Arrhenodes minutus

Current scientific name: Arrhenodes minutus (Drury) Class: Insects Order: Coleoptera Family: Brentidae Genus: Arrhenodes Species: Arrhenodes minutus Synonyms: A. minuta, Arrenodes minutus, Brentus brunneus, B. minutus, B. mucillosus, B. septentrionis, Curculio minutus, Eupsalis lecontei, E. minuta, E. sallei, Platysystrophus minutus EPPO Code:  ARRHMI  Common name: oak timberworm Taxonomic rank: species

Arrhenodes minutus is a Union quarantine pest, listed in Annex II (Part A ‘Pests not known to occur in the Union territory’, section 3 'Insects and mites', Point 15) of  Commission Implementing Regulation (EU) 2019/2072 . While there are no specific import requirement for the pest, the same Regulation prohibits the introduction of some of the plant taxa known to be its hosts: Quercus L. and Populus L. with leaves, other than fruit and seeds, coming from non-EU countries (with some exceptions) and plant products such as isolated bark of Quercus, other than Q. suber L., coming from Canada, Mexico and the United States (Annex VI, points 2 and 5). Restrictions also apply to the importation of raw wood of Quercus from the United States (point 90) and that of Populus from the Americas, which has special requirements (point 96).

Arrhenodes minutus is a potential vector of the Union quarantine pest Bretziella fagacearum (Curl 1954; Solomon, 1995), also listed in Commission Implementing Regulation (EU) 2019/2072. The introduction of plants of Quercus L. other than fruit and seeds into the EU from the United States is subject to special requirements and must be accompanied by an official statement declaring that they originate in areas known to be free from B. fagacearum (Point 34 of Annex VII).

The general requirements for survey of quarantine pests in the EU territory are laid down in  Regulation (EU) 2016/2031  and  Commission Implementing Regulation (EU) 2020/1231 .

Arrhenodes minutus is not known to occur within the EU; its distribution is limited to its native range in southeastern Canada and the eastern USA down to Florida and Mexico (Frost, 1966; Thomas, 1996).

Note: the information included in this section is aligned with the EPPO map updated on 13-10-2022

The biology of A. minutus is described mainly in Buchanan(1960), Bright (1993),  EPPO (online-b) , Dajoz (2005), Sanborne (1983) and Solomon (1995).

Adults emerge in May and are active until late August over most of the species’ range, with a peak between mid-May and mid-June (Buchanan, 1960), but in Florida, adults are also captured in late March and December (Frost, 1966; Bloem et al., 2002). The adults feed on sap leaking from fresh wounds on injured, dying or felled trees (Buchanan, 1960; Solomon, 1995). They have also been observed colonising unseasoned timber, stave bolts and squared wood (Solomon, 1995). Adults of both sexes aggregate under loose bark near wounded tissues for feeding (Sanborne, 1983; Dajoz, 2005). Under laboratory conditions, the adults were observed to mate on the same log they fed on (Sanborne, 1983). However, information on the place of mating under natural conditions is not available.

After mating, females chew cylindrical oviposition ‘hair-sized’ holes with their proboscis, often at or near wounds where sap is leaking from the wood. It takes approximately 2 h for a female to chew an oviposition hole. Then she oviposits one egg per hole and plugs it with frass and a secreted sticky substance (Buchanan, 1960). In Ontario, there are two periods of oviposition, from mid-June to late July and from early to mid-September (Sanborne, 1983).

The larvae hatch in a few days to three weeks depending on the location (Sanborne, 1983; Solomon, 1995) and then bore a tunnel almost straight across the grain of the wood (into the xylem) with very little slope either up or down until they almost reach the other side of the trunk before making a sharp U-turn either up or down and tunnel back across the grain of the wood towards their original entry point (Buchanan, 1960) (figure below). Information on the instars and timing of the larval development is not available.

Pupation occurs near the gallery’s exit (Buchanan 1960), and the newly developed adult emerges from an exit hole often located near the oviposition hole initially excavated by the female ( EFSA PLH Panel et al., 2019 ). Information on the timing of the pupal development is not available.

In Missouri, the life cycle generally completes in three years, but some individuals develop in two years, and a few require four years (Buchanan, 1960) (figure below).

Role as vector for oak wilt disease

Buchanan (1957) reported observations of A. minutus adult activity on oak wilt fungus mats on several host trees. He suggested that the larvae boring deep into the xylem could come into contact with Bretziella fagacearum spores if the tree has oak wilt. Adults that develop from such larvae would then have the fungus on and in their bodies, and carry it to healthy trees (Buchanan, 1957). However, there is no experimental evidence for this ( EFSA PLH Panel et al., 2018 ).

This section provides the information needed to characterise the population of host plants to target in a survey, as described in the  ‘General guidelines for statistically sound and risk-based surveys of plant pests’  (EFSA et al., 2020). This includes the pest’s host range and main hosts in the EU ( Section 2.1 ), the suitability of EU environments to the pest’s establishment ( Section 2.2 ), the ability of the pest to spread ( Section 2.3 ), and the identification of risk factors associated with an increased probability of presence ( Section 2.4 ).

Once the above parameters have been defined, the target population can be structured in multiple levels. At level 1 is the survey area, which corresponds to the entirety or part of the Member State. At levels 2 and 3 are the epidemiological units that can be distinguished within the survey area. Epidemiological units can be chosen as administrative regions (e.g. EU NUTS areas or Member State-level regions) if they are homogeneous, or further subdivided into the environments where host plants are present using a land-use categorisation (e.g. urban, agricultural and natural areas, nurseries). At level 4, if risk factors are identified, the risk areas are defined around the risk locations. At level 5 are the inspection units, the elementary subdivisions of the target population that are inspected for the detection of the pest (e.g. host plants), depending on the pest detection method ( Section 3 ). For the definitions of the target population, epidemiological units and inspection units, see also the  glossary  of terms available at the end of this document.

The hierarchical structure of the target population should be tailored to the situation in each Member State. A possible structure of the target population for surveys of A. minutus within the EU is proposed in  Section 2.5 .

The main hosts of A. minutus are trees in the genera Quercus, Fagus, Populus and Ulmus ( EFSA PLH Panel et al., 2019 ;  EPPO, online-b ; Solomon, 1995). A number of other tree species are reported as potential hosts (table below) but whether A. minutus can complete its life cycle on all of them is unclear.

Detection surveys in the EU should focus on Quercus, Fagus, Populus and Ulmus species while delimiting surveys following a detection could also include all potential hosts, including European species within the genera reported as potential hosts in North America (table below).

Climatic suitability

Climatic conditions in EU territories are suitable for the establishment of A. minutus if introduced ( EFSA PLH Panel et al., 2019 ). Climate types suitable to the survival of A. minutus also overlap to a large extent with the distribution of native European Quercus, Fagus, Populus and Ulmus species ( EFSA PLH Panel et al., 2019 ).

Host availability

Species belonging to all the main host genera are present in the EU. North American species Q. coccinea, Q. rubra and Q. shumardii, which are among the most common hosts, have been introduced into the EU as ornamental and urban trees. Species belonging to genera reported as potential hosts are also present in Europe. Quercus spp. are widely distributed throughout the EU except the northern Scandinavia ( EFSA PLH Panel et al., 2019 ; Gellini and Grossoni, 1997) (figure below). Oak species can be found in pure or mixed forests (Bernetti, 1995), pure or mixed plantations (e.g. for production of timber) (Ravagni et al., 2015; Niccoli et al., 2020), or as ornamental trees in cities and parks (Bonner and Karrfalt, 2008). The distribution of Q. petraea and Q. robur largely overlaps and covers most of Europe ( EFSA et al., 2019 ). Quercus pubescens has a widespread distribution in central and southern Europe, and it is one of the most important forest tree species in south-central and southeastern Europe and Anatolia (Bordács et al., 2019). The North American species Q. palustris and Q. rubra are commonly planted in cities and parks, especially in western and central Europe ( EFSA et al., 2019 ; Euforgen, 2022).

Other confirmed host genera are also widespread in the EU territory (figure below). Beeches, F. sylvatica and F. orientalis, can be found from southern Scandinavia to Sicily, from Spain in the west to northwest Turkey in the east in pure and mixed forests, and urban parks (San-Miguel-Ayanz et al., 2016). Populus nigra and P. tremula, are widely distributed throughout the EU from warm temperate Mediterranean zones to northern boreal regions. They can be found in pure or mixed forests and plantations along with urban parks throughout Europe (San-Miguel-Ayanz et al., 2016). Elms, Ulmus glabra, U. laevis and U. minor, can be found across almost the whole of Europe in mixed broadleaved forests and urban parks (San-Miguel-Ayanz et al., 2016).

Natural spread

There are no data on the flight ability of A. minutus. As a reference, adult males of the sweet potato weevil Cylas formicarius (a well-studied species belonging to the same beetle family, Brentidae, but smaller in size than A. minutus) can cover a mean distance of 53.6 m in a 23 h flight trial in laboratory conditions (Moriya and Hiroyoshi, 1998), and 55–64 m (Sugimoto et al., 1994) and 32–162 m (Miyatake et al., 1997) per day in field conditions. The ability of C. formicarius females to disperse is low as they mainly travel by walking. Considering this, A. minutus might be expected to have a similar flight ability, covering a few hundred metres per year by natural spread.

Human-assisted spread

Long distance spread could occur through the movement of wood, woody materials and plants for planting. Majka et al. (2007) reported that A. minutus emerged from wooden furniture imported to Nova Scotia, Canada, from Indiana, USA. Hitchhiking in containers is also possible (Stanaway et al., 2001; Meurisse et al., 2019).

Individuals of Arrhenodes spp. were intercepted in France in 2005 in a consignment of wood and bark of Q. alba from the USA (EUROPHYT, online).

Identification of risk factors and their relative risk estimation are essential for performing risk-based surveys. A risk factor is a biotic or abiotic factor that increases the probability of infestation by the pest in the area of interest. The risk factors that are relevant for surveillance need to be characterised by their relative risk (should there be more than one level of risk for the target population) and the proportion of the overall target population to which they apply. The identification of risk factors needs to be tailored to the situation in each Member State. This section presents examples of risk factors for A. minutus but they are not necessarily exhaustive.

For the identification of risk areas, it is first necessary to identify the activities that could contribute to the introduction or spread of the insects. These activities should then be connected to specific locations. Around these locations, risk areas can be defined, bearing in mind that their size depends on the spread capacity of the target pest and the availability of host plants around these locations.

Example 1: Trade of infested commodities

The trade of timber into the EU territory from North America is closed for Quercus and Populus species (see  Section 1.2 ). However, the risk of introducing individuals of A. minutus into the EU from infested areas through wood and woody materials in general (e.g. unseasoned timber, stave bolts and squared wood) cannot be excluded considering the extent of the host range (see  Section 2.1 ). Thus, timber importers and sawmills should be considered as risk locations and their surrounding natural areas as risk areas (Rassati et al. 2015). Annex VII, point 34 of Implementing Regulation (EU) 2019/2072 defines the import requirements for the oak wilt pathogen B. fagacearum. In addition,  EPPO (2017)  recommends that importation of raw wood from the countries where B. fagacearum occurs should be conducted under the condition that it is transported outside of A. minutus flight periods or not transported through areas infested with A. minutus. This procedure might help to reduce the likelihood of introduction via this pathway.

Example 2: Hitchhiking in containers

Commercial trade of any kind between Europe and the native range of the beetle can present a possible pathway. In fact, adult individuals of A. minutus could be introduced as hitchhikers in containers (Stanaway et al., 2001; Meurisse et al., 2019) originating from areas where they occur. In this case, entry sites such as ports and airports should be considered as risk locations and their surrounding natural areas, including urban parks in urban-dominated landscapes, as risk areas (Rassati et al. 2015).

The figure on the right panel gives examples of the components of a target population for A. minutus and is not necessarily exhaustive.

For detection surveys of A. minutus, light traps with black light fluorescent lamps should be used in risk areas. For delimiting surveys following the finding of adult beetles in traps or that of B. fagacearum on oaks, traps could be complemented with other strategies, i.e. looking for adults aggregating under loose bark near sap-leaking wounds on dying or injured trees present at the same location where traps are set up and cutting branches or trees showing sign of infestation (i.e. oviposition and exit holes) to check for the distinctive larval galleries. Adults can be identified morphologically and confirmed molecularly (sequences are available) if needed.

3.1.1. Visual examination

Symptoms and signs

• Oviposition and exit holes: often located close to each other at or near wounds where sap is leaking from the wood (Buchanan, 1960). The oviposition holes are ‘hair-sized’ and thus more difficult to observe than the exit holes.

