From plant-eating, flying beetles, to parasites which themselves attach to beetles and other bugs, to those species which can affect wooden items, such as pallets, there is a range of potential pests which could devastate Estonia's forests and gardens.

This would be particularly poignant given Estonia has for over 20 years held the EU's rare protected zone status, which means strict restrictions on plants entering the country from other EU member states and the use of "plant passports."

Alien species rarely reach Estonia of their own accord. Instead, they arrive with the aid of human activity. Global supply chains, timber imports and busy international travel all provide ideal pathways for insects and plant diseases to spread. The authorities' watchlist now has nearly 500 different threats.

Speaking to radio program "Ökoskoop," Ülle Metsmann, plant health adviser at the state Agriculture and Food Board (PTA), stressed how critical prevention is, starting at Estonia's borders.

"One of the most important plant health measures is barring new pests from reaching Estonia via trade. Border crossing areas are naturally high-risk locations for surveillance, where it would make sense to set traps and monitor trees and shrubs," she said.

Changing weather conditions have also been leading to an expansion of the workload facing customs inspectors, and also plant health inspectors, particularly the milder winters and longer warm periods, which have allowed species from more southerly climes to survive more easily than they could previously.

As a result, many pests and bacteria that would have perished during the Estonian winter are increasingly successfully overwintering, for instance in the Estonian soil or beneath tree bark.

"Climate change is definitely affecting the spread of plant pests as a warmer climate favors their survival. The warmer it gets, the more suitable the conditions get for species that have not previously established themselves here," Metsmann noted.

One of the greatest concerns has been the emerald ash borer, a beetle native to northeastern China which is responsible for the rapid destruction of native ash trees. It has not been found in Estonia yet, but it is making steady progress into the EU more broadly.

After getting established in Ukraine some years ago, this tiny species of beetle somewhat unexpectedly began moving further towards Central Europe, and has now also been detected in the EU nations of Hungary and Slovakia.

"Given the current situation, you can imagine how little capacity Ukraine's national plant protection service has to monitor this pest. Unfortunately, it has now reached the European Union, which means a great deal of additional work for the countries where it has been found," the adviser said.

In addition to the bug being found to Estonia's south, it has also turned up to the east as well, and close to Estonia. Four years ago, a viable population of the pest was discovered near St. Petersburg, just 150 kilometers from the Estonian border, putting local specialists on high alert.

As a beetle, the species can fly too, of course. "It is an insect with excellent flying ability, so its arrival here from the St. Petersburg area is highly likely. At some point it will make that journey. Our task is to detect it as early as possible so we are prepared," Metsmann explained.

The emerald ash borer measures just 1.4 centimeters in length, maximum, so is extremely difficult to spot in the wild with the naked eye. To improve early detection, the state maintains 90 special sticky traps along the eastern border and along major highways, replacing them every couple of weeks.

Another invasive pest is the Japanese beetle, which often travels long distances by hitching rides on human transport, hence the precautions taken at major highways and the border.

"The Japanese beetle is indeed one of the species we monitor because it is what we call a hitchhiking insect that can arrive on vehicles. It can hide in aircraft cargo holds, which is why we conduct extensive surveillance around airports," the adviser went on.

As to why it is of concern, the beetle targets any ash trees which have already been severely weakened by ash dieback, a chronic fungal disease. Once beneath the bark, its larvae bore through the tree's nutrient-carrying layer, eventually causing the tree to dry out completely and die.

Ash trees are not the only plants requiring protection from the Japanese beetle either. Many ornamental and fruit trees are also threatened by the Japanese beetle, which has an enormous appetite for the leaves and flowers of grapevines, apple trees, maples and roses.

After feeding through the summer, the beetle burrows into the soil to spend the winter, where as noted it has more scope for doing so in a milder climate. Once underground, its larvae feed on plant roots and damage grasslands.

Alongside flying beetles, specialists also monitor even smaller, often microscopic threats. Another major concern is the pine wood nematode, which is completely invisible to the naked eye. In addition comes the tiny roundworm, mainly found in Portugal and parts of Spain and which has recently spread into France's conifer forests.

To colonize new areas, the nematode secretes itself inside longhorn beetles, which themselves travel between countries in international wooden packaging material. Once the large beetle begins feeding on conifer trees at its destination, it delivers its hidden passenger directly into the living tree's vascular system.

The dangerous Asian longhorn beetle spreads in a similar way and has already been discovered several times in wooden pallets arriving at Estonia's border.

Ensuring the safety of wooden pallets used in global trade requires constant and strict inspections. Although their quality has improved over the years in eastbound trade, exporters still do not treat shipments thoroughly enough, allowing destructive wood pests to move freely between countries.

"There are many other threats as well, especially those associated with wooden packaging material, because it is used so widely and many countries fail to comply with the established requirements. We have to remain extremely vigilant at the border and everywhere else because these packages circulate around the world so extensively," Metsmann said.

A fourth major threat neither flies nor tunnels through wood, but spreads as a bacterium. Fire blight, a destructive bacterial plant disease caused by Erwinia amylovora, is considered the world's most destructive disease affecting plants in the rose family.

Since 2005, Estonia has held the EU's rare protected zone status, imposing strict restrictions on plants entering the country from other member states.

"There are very few protected zones left — only Finland, Latvia and Lithuania. In the case of this bacterium, we can protect ourselves by ensuring that plants come only from safe areas," Metsmann went on.

To maintain this protective barrier, all host plants crossing the border — including pear, apple and rowan trees — must originate entirely from disease-free nurseries. The requirement applies to every shipment without exception.

The system relies on official documentation certifying plant safety. "Every single plant, regardless of how many an individual or landscaper brings in, must be accompanied by a plant passport confirming that it is safe for a protected zone," Metsmann stressed.

Official inspections alone, however, are not enough, so the authorities are increasingly relying on the public to detect new threats. Gardeners are also encouraged to monitor fruit trees and report browned branches or shoots bending downward to specialists. Forest visitors, meanwhile, can use the PlutoF Go app to map living ash trees and carefully inspect larval tunnels beneath the bark.

Only through joint monitoring can Estonia continue to keep these potentially dangerous unwelcome guests out of its natural environment.

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