Box tree caterpillar has knocked slugs and snails off the top spot for the first time in nearly a decade, according to a chart of Britain’s worst plant wreckers.
The “voracious feeders of box,” a native to east Asia, were first discovered in the UK in 2011.
Now prevalent in the home counties, it is spreading across south-east England.
Larvae of the box tree moth feed on leaves of the host plant under a blanket of pale, fine webbing, which covers infested plants.
For the 20th year running, honey fungus was the garden disease that was most commonly reported to the Royal Horticultural Society’s Advisory Service.
Honey fungus attacks and kills the roots of many woody and perennial plants. Symptoms can include white fungal growth between bark and wood, usually seen at ground level.
The second most reported plant disease during 2015 was box blight which, like box tree caterpillar, also targets box plants, but leaves plants covered in bare patches and suffering from dieback.
The RHS said: “Box blight, caused by the fungus Cylindrocladium, is the scourge of hedges, parterres and topiary.
“Box blight poses a serious threat to the UK’s horticultural heritage as box plants provide the structure for many historic formal English gardens.”
According to the RHS, mild and wet weather in December saw an “unexpected increase” in enquiries about box blight at a time when cold weather would normally suppress the disease.
And high temperatures and rainfall during spring led to a “spike” in enquiries from gardeners concerned about box blight.