Fighting tree killers: advice on tree pests and diseases

Healthy ash leaves and those affected by ash dieback

Healthy ash leaves and those affected by ash dieback - Credit: cms

The health of Herts’ trees is under threat from new diseases and pests. Angela Forster of the Countryside Management Service outlines the problems and what we can do to prevent their spread

Oak processionary moth caterpillar

Oak processionary moth caterpillar - Credit: cms

Many of you will have heard in the national news about ash dieback. The disease has affected millions of trees in continental Europe and arrived here in 2012. It is spreading across the UK and is already causing significant problems in eastern areas, damaging and killing ash trees. It’s one of a number of pests and diseases that threaten our trees, the landscape they enrich and the wildlife that depends on them. So what are the impacts likely to be in Hertfordshire and what, if anything can we do about it?

Turning to ash

Ash dieback is a fungal disease that affects several species of ash including the common ash native to the UK. It arrived in Europe in the 1990s and has since affected up to 90 per cent of trees. There is currently no cure and in many cases the trees will eventually die, either from the disease itself or from other pathogens, such as honey fungus, attacking them in their weakened state.

Ash is one of Hertfordshire’s most common trees, found in a wide range of habitats including woodlands, hedgerows, highway verges, parks and gardens. The impact of the disease is likely to be considerable, affecting our landscape and biodiversity dramatically.

The oak’s nemesis

The caterpillar of the oak processionary moth strips the leaves from oak trees, weakening the trees. The caterpillars also have thousands of tiny hairs which carry an irritant that can cause health problems for people and animals, including skin rashes and eye problems. These insects form nests of silken web in which to pupate and hatch into adult moths, sometimes quite low on the tree where they may attract attention, especially from children. The nests can contain shed hairs even empty.

Horse chestnut with leaf miner moth larvae damage

Horse chestnut with leaf miner moth larvae damage - Credit: cms

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The species arrived in London in 2005, as eggs on a semi-mature imported tree. Despite efforts to control the pest, it is spreading slowly and is likely to arrive in Hertfordshire in the next couple of years.

Brown chestnuts

Our horse chestnut trees are affected by other problems. You may have noticed their leaves turning brown prematurely, the effect of the leaf miner moth larvae and/or the fungal disease guignardia leaf blotch. An increasing number are also be affected by chestnut bleeding canker, which can be fatal.

These are just a few of the pests and diseases that threaten our trees. Their arrival is in many cases due to an increase in world trade, which spreads them into areas that have not evolved to coexist with them, perhaps exacerbated by climate change. In addition, mammalian pests such as squirrels and deer cause further problems.

So, what can we do?

Oak processionary moth caterpillar nest. The insects hairs can cause an allergic reaction

Oak processionary moth caterpillar nest. The insects hairs can cause an allergic reaction - Credit: cms

Although we cannot stop the spread of many pests and diseases that are already here, we can do our best to reduce their impact on the trees in our care. We can also do our bit to prevent new ones arriving. Departments across Hertfordshire County Council are working together to implement a plan, along with partners and the local community, to respond to the spread of tree disease in the county.

Residents can also play their part by reporting trees they suspect are affected to the Forestry Commission Tree Alert system ( Many problems are relatively easy to spot and there is lots of useful information on the website to help with identification. This will help to monitor the spread around the county. It may even help to nip a new problem in the bud, as was the case when a volunteer spotted a new pest in St Albans last year, enabling swift action to eradicate it before it could spread.

You can also take care when buying plants. Ask questions about where plants were sourced and grown – one of the routes for the arrival of ash dieback was on young nursery plants brought in from the continent. If you can’t get what you want, encourage suppliers to change their practices. The Countryside Management Service already ensures its plants are sourced and grown in the UK. This is something you can do too. Tree and woodland owners can follow best practice in their conservation and management, whether they are affected or not.

Finally, report and keep away from oak processionary moth caterpillars and nests and warn children to do the same. If you think you have reacted to contact with the hairs, see your GP.

If we all play our part, we can help to make the future brighter for our trees, the wildlife they support, the landscapes they populate and, ultimately, ourselves.

More information can be found on the Forestry Commission website,, or on the tree health page of the Countryside Management Service website.