Inviting trouble: How the tree of heaven promotes the advance of the stink bug

All over the world, plants are growing in places where they do not actually belong - because humans have intentionally introduced or spread them accidentally. This is a decisive factor in the increasing occurrence of harmful, invasive insect species, which can cause major damage to the environment, biodiversity and the economy. This is the conclusion of an international research team including the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.

For agriculture, forestry or as ornamental plants, humans have brought countless plants to countries or even continents where they did not originally occur. Some of these have become invasive and are spreading at the expense of the native flora. Researchers have now discovered that this favours invasive insects.

The research team – including experts from Switzerland, the US, and South Africa – analysed how the spread of such plants supports insect invasions and identified global patterns involved. They also reviewed numerous case studies of introduced insects. Their findings underscore that the growing prevalence of these plants is intensifying insect invasions worldwide. This is because insects tend to settle in new areas when their food plants already exist there. They are the “stepping stones”, so to speak, for the insects to establish themselves and spread further.

Stink bugs on tree of heaven

One example from Switzerland is the brown marmorated stink bug (Halyomorpha halys), a fruit and vegetable pest originating from East Asia, which was first discovered in Europe in Zurich in 2004. Now, in autumn, it often enters warm homes and is known as a “stink bug” due to its foul-smelling defensive secretion. “The brown marmorated stink bug probably invaded in part because several of its preferred host plants, including the tree of heaven (Ailanthus altissima) and the butterfly bush (Buddleja davidii), were planted extensively. These are garden plants that also originate from East Asia,” says Eckehard Brockerhoff, an insect expert at the Swiss Federal Institute for Forest Snow and Landscape Research WSL and co-author of the study.

The research team further warns that the number of invasions may continue to rise. This relates to the concept of “invasion debt”: A large number of plant species that already thrive outside their natural distribution areas could give many other insects from the same regions a head start in the future. The problem is therefore likely to become even worse in the future.

The researchers thus emphasise the importance of biosecurity measures for both plants and insects, such as regulations and treatment with pesticides for imports. “Although Switzerland and Europe have implemented many such measures, it is impossible for them to be 100 percent effective, given the large volume of intercontinental trade. Therefore, it is also important, that garden owners choose native plants over non-native ones, to avoid the spread of invasive plants and insects,” Brockerhoff says. A recent sales ban on various non-native plants, such as the Chinese windmill palm (Trachycarpus fortunei), known as the “Ticino palm,” also serves this purpose.