The box tree caterpillar and fuchsia gall mite will continue their march across British gardens in 2018, experts warn, after the fast-spreading bugs topped the Royal Horticultural Society’s (RHS) pest list for 2017.

The RHS also warned that a “game-changing” bacterial disease called xylella, which is devastating parts of southern Europe and has already been intercepted at the UK border, is a very serious danger to UK plants and trees.

Global trade and changing climate has led to plant pests and diseases spreading around the world in recent years, but currently UK citizens can bring back any garden plants grown in EU countries. The RHS says both regulations and border controls must be strengthened to protect the nation’s plants.

The RHS is the only group that undertakes surveillance of garden plant health through the 10,000 pest and disease inquiries it gets each year to its advice service. In 2017, the box tree caterpillar topped the list of pest inquiries.

These moth larvae were first seen in gardens in 2011, probably arriving via the plant trade from continental Europe. It spins a protective web and then strips the plant of leaves. Box has also been hard hit by two types of blight, both in the separate list of the top 10 diseases.

Quick guide The top garden pests and diseases reported in 2017 Show Hide The top 10 pests 1. Box tree caterpillar 2. Fuchsia gall mite 3. Vine weevil 4. Slugs and snails 5. Alder leaf beetle 6. Viburnum beetle 7. Tortrix moth 8=. Glasshouse mealybug 8=. Pear blister mite 10. Woolly aphid The top 10 diseases 1. Honey fungus

2. Phytophthora root rots 3. Rusts 4. Powdery mildews 5. Box blight 6. Volutella blight of box 7. Leaf spot and canker of Prunus 8. Verticillium wilt 9. Blossom wilt of fruit trees 10. Kerria twig and leaf blight

“They are causing very significant damage,” said Dr Gerard Clover, head of plant health at the RHS. The box pests and diseases are mainly in London and the home counties but were also found in Suffolk and Gloucestershire in 2017. “There is some suggestion that it might not go too far north, but who knows with climate change,” he said. “We are very wary that it might continue to spread.”

Facebook Twitter Pinterest A healthy fuchsia (right) and a plant affected by the gall mite (left). Photograph: Andrew Halstead/Andrew Halstead/RHS

The second most reported pest was the fuchsia gall mite, which is believed to have arrived in the UK on cuttings brought by a private enthusiast from South America. “It is a new pest people are not familiar with and gives really quite horrendous symptoms – very distorted growth,” said Clover. “People look at it and think what on Earth is that?”

For 2018, Clover highlights the box caterpillar and fuchsia mite but said the greatest potential threat is the killer bacteria xylella. Originating in the Americas, it arrived in Italy in 2014 and has spread to France and, in 2017, mainland Spain, killing many old olive trees and grape vines.

“It is on the move,” said Clover. “It affects over 350 host plants, everything from crops through to weeds, like timothy grass and ivy and shepherd’s purse, through to trees like prunus, acers, magnolias, and to ornamental plants.”

Xylella is transmitted by the spittle bugs that produce the foam seen on plant stems in summer and there are at least 10 suitable species in Britain. “The fact it can be transmitted so easily, and that it kills plants, really does mean that this is a disease like no other we have faced,” Clover said. “It is unfortunately likely to come here eventually. I hope it is not in 2018 but we need to be prepared.

Facebook Twitter Pinterest Olive trees felled in Bari, Italy, after becoming infected with xylella. Photograph: Alamy

“There is clearly a danger of people potentially coming back in their cars with high-risk plants in pots from Italy or France,” he said. “It is really not something that people should be doing any more.” The government has introduced tighter restrictions for the commercial plant trade, but the RHS is calling for similar moves for members of the public.

While new diseases are growing, the most reported in the RHS disease list for 2017 was honey fungus, which has been top every year since 1998 when the lists were first published. This produces fungus at the base of the stems or trunks of shrubs and trees and is therefore easy to spot. The RHS is now investigating whether more benign honey fungi could be used to keep out the highly pathogenic ones.

Gardeners can reduce the risk of pests and diseases, said Clover: “A lot comes back to the idea of ‘right plant, right place’.” Having the right conditions for the plant reduces stress and makes it less vulnerable, but he said new plants should be properly sourced and ideally grown in the UK.

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Having a diverse range of plants in the garden also make it more resilient, Clover said: “There’s a long list of pests and diseases out there, but there’s a also huge diversity of plants that people can grow. There are an awful lot of plants out there that are healthy as well.”

However, the current freezing weather, which can help kill off pests and diseases, has come too early in the year to make much difference, he said.