mostly obtained by the felling of affected trees, then chopping logs to obtain collections. When adults were not present, logs 1.0–1.5 m in length were transported to the laboratory in Hanoi. Some logs were dissected to capture the adults and/or larvae. The cuts of other logs were sealed with Parafilm^® (Bemis Company Inc., Neenah, WI, USA) and were taken to the laboratory for rearing adults for identification.

In addition, more intensive sampling was undertaken through trapping in the field for adult ambrosia beetles. Black funnel Lindgren traps (BioQuip Products, Inc., Compton, CA, USA) and self-made plastic-bottle traps baited with 70% ethanol and para-menthenol (1S, 4R)-p-menth-2-en-1-ol) (Synergy Semiochemicals Corp. (Burnaby, BC, Canada) were used to attract adults. Propylene glycol (Merck, Darmstadt, Germany) and water (50:50) solution was used in the collection cups. Each trap was suspended at least 10 m apart in a plantation and 1.5 m above the ground to avoid damage by wild animals. The baits were replaced once a week. Baiting was undertaken from April to June. Trap collections were stored in 70% ethanol in Eppendorf^® (Eppendorf Manufacturing Corp., Hamburg, Germany) tubes and then sorted in the laboratory.

4. Change over Time

Forest health surveys undertaken from 2011 to 2020 on the plantations of different tree species in Table 1 revealed 14 new or emerging insect pest species and major disease threats from two plant pathogens (Table 2). The total number of new or emerging insect pest species and pathogens increased from 2 in 2011 to 17 in 2020 (Figure 2). Hence, on average, 1–2 new pests per year were recorded damaging forest plantations in Vietnam. The temporal and geographical occurrence of the observed pests and pathogens is detailed in Table 3. Of particular note is the apparently rapid spread of Aulacapsis tubercularis in Cinnamomum cassia, Euwallacea fornicatus in Acacia spp. and C. cassia, Tapinolachrus lacordairea in Chukrasia tabularis, and Xyleborus perforans in Acacia and Eucalyptus. The two main types of damage from insect pest species are from foliar feeding by folivores (5 species of Lepidoptera), and bark and/or wood feeding (6 species of Coleoptera). The fungal pathogen Ceratocystis manginecans has extended its host range from exotic Acacia and Eucalyptus to the native Dalbergia tonkinensis and C. tabularis.

Figure 2. Change in the number of pest and pathogen species recorded each year.

Table 2. Pests and pathogens recorded in forest health surveys in Vietnam causing significant damage to host trees.

Group	Species	Tree Species	Damaged Part
Pests
Coleoptera	Batocera lineolata	Eucalyptus hybrids	Boles
	Euwallacea fornicatus	Acacia auriculiformis Acacia mangium Acacia hybrids Cinnamomum cassia	Boles
	Lycaria westermanni	Fernandoa brilletii	Leaves
	Tapinolachnus lacordairei	Chukrasia tabularis	Boles
	Xyleborus perforans	Acacia hybrids Eucalyptus urophylla	Boles
	Xystrocera festiva	Acacia mangium	Boles
Hemiptera	Aulacaspis tubercularis	Cinnamomum cassia	Leaves
Hemiptera	Helopeltis theivora	Acacia auriculiformis Acacia mangium Acacia hybrids Cinnamomum cassia Melaleuca cajuputi Melaleuca leucadendra	Young leaves, shoots
Lepidoptera	Antheraea frithi	Dipterocarpus alatus Hopea odorata	Leaves
	Arthroschista hilaralis	Neolamarckia cadamba Nauclea orientalis	Leaves
	Atteva fabriciella	Ailanthus triphysa	Young leaves, shoots
	Krananda semihyalina	Cinnamomum cassia	Leaves
	Moduza procris	Neolamarckia cadamba Nauclea orientalis	Leaves
Orthoptera	Hieroglyphus tonkinensis	Dendrocalamus barbatus Dendrocalamus latiflorus	Young leaves
Pathogens
Fungi	Ceratocystis manginecans	Acacia auriculiformis Acacia mangium Acacia hybrids Chukrasia tabularis Dalbergia tonkinensis Eucalyptus camaldulensis Eucalyptus urophylla	Boles
Fungi	Fusarium solani	Dendrocalamus latiflorus	Shoots, roots
Oomycete	Phytophthora acaciivora	Acacia mangium Acacia hybrids	Roots and dieback of seedlings

Table 3. Temporal and geographical occurrence of pest and pathogen species in forest health surveys in Vietnam from 2011 to 2020.

Organism	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020
Pest
Antheraea frithi							SE	SE, SW	SE, SW	SE, SW
Arthroschista hilaralis					SW	SW	SW	SW	SW	SW
Atteva fabriciella						NE	NE	NE	NE	NE
Aulacaspis tubercularis					SW	SW, SC	SW, SC, NE, NP	SW, SC, NE, NP	SW, SC, NE, NP	SW, SC, NE, NP
Batocera lineolata									NW	NE, NW
Euwallacea fornicatus			NE, SC, HC	NE, SC, HC	NE, SC, HC	NE, NW, NC, SC, SE, HC	NE, NW, NC, SC, SE, HC	NE, NW, NC, SC, SE, HC	NE, NW, NC, SC, SE, SW, HC	NE, NW, NC, SC, SE, SW, HC
Helopeltis theivora										NC, NW, NE, HC, SE, SW
Hieroglyphus tonkinensis	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC	NW, NE, NC
Krananda semihyalina										NE, NP
Lycaria westermanni								NC	NC, NE, NW	NC, NE, NW
Moduza procris					SW	SW	SW	SW	SW	SW
Tapinolachnus lacordairei									NW	NE, NW, NC
Xyleborus perforans									NE	NE, SE, SW
Xystrocera festiva			HC	HC	HC	HC, SC	HC, SC	HC, SC	HC, SC	HC, SC
Pathogen
Ceratocystis manginecans	NE, NW, NC	NE, NW, NC	NE, NW, NC	NE, NW, NC, SE	NE, NW, NP, NC, SE, SW	NE, NW, NP, NC, SC, SE, SW	NE, NW, NP, NC, SC, HC, SE, SW	NE, NW, NP, NC, SC, HC, SE, SW	NE, NW, NP, NC, SC, HC, SE, SW	NE, NW, NP, NC, SC, HC, SE, SW
Fusarium solani									NE	NE
Phytophthora acaciivora			NE	NE	NE	NE	NE	NE	NE	NE

Note: NE. North East; NW. North West; NP. North Plain; NC. North Central; SC. South Central; HC. Highland Central; SE. South East; SW. South West.

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