Hi welcome to Taiga Bonzai, we continue our discussion on pests and disease with a selection of beetles whom bore their way into the trunk of trees. The damage caused by these insects is often results in the tree’s demise.

Introduction – borers are perhaps the most harmful to trees, they once were endemic to certain parts of the globe, but are now found in other parts of the world. Eradication of these insects is an on going battle with no conclusive results.

Airborne invasion – we know that many pests and disease have migrated throughout the world by conventional methods, in the packaging containing merchandise and sometimes in the merchandise itself; mainly via land sea and air.

However, these modes of transportation are not the only way for pests and disease to migrate to other realms. There are those whom are able to take to the wing and reach altitudes of 2,000m some actually fly, whilst other drift on the air currents.

Dutch elm disease (DED) – first appeared in the north-west of Europe about 1910 and between 1914 and 1919, several Dutch scientists carried out influential research on the cause of the disease. According to Forest research.gov.uk this disease “Is one of the most serious tree diseases in the world.” The fungus that causes the disease is spread by bark beetles triggering foliage and tip dieback in all of Britain’s native elms: Ulmus procera, U. carpinifolia and U. glabra. https://www.forestresearch.gov.uk

The disease first spread to Britain in the 1920s, where it killed 10-40% of elm trees. Although the initial epidemic died down, a more aggressive species of Dutch elm disease fungus was accidentally introduced into Britain in the 1960s.

This second epidemic took hold of lowland central and southern Britain where there were English elms in the early to mid-1970s and by 1980, most mature English elms had died. Scattered pockets of mature elm occasionally survived where the geographic situation has facilitated an effective and continuing sanitation control programme.

By the late 1980s the bark beetles used up most of the mature elms that they relied on for breeding material, so beetle populations declined and the disease virtually disappeared from many southern and south-western areas.

In 1982, Forestry Commission research on the biology of Ophiostoma novo-ulmi, an extremely virulent species from Japan has devastated elms in Europe, North America, Asia and now is spreading across across Eastern Europe. (Romania to Poland)

This suggested that the disease would not decline in intensity or contrast to the first epidemic, caused by O. ulmi. The new pathogen, O. novo-ulmi, would return in a continuing cycle to attack the following generation of small elms once they were large enough to support beetle breeding.

Cryphonectria parasitica  – a pathogenic fungus a member of the Ascomycota (sac fungi) native to East Asia and south-east Asia was introduced into Europe and North America in the early 1900s spreading rapidly causing significant tree loss in both regions.

This disease came to be known as ‘chestnut blight’ due its infestation of Chestnut trees Castanea dentata, it has had a devastating economic and social impact on communities in the eastern United States. Once a tree begins to decline it is often dead within a few years and eradication efforts by cutting and burning the infected plants/trees have mostly failed. At this present juncture there are no chemical management options for control.

Sirex wood wasp – (Sirex noctilio) a species of horntail native to Europe, Asia, and North Africa is an invasive species in other realms. These include Australia, New Zealand, North and South America and South Africa, where it has become a significant economic pest of pine trees especially Pinus radiata.

The wasp can attack a wide variety of pine species, although some species seem to be more susceptible than others and stressed trees are often attacked. It is believed that this insect was introduced on unprocessed pine logs imported from Europe.

P. radiata were first planted in the late nineteenth century in Australia, Chile, New Zealand and in South Africa during the early 1900’s. Their excellent growth provided the basis for thriving lumber and paper industries. During 1920’s and 30’s the lumber industry stagnated, because the demand for small logs from thinning operations decreased.

Hence thinning ceased which made plantations susceptible to S. noctilio and its associated fungus, Amylostereum areolatum. By 1947, high levels of tree mortality were occurring, primarily in the un-thinned plantations causing devastation to the lumber and paper industries.

Adult sirex wood wasps vary in size from 9 to 36 mm (0.35 to 1.42 in), during oviposition the female will lays 2 eggs often with a mucoid substance and a symbiotic fungus to feed on once hatched.

This mucoid substance is toxic to trees as are the ascospores from symbiotic fungus Amylostereum areolatum a species of crust fungus originally called Thelephora areolata. It was given its current name by French mycologist Jacques Boidin in 1958.

The Asian Longhorned beetle – Anoplophora glabripennis native to eastern China, and Korea has been introduced into the United States, where it was first discovered in 1996. Also in Canada and several countries in Europe including, Austria, France, Germany, Italy and UK.

This beetle is believed to have been spread from Asia in solid wood packaging material. A. glabripennis primarily infest maple, poplar, willow, and elm trees. In the United States it has attacked birch, katsura, ash, planes and Sorbus. In Canada on maple, birch, poplar and willow and in Europe on maple, alder, birch, hornbeam, beech, ash, planes, poplar, Prunus, Sorbus and willow.

The Bronze Birch borer – Agrilus anxius is a wood-boring Buprestid beetle native to North America, numerous in warmer parts of the continent where it thrives. It is a serious pest on birch trees Betula frequently killing them. If this insect came to Europe there would be no hope for Birch forests, as the trees have no resistance against this species of insect; hence the effect on Scandinavia’s Birch industry would be catastrophic.

The Emerald Ash Borer – Agrilus planipennis, a devastating alien pest of ash trees was first detected in Europe in Moscow in 2003. Its outbreak in the cities of Western Russia seriously damaged plantations of Ash trees Fraxinus pennsylvanica introduced from North America.

This alien pest posing a major threat to ash trees all over Europe, has spread to Ukraine and the south of Western Russia. It severely damages the green ash F. pennsylvanica, research indicates that will appear in other European countries soon with the potential to destroy F. pennsylvanica plantations.

The Chinese emerald ash borer often referred to as The green menace found its way to America via international shipping. When it gained its freedom it was greeted with a fresh new ‘smorgasbord’ of North American ash trees Fraxinus americana. To date the amount of devastation to millions of ash trees is now in the tens of millions across 25 states.

European spruce bark beetle – Ips typographus, is a species of beetle in the weevil sub family Scolytinae, they are endemic from Europe to Asia Minor and some parts of Africa. Bark beetles are so named because they reproduce in the inner bark of living and dead phloem tissues of trees. In ideal conditions they can travel up to almost a kilometre (½ a mile) in search of a vulnerable host.

Once the host is located, the adult burrows through the weakened bark in order to build tunnels where they can mate and lay eggs. They release pheromones to attract more individuals to the host tree. Two to five weeks after contamination, they may migrate to another host and repeat the process.

European bark beetles have the ability to spread rapidly over large areas and trees in the genera Picea (spruce), Abies (fir), Pinus (pine), and Larix (larch) are this beetles’ trees of choice. Healthy trees use defences by the production resin or latex, which may contain several insecticidal and fungicidal compounds; that kill or injure attacking insects. However in many cases this form of defence is overwhelmed by this pest. 

The insects (from Latin insectum) mentioned here are hexapod invertebrates, the largest group within the arthropod phylum. They are able to create their own colonies if conditions allow, attacking vulnerable vegetation of all types. At this juncture methods of control are inadequate, the old adage that ‘Prevention is better than cure’ is meaningful. But the devastation of ‘unseen invasion’ only becomes visible when it is too late to react.

What we have mentioned here is but a very brief glance into the world of hexapod invertebrates. In the next article of ‘Unseen invasion’ we continue our discussion on more of these harmful pests that are a constant threat to horticulture and bonsai. Until next time, BW, Nik.