Bringing trees to potential invasive species

The expansion of international trade, rapid transport, increasing sales of decorative plants and agricultural goods, and global warming are all factors which contribute to the unintentional introduction and survival of organisms into new geographical zones often far from the region of origin.  Although only a small fraction of accidentally introduced species become invasive and harmful, the financial costs for agriculture and forestry industry can be significant, not to mention that some species represent risks to human health and other severely impact local biodiversity.

Quarantine measures to prevent insect invasions tend to focus on well-known pests but a large proportion of the recent invaders were not known to cause significant damage in their native range, or were not even known to science before their introduction.

The majority of exotic species which are potentially harmful to plants in Europe come from Asia. Consequently, European scientists started working with colleagues in China to study the ability of Chinese insect and fungal pathogens to colonize European trees. They planted seven species of European trees at two sites in China. They chose five broadleaved species (hornbeam, beech and three species of oak) and two conifers (cypress and pine). One hundred trees of each species, initially each measuring about 1.5 m, were planted at each site in adjoining lots of 25 plants each. In total, 1100 trees were planted. Between 2007 and 2011, researchers regularly monitored the colonization of these trees by local insects and fungi closely.

Over the course of this four-year period, each tree was examined on a regular basis to identify and count adult insects and larvae and any damage found. The insects were then collected. Different types of damage to foliage, buds, branches or trunks were noted and photographed. Using reared insects, researchers then tried to link each type of damage to the insects present, and larvae and adult insects were kept for morphology based taxonomy and DNA Barcoding.

In total, 104 insect species were observed on the new host trees. Many simply fed on leaves on an occasional basis, but multiple colonization events were recorded for 38 species primarily on sessile oak (Quercus petraea). At least six species could successfully reproduce on European trees as their larva colonized the new trees. All 38 species are considered to be potentially invasive in case they are introduced to Europe. Surprisingly, most of these species appear to be originally linked to agriculture and fruit trees rather than the surrounding local flora.

The maximal rate of colonization was three years. Nearly all the trees survived the first year. After that, the mortality rate was significantly high at both sites, though important differences were noted between varieties. After three years of testing, only 99 of the 400 trees planted at a site in Beijing were still alive: all but four of the conifers were dead but half of the oaks survived. At the other site in Fuyang, the sessile oak was the only species with a survival rate of nearly 50%.

This sentinel tree method appears to be very promising, and its possible use in other contexts is being examined within another EU framework (Global Warning). One major road block is the difficulty to identify insects, particularly at early life stages, and as well as other pathogens using standard methods. This is clearly an issue that could be solved by intensifying our efforts to build DNA Barcode reference libraries.