• Galleries: 0.2–4.0 mm wide larval galleries, which are initially slightly larger in diameter than the larvae but then increase as the larvae grow, develop straight across the grain of the wood with very little slope either up or down. The galleries almost reach the opposite side of the tree, and then make a sharp U-turn toward the entrances (Buchanan, 1960). The observationof galleries is a destructive and time-consuming operation as it requires cutting and splitting the wood. It can be considered as a possible approach in addition to trapping only for delimiting surveys.

Pest

Direct observation of adults aggregating under loose bark near wounded tissues is possible.

• Eggs: Round, less than 1 mm in diameter, translucent at first, but they gradually become opaque, and just before hatching, the outline of the larva can be seen inside (Buchanan, 1960).
• Larvae: 12–24 mm long when fully grown, with a white, cylindrical body, three pairs of thoracic legs and one pair of prolegs at the end of their abdomen (figure in the right panel) ( EFSA PLH Panel et al., 2019 ).
• Adults: 7–35 mm long, shiny, elongated, reddish brown to almost black with yellow spots on the elytrae (Sanborne, 1983; Solomon, 1995; Arnett et al., 2002;  EPPO, online-b ). The females have long and slender snouts, while the males show broad and flattened mandibles. A full morphological description is provided in Anderson and Kissinger (2002), Arnett (1968), Buchanan (1960), Davis (2017), Hayes and Kearns (1934), Sanborne (1983) and Schaeffer (1911).

3.1.2. Trapping

Adult beetles can be collected using light traps with black light fluorescent lamps (Frost, 1966, Steury et al., 2020). Light traps can be hung at a standard 2.5 m above the ground. When targeting natural areas surrounding entry points, traps should be set up in small clearings to be fully exposed on all sides (Frost, 1966). As the adults are attracted to wounded trees, whenever possible the clearing where the traps are to be hung should be close to trees with fresh (younger than a year) wounds. The other methodologies listed in the literature that were able to catch at least one A. minutus individual, such as Malaise traps (Bloem et al., 2002; Steury et al., 2020), are more complicated to use in surveillance activities due to the higher efforts needed to set them up, especially in anthropised environments such as entry points. Trap effectiveness and attraction range are not known.

3.1.3. Sample collection

Collected adult beetles should be preserved in 95% ethanolboth for further identification and for molecular analyses. For detection of B. fagacearum on the collected insect samples, refer to the specific EFSA Pest survey card relative to this pathogen ( EFSA et al., 2022 ).

3.1.4. Timing of detection and identification

• Trapping: trapping should coincide with the flight activity period of the adult beetles, which is May–August.
• Sampling adult beetles: adult beetles congregating under loose bark near wounds can be sought in May–August.
• Galleries: after beetle colonisation and larval development, galleries are visible all year round, but this requires cutting and splitting the wood.

3.2.1. Morphological identification

Morphological keys (Anderson and Kissinger, 2002; Arnett, 1968; Davis, 2017; Hayes and Kearns, 1934) allow accurate identification of the adults at species level (figure in the right panel). The genus Arrhenodes is not present in the EU; therefore, identification of the samples to genus level would provide important initial information to be further confirmed either morphologically or molecularly.

3.2.2. Laboratory testing and other methods of identification

The identity of A. minutus can be confirmed using molecularmethods. There are 25 COI sequences (14 in BOLD systems, and the other 11 in NCBI) and two RNA sequences (NCBI) belonging to A. minutus in the public databases.

Information on what, where, when and how to conduct survey activities for A. minutus is summarised in the Table on the right panel. The identification of the target population needs to be tailored to the situation in each Member State (example shown below).

The figure below shows the next steps after the survey preparation for designing statistically sound and risk-based detection and delimiting surveys of A. minutus. Guidance on selecting the type of survey, related survey preparation and design, is provided in the the  EFSA general guidelines for pest surveys  on the right panel (EFSA et al., 2020).

Scroll down the right panel to access the definitions included in the glossary.

 General guidelines for statistically sound and risk-based surveys of plant pests 

 Index of EFSA Plant Pest Survey Toolkit 

 Plant Pest Survey Cards Gallery 

 Pest survey cards: what, when, where and how to survey? 

 The statistical tool: RiBESS+ 

 The RiBESS+ manual 

 The RiBESS+ video tutorial