Agrilus biguttatus

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Agrilus biguttatus
Agrilus biguttatus from France.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Buprestidae
Subfamily: Agrilinae
Tribe: Agrilini
Subtribe: Agrilina
Genus: Agrilus
Species:
A. biguttatus
Binomial name
Agrilus biguttatus
Synonyms
  • Agrilus (Anambus) biguttatus (Fabricius, 1776)
  • Agrilus bicolor Fleischer, 1930
  • Cucujus octoguttatus Geoffroy in Fourcroy, 1785
  • Buprestis pannonicus Piller & Mitterpacher, 1783
  • Agrilus pannonicus (Piller & Mitterpacher, 1783)
Agrilus biguttatus laying eggs

Agrilus biguttatus is a species of beetle in the family Buprestidae, the jewel beetles. Common names include oak splendour beetle, oak buprestid beetle, and two-spotted oak borer. [1] This beetle is known as a pest that causes damage to oak trees and is a major factor in oak decline. [2]

Contents

The adult beetle is 8 to 13 millimeters long. [1] It is metallic green in color with a black or yellow cast. [3] There is a pair of white spots on the inner edges of the elytra. The larva is a creamy white color and measures up to 43 millimeters in length. The first thoracic segment is enlarged. The grub is legless and has a pair of horns on its last abdominal segment. [2] The beetles have an average lifespan of 2 months, but some may live upwards of 5 months. [3]

Taxonomy

Agrilus biguttatus is part of the subfamily Agrilinae, within the family Buprestidae in the order Coleoptera. Agrilinae is the largest, most genetically diverse subfamily in Buprestidae. It is made up of four tribes: Coraebini, Aphanisticini, Tracheini and Agrilini. [4] Agrilus biguttatus is part of tribe Agrilini under the subtribe Agrilina. Based on when Buprestidae was estimated to have diverged, it suggests that its ancestors had a habit of wood-boring larval feeding. Nonetheless, some groups in Agrilinae shifted to eating leaves. [5]

The Agrilus genus of buprestid beetles is known for its remarkable species diversity, with over three thousand species identified worldwide. [6] These beetles' larvae typically mature inside the vascular tissues of trees or woody plants, showing a strong preference for specific host plants. Their method of development has been found to contribute to significant oak decline.

Distribution and habitat

Agrilus biguttatus is widespread throughout Europe, reaching the northern borders of southern Sweden and the UK. North Africa, and Northern Asia. A biguttatus can fly several kilometres but their distribution is restricted by cool temperatures. [5] Until the 1970s, these beetles were considered rare in the UK. In the mid 1990s, A. biguttatus had begun expanding from its native range in Denmark and the Netherlands. Researchers had proposed that the expansion is largely due to global warming because it tends to live in more temperate areas. The beetle inhabits many environments, including deserts, xeric shrublands, Mediterranean scrub, mixed forests, and temperate coniferous forests. [5] It can also be transported to new areas with shipments of wood. [2] While Agrilus biguttatus is not currently found in the United States, its introduction could pose a significant risk to US natural environments. [5]

The main hosts of this insect are oak species, including English oak (Quercus robur), sessile oak (Q. petraea), downy oak (Q. pubescens), evergreen oak (Q. ilex), cork oak (Q. suber) and turkey oak (Q. cerris). The beetle can also be found on European beech (Fagus sylvatica) and chestnut (Castanea sativa). [2]

In Europe, species such as A. Sulcicollis Lacordaire, A. angustulus Illiger, and A. biguttatus have been identified as significant contributors to oak decline due to their development pattern. [6] While some species prefer infesting trees with smaller stem diameters or developing in branch bark, A. biguttatus larvae typically grow within the bark of the main trunk. [7]

Lifecycle

Mating and host selection

Adult beetles usually complete pupation and emerge from May to July. These newly emerged adults typically move up to the canopy of mature oak trees to feed on oak foliage, sycamore and beech trees. [7] Males choose their mates while flying. Laboratory studies have shown that they also use volatile cues, known as pheromones, to inform their choices and locate feeding and mating sites. The beetles were found to follow pheromones from fresh oak foliage to locate their host trees. [8] Their antennae are able to respond to specific components of oak leaf odor—namely, (Z)-3-hexenal, (Z)-3-hexen-1-ol, (E)-2-hexenal, and (Z)-3-hexenyl acetate—to help them navigate their environments. [8] After mating, mated females use olfactory signals to descend to the trunks and lay their eggs. They deposit eggs within cracks and crevices in the bark of suitable host trees, often between bark plates where larvae can easily tunnel through the oak tree to reach its vascular tissue. A majority of the beetle's life cycle occurs within the tissue of the mature oak. [8]

Agrilus biguttatus prefers to reside in old oak trees of 30-40 cm in diameter [6] with an inner bark thickness of 10-13 mm. [9] While healthy trees can generally resist A. biguttatus infestations, female beetles tend to live in trees that are physiologically stressed, often as a result of drought conditions. Moisture within the inner bark tissues is critical for A. biguttatus larval development. Larvae typically feed toward the ground, because there is greater water content the lower it feeds in the tree stem. Additionally, they have been observed to lay new generations of eggs under those laid by previous generations on their host trees. [10] It is important to note that the beetle has a brief period of time to invade the host. The window occurs between the time when drought conditions reduce the tree's resistance to infestation, and the time of the death of the tree, which severely reduces water content in vascular tissue. Ultimately, careful host selection can promote optimal larval development.

Larval development to adult emergence

Agrilus biguttatus larvae.jpg

Under artificial conditions, beetles would lay soft-shelled eggs in clusters of up to thirty. Agrilus biguttatus show a preference for laying their eggs and allowing initial larval development to occur on the warmer, south-facing side of host trees. [7] Temperature determines the rate of egg development, which has been observed to range from 2 to 6 weeks under laboratory conditions. [11]

This species requires warmer temperatures for its development. A study measured the development rates of A. biguttatus in terms of the time it takes for the eggs, larvae, and pupae to reach adulthood. [3] The development rates of eggs, larvae, and pupae have lower developmental thresholds of 12.1, 11.9, and 15.1 °C, respectively. [11] Development rates displayed a positive relationship with temperature, meaning that as temperature increases, development rate also increases. Likewise, as the temperature decreases, development rate decreases. The study found evidence to support that the beetle's distribution is limited by its thermal requirements, and it may not be able to survive or reproduce effectively in cooler regions.

Newly hatched larvae burrow into the inner bark to feed. Their feeding behavior hollows out the tree, and they can create zigzag galleries up to 1.5 meters long, which results in partial girdling of the tree. [7] As the larvae grow, A. biguttatus can widen from less than 1 millimeter to 3 to 4 millimeters in diameter. Larvae create galleries that twist irregularly in all directions. This behavior leads to structural abnormalities in the tree's cambium tissue. [7] Moreover, branches and twigs at the top of the tree may die off, causing the crown to appear sparse with scantily clustered foliage. Eventually, if the damage is too severe, the tree can succumb to the larval activity and die. [7]

The larva undergoes five instars during its development. Generation time, the time it takes for A. biguttatus to produce a generation, can change depending on location and varies between one or two years. [7] Their life cycle is prominently influenced by local conditions, with temperature and health of the host tree being crucial factors. In laboratory settings, all larval samples must undergo a period of cold temperatures, to mimic overwintering, to progress in their development. This indicates that A. biguttatus must enter a state of "obligatory prepupal diapause", which they achieve through overwintering; [3] otherwise they would die. The pupal overwinters by curling up inside pupal chambers located in the bark, which are 10.4 to 14.8 by 3 to 4.5 mm in size. [6] Pupation happens in the spring, lasting about 14 days. Following pupation, the adult beetle emerges through a D-shaped exit hole, typically measuring 2.5 to 4 mm in diameter. [7]

Relationship with oak hosts

Interactions with humans and oak trees

Damage of Acute Oak Decline shown on an Oak tree. Acute oak decline fig01.jpg
Damage of Acute Oak Decline shown on an Oak tree.

Beetles have a considerable impact on the decline of oak trees across Europe, a phenomenon known as acute oak decline (AOD). One of the major symptoms of AOD is the presence of black fluid seeping from cracks in the bark, which often leads to the tree's death within a few years. [7]  If given enough time without further infestation, they are likely to recover. However, A. biguttatus, considered a secondary pest to oak, poses a serious threat by invading weakened trees before they can undergo the process of recovery. [12] Symptomatic oak trees often exhibit distinctive D-shaped exit holes from emerging A. biguttatus adults. The decline of oak trees in Europe is believed to have been exacerbated by warmer climates and drought, as those conditions are optimal for A. biguttatus growth and host invasion. Documented evidence shows Agrilus pest incidence in Germany, France, Hungary, Poland, and England, contributing significantly to the increase in tree mortalities. [6]

Host defense

Oak trees have established physical defenses against wood-boring pests. Agrilus biguttatus often exploits weakened trees, and dark fluid is produced when bacteria enter the wounded oak bark. Presence of A. biguttatus has also been found to invite pathogenic honey fungi, Armillaria , to grow in the stems of oak trees. [6] Despite fluid leakage being a symptom of oak decline, mid-instar larvae have been observed to drown in their galleries from these excretions, thus minimizing their spread through the tree's vascular tissue. [13] Robust callusing is a sign of the tree's health and its capacity to fend off attacks and recover. Furthermore, this defense is particularly effective against early larval instars, which have slower feeding and movement rates, enabling the host's calluses to suffocate them within the galleries. [14]

Relationship with surrounding environment

Due to its damaging effects on oak trees, efforts have been made to manage the A. biguttatus population. Common strategies for controlling this beetle in Britain include the following:

Removal of infested stems

Removing heavily infested stems, particularly those exhibiting sudden and rapid decline, significant foliage and twig loss, and signs of Agrilus presence like exit holes and galleries beneath the bark can help reduce the beetle population. This should ideally be carried out before the spring when new A. biguttatus adults emerge after they have completed larval development. [7] Notably, to avoid creating sunny open areas that attract female beetles seeking oviposition sites, planting trees that can provide shade, particularly along the southern side of woodlands, can be beneficial. This strategy aims to discourage A. biguttatus beetles from laying eggs there. [7]

Insecticides

When insecticides are applied to the tree's surface before adult emergence, there is evidence supporting its efficacy in controlling beetles in their pupal chambers within the outer bark. [7] However, the need for repeat applications and the cost of the treatment can be burdensome. Additionally, insecticides used on oaks could lead to the death of many other non-pests. Research is being conducted on the use of semi chemicals, natural behavior-modifying chemicals that mimic the volatiles emitted by oak tree hosts, to more effectively manage the beetle population, as there are no known baits or traps for A. biguttatus. [8]

Predation

Woodpeckers have been shown to prey on older larvae. Creating environments that appeal to woodpeckers can aid in controlling buprestid populations. Additionally, larvae of A. biguttatus can be parasitized by wasps. [6] The species locates larvae within the bark before laying multiple eggs near them, which provides the wasp larvae a food source after hatching. Common predators of A. biguttatus include: the European red-bellied clerid ( Thanasimus formicarius ), and the wasp Dolichomitus imperator from the family Ichneumonidae . [15] Wasps are currently being researched as a potential biocontrol method in the USA. [14]

Related Research Articles

<span class="mw-page-title-main">Buprestidae</span> Family of insects

Buprestidae is a family of beetles known as jewel beetles or metallic wood-boring beetles because of their glossy iridescent colors. Larvae of this family are known as flatheaded borers. The family is among the largest of the beetles, with some 15,500 species known in 775 genera. In addition, almost 100 fossil species have been described.

<span class="mw-page-title-main">Emerald ash borer</span> Species of beetle

The emerald ash borer, also known by the acronym EAB, is a green buprestid or jewel beetle native to north-eastern Asia that feeds on ash species. Females lay eggs in bark crevices on ash trees, and larvae feed underneath the bark of ash trees to emerge as adults in one to two years. In its native range, it is typically found at low densities and does not cause significant damage to trees native to the area. Outside its native range, it is an invasive species and is highly destructive to ash trees native to Europe and North America. Before it was found in North America, very little was known about the emerald ash borer in its native range; this has resulted in much of the research on its biology being focused in North America. Local governments in North America are attempting to control it by monitoring its spread, diversifying tree species, and through the use of insecticides and biological control.

<span class="mw-page-title-main">Bark beetle</span> Subfamily of beetles

A bark beetle is the common name for the subfamily of beetles Scolytinae. Previously, this was considered a distinct family (Scolytidae), but is now understood to be a specialized clade of the "true weevil" family (Curculionidae). Although the term "bark beetle" refers to the fact that many species feed in the inner bark (phloem) layer of trees, the subfamily also has many species with other lifestyles, including some that bore into wood, feed in fruit and seeds, or tunnel into herbaceous plants. Well-known species are members of the type genus Scolytus, namely the European elm bark beetle S. multistriatus and the large elm bark beetle S. scolytus, which like the American elm bark beetle Hylurgopinus rufipes, transmit Dutch elm disease fungi (Ophiostoma). The mountain pine beetle Dendroctonus ponderosae, southern pine beetle Dendroctonus frontalis, and their near relatives are major pests of conifer forests in North America. A similarly aggressive species in Europe is the spruce ips Ips typographus. A tiny bark beetle, the coffee berry borer, Hypothenemus hampei is a major pest on coffee plantations around the world.

<span class="mw-page-title-main">Citrus long-horned beetle</span> Species of beetle

Anoplophora chinensis, the citrus long-horned beetle, is a long-horned beetle native to Japan, China, Korea, Taiwan and Southeast Asia where it is considered a serious pest. This beetle has invaded several countries in Europe, including Italy, Switzerland, Turkey, France, Germany, and Croatia.

<i>Agrilus anxius</i> Species of beetle

Agrilus anxius, the bronze birch borer, is a wood-boring buprestid beetle native to North America, more numerous in the warmer parts of the continent and rare in the north. It is a serious pest on birch trees (Betula), frequently killing them. The river birch Betula nigra is the most resistant species, while other American birches are less so. European and Asian birches have no resistance to it at all and are effectively impossible to grow in the eastern United States as a result.

<i>Fraxinus pennsylvanica</i> Species of ash

Fraxinus pennsylvanica, the green ash or red ash, is a species of ash native to eastern and central North America, from Nova Scotia west to southeastern Alberta and eastern Colorado, south to northern Florida, and southwest to Oklahoma and eastern Texas. It has spread and become naturalized in much of the western United States and also in Argentina and Europe, from Spain to Russia.

<i>Thanasimus formicarius</i> Species of beetle

The ant beetle, also known as the European red-bellied clerid, is a medium size insect, rather soft-bodied, with strong mandibles that can tear between the hard sclerotized integument of bark beetles. Larvae and adults are common predators of bark beetles in Europe.

<span class="mw-page-title-main">Woodboring beetle</span> Common name for a beetle that eats wood

The term woodboring beetle encompasses many species and families of beetles whose larval or adult forms eat and destroy wood. In the woodworking industry, larval stages of some are sometimes referred to as woodworms. The three most species-rich families of woodboring beetles are longhorn beetles, bark beetles and weevils, and metallic flat-headed borers. Woodboring is thought to be the ancestral ecology of beetles, and bores made by beetles in fossil wood extend back to the earliest fossil record of beetles in the Early Permian (Asselian), around 295-300 million years ago.

<span class="mw-page-title-main">Cleridae</span> Checkered beetles

Cleridae are a family of beetles of the superfamily Cleroidea. They are commonly known as checkered beetles. The family Cleridae has a worldwide distribution, and a variety of habitats and feeding preferences.

<span class="mw-page-title-main">Wharf borer</span> Species of beetle

The wharf borer, Nacerdes melanura, belongs to the insect order Coleoptera, the beetles. They belong to the family Oedemeridae, known as false blister beetles. Wharf borers are present in all the states of the USA except for Florida. It takes about a year to develop from an egg to an adult. The name 'wharf borer' comes from the larval stage of this insect, which often lingers on pilings and timbers of wharves, especially along coastal areas. The adult beetles are identifiable via a black band across the end of both elytra. In addition, wharf borers are distinct from other members of the family Oedemeridae due to the presence of a single spur on the tibia of the forelegs and the distance between both eyes. The female beetle oviposits eggs on rotten wood, on which the larvae hatch, burrow, then feed. Adults do not eat and depend on stored energy reserves accumulated as a larva. They are considered a pest because they damage wood used in building infrastructures.

<span class="mw-page-title-main">Acute oak decline</span> Disease of oak trees

Acute oak decline (AOD) is a disease that infects oak trees originally described in the UK. It mainly affects mature oak trees of over 50 years old of both Britain's native oak species: the pedunculate oak and the sessile oak. The disease is characterised by the trees bleeding or oozing a dark fluid from small lesions or splits in their bark. Unlike chronic oak decline, acute oak decline can lead to the death of trees within 4 to 5 years of symptoms appearing. The number of trees affected is thought to number in the low thousands, with a higher number of infected trees being found in the Midlands.

<i>Dinoderus minutus</i> Species of beetle

Dinoderus minutus, the Bamboo borer, is a species of wood-boring beetle. In tropical regions, it is one of the main pests of bamboo, attracted by the internal starch. It is native to Asia but has spread widely with the trade of infested bamboo wood and bamboo products.

<i>Tetrastichus planipennisi</i> Species of wasp

Tetrastichus planipennisi is a parasitic non-stinging wasp of the family Eulophidae which is native to North Asia. It is a parasitoid of the emerald ash borer, an invasive species which has destroyed tens of millions of ash trees in its introduced range in North America. As part of the campaign against the emerald ash borer (EAB), American scientists in conjunction with the Chinese Academy of Forestry searched since 2003 for its natural enemies in the wild leading to the discovery of several parasitoid wasps, including Tetrastichus planipennisi which is a gregarious endoparasitoid of EAB larvae on Manchurian Ash and has been recorded to attack and kill up to 50 percent of EAB larvae.

<i>Spathius agrili</i> Species of wasp

Spathius agrili is a parasitic non-stinging wasp of family Braconidae which is native to North Asia. It is a parasitoid of the emerald ash borer, an invasive species which has destroyed tens of millions of ash trees in its introduced range in North America. As part of the campaign against the emerald ash borer (EAB), American scientists in conjunction with the Chinese Academy of Forestry began searching in 2003 for its natural enemies in the wild, leading to the discovery of several parasitoid wasp species, including Spathius agrili. S. agrili was discovered in Tianjin, China where it is a prevalent parasitoid of EAB larvae in stands of an introduced ash species, and an endemic ash species. S. agrili has been recorded to attack and kill up to 90 percent of EAB larvae.

<i>Agrilus auroguttatus</i> Species of beetle

Agrilus auroguttatus is a species of jewel beetle in the United States, known by the common name goldspotted oak borer. Native to southeastern Arizona, it is best known for its probably invasive presence in California, destroying stands of oak trees in the Cleveland National Forest in San Diego County. It was originally considered a subspecies of the Central American species Agrilus coxalis, and much of the literature refers to it by this name, but now it is regarded as a separate species, known only from Arizona and California.

<i>Phoracantha semipunctata</i> Species of beetle

Phoracantha semipunctata, the Australian Eucalyptus longhorn, is a species of beetle in the family Cerambycidae. Native to Australia, it has now spread to many parts of the world, including practically all countries where tree species of Eucalyptus have been introduced. It has been classified as an invasive pest species of Eucalyptus outside Australia.

<i>Platypus cylindrus</i> Species of beetle

Platypus cylindrus, commonly known as the oak pinhole borer, is a species of ambrosia beetle in the weevil family Scolytinae. The adults and larvae burrow under the bark of mature oak trees. It is native to Europe.

<i>Dendroctonus micans</i> Species of beetle

Dendroctonus micans, the great spruce bark beetle, is a species of bark beetle native to the coniferous forests of Europe and Asia. The beetles burrow into the bark of spruce trees and lay eggs which develop into larvae that feed on the woody layers under the bark.

<i>Arrenodes minutus</i> Species of beetle

Arrenodes is a genus of primitive weevils belonging to the family Brentidae, containing a single described species, Arrenodes minutus, commonly known as the oak timberworm. These beetles are pests of hardwoods in North America. Adult oak timberworms are shiny, elongate, and range 7 to 25 mm in length. They are reddish-brown to brownish-black in coloration, with yellow spots on their elytra. Adults display strong sexual dimorphism; females have long, slender, straight mouthparts, while males possess flattened, broadened mouthparts with large mandibles. Males are known to be aggressive and use these large mandibles for combat. These mandibles are also used in courtship. Larvae are elongate, cylindrical, white, and curved. They have three pairs of jointed legs on the thorax and one pair of prolegs near the end of the abdomen.

<i>Synanthedon resplendens</i> North American clearwing moth

Synanthedon resplendens, also known as the sycamore borer moth, is species of a clearwing moth native to western North America. Larva of this moth live under the bark of sycamore, ceanothus, coast live oak, and, on rare occasions, avocado trees. Sycamore hosts include California sycamore, Arizona sycamore, and American sycamore trees. Coast live oak is a confirmed host, but this moth likely uses many or most other western oaks as well.

References

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Further reading