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Xylosandrus germanus, known generally as the alnus ambrosia beetle or black stem borer, is a species of ambrosia beetle in the family Curculionidae. The black stem borer is native to eastern Asia, but is an invasive species in Europe and North America. This species carries and feeds on associated ambrosia fungus, Ambrosiella grosmanniae.

Biology and ecology
Xylosandrus germanus are tiny insects, where females measure up to approximately two millimeters. These insects are known for boring into trees, especially those that are under distress. Trees provide protection for the beetle which ensures the progression of their life cycle. Throughout the winter months, beetles accumulate in tree trunk galleries for protection. Once the winter months pass, adults leave their galleries in search of new hosts. The beetle's search is guided by the presence of ethanol, which is synthesized in larger quantities by dead or distressed woody material. Field research conducted in Slovakia suggests that cold winter temperatures have little impact on the persistence of X. germanus.

Sex and Reproduction
Xylosandrus germanus females lay eggs in the protective galleries of host plants, and many plants serve as viable media for these insects to lay their eggs. Common sawdust types that enable X. germanus reproduction include pear, oak, white ash, European buckthorn, and red maple. Research suggests that offspring of this species hatch in a female-biased ratio, where this ratio can be influenced by the type of host species. The reproductive strategy of X. germanus follows a pattern of arrhenotoky, where only male offspring form from unfertilized eggs, and females or males can form from fertilized eggs. Given that females typically outsize males, X. germanus are also sexually dimorphic. Additionally, given the close-quartered gallery space, inbreeding is common for this species.

Symbiotic Relationships
Ambrosia beetles engage in fungal symbiosis with genera Ambrosiella and Raffaelea. Beetles carry fungi into established galleries. The transport of the fungus is achieved through the mycangia of females, a carrying structure that serves specifically for this purpose. X. germanus engages in symbiosis with Ambrosiella grosmanniae. The insect provides adequate substrate inside their galleries for fungal growth, while the mycelia of the fungus provides nutrition for the insect. Researchers have noticed that X. germanus solely appears to lay eggs succeeding the establishment of A. grosmanniae. This understanding has important implications for how this species may be managed as a pest.

For more information, see Pest management.

Invasion
Many studies based in North America and Europe have declared the invasiveness of Xylosandrus germanus.

European forests
Evidence for the invasion of X. germanus has been noted in Austria, the Czech Republic, Germany, Slovakia, Slovenia, and other countries within Europe.

It is believed that X. germanus initially entered Europe through the transport of wood from Japan to Germany in the middle of the twentieth century. Researchers now theorize that climate change and the global transport of lumber continues to bring X. germanus into new environments. The presence of X. germanus is generally inferred using ethanol traps, where the number of individuals trapped can be used to estimate species abundance. The invasion of X''. germanus'' in Europe is detrimental to the logging industry, as downed trees inhabited by the pest lose value, or are rendered unsellable.

New York apple orchards
Xylosandrus germanus invasions of New York apple orchards are detrimental to tree growth and health. Word of the damage reported in New York has also spread to Canada, where the Government of Ontario has warned orchards in the province of the damage caused by ambrosia beetles. Given this invasion, orchards are searching for optimal management strategies to reduce the economic impact of this pest.

Pest management
The persistence of X. germanus appears to be unimpacted by applying spray insecticides to the trunks of trees. The trialing of insecticides on apple orchards in New York was suggestive of their inconsistency.

Given the ineffective nature, cost, and danger associated with using chemicals for pest management, researchers have began to search for better ways to manage X. germanus. It was found that fungi in genus Trichoderma could reduce the growth of A. grosmanniae, the symbiont of choice for X. germanus. The hindered growth of A. grosmanniae ultimately causes X. germanus egg production to cease. Utilizing Trichoderma may be an adequate alternative to gain control of X. germanus damage and invasion.