Integrated Management of Xylosandrus Germanus in Plant Nurseries

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According to Weber and McPherson, over 200 species in 51 families are attacked were by X. germanus. These include oak (Quercus spp.), elm (Ulmus spp.), maple (Acer spp.), beech (Fagus spp.), redbud (Cercis spp.), hickory (Carya spp.), chestnut (Castanea spp.), magnolia (Magnolia spp.), dogwood (Cornus spp.), and black walnut (Juglans nigra L.). Infestations in apple (Malus domestica Borkh.) orchards were first reported in Ohio in 1982.

In some case, X. germanus may attack on apparently healthy trees but physiologically stressed trees, unhealthy, or dying trees are mostly targeted for colonization. Ambrosia beetles have efficient host-seeking capabilities, able to differentiate among slight differences in host vigor. In fact, compared with healthy specimens, stressed trees produce greater concentrations of several volatile organic compounds, including acetaldehyde, acetone, ethane, ethanol, ethylene and methanol. X. germanus is attracted to these volatiles especially ethanol emitted from those plants. Moreover, while stressed species are mostly attacked, reports from ornamental plant nurseries and tree fruit orchards also involve thin barked deciduous species. Generally, X. germanus prefers stems of <10 cm in diameter and trunks over branches.

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Adult female X. germanus tunnel horizontally into the xylem of stems or trunks and sometimes branches and exposed roots and create 1-mm-diameter holes to form galleries. Tunnels widen into brood chambers and additional tunnels may extend vertically with and across the grain of the wood. These tunnels may extend into the pith in stems with small diameter or may branch once or twice in association with a horizontal brood chamber. Galleries are excavated by the foundress, and comprise entrance tunnels, brood chambers containing eggs and immatures, and branch tunnels where the young develop; this arrangement accommodates all life stages and developmental processes of the insect’s life history

Adult females carry spores within an invaginated, membranous, and pouch-like structure called a mycangium, located between the pronotum and mesothorax. Spores of the symbiotic fungi, called Ambrosiella grosmanniae are transferred from the beetles into their host tunnels during excavation. The beetles feed directly on this fungal growth that is visible in the galleries as an abundant grayish-white mycelial growth, rather than the host plant tissue. Notably, adult female X. germanus initiates oviposition only after their symbiotic fungus is established within the gallery. Old galleries contain the mycelial form of the fungus and not the white ambrosial form.

Socio-economic impacts of X. germanus

Mass attacks by X. germanus and other opportunistic generalist ambrosia beetles can quickly result in extensive losses within ornamental nurseries. Plants can exhibit rapid dieback following attacks by X. germanus especially during spring months, partly attributed to symbiotic and opportunistic microorganisms associated with X. germanus. Attacks do not always result in plant death, but growth, aesthetic, and economic value of ornamental nursery plants can be negatively affected. It is sometimes not easy to detect a plant attacked by X. germanus due to a very small gallery entrance, but toothpick-like extrusions of chewed wood material projecting up to 3–4 cm out from the stems are a characteristic symptom of attacks by X. germanus. Sap stains on bark, wilting foliage and branch dieback are some of other symptoms of the attack. X. germanusis a serious pest in tree nurseries and can cause losses in excess of US $5,000 per nursery for nurseries across the southeastern United States and in Texas.

Control of this pest is a challenge, as it is not possible to determine whether or when an orchard might be susceptible to X. germanus attack or to identify which trees will be targeted. Moreover, a variety of other factors including a broad host range, wood-boring habit, fungal symbionts that serve as their food source, and haplodiploid reproduction make it even harder to manage. Despite the repeated application of insecticides, growers still lose substantial numbers of trees to ambrosia beetle damage each year, which proves incomplete insecticide coverage of tree trunks or insecticides losing efficacy between applications.

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