The plum moth (Grapholita funebrana) has appeared for the first time in the RHS's top 10 garden pests for 2013, though it remains unclear whether last year was just a one-off for the innocuous but damaging fruit pest.
RHS senior entomologist Andrew Salisbury said: "We have no idea why they're suddenly such a problem."
The plum moth adults lay their eggs on unripe fruits and when the maggots hatch they burrow inside the ripening fruit.
"The fruits that look the most ripe and juicy are often the ones most affected," Salisbury explained. "People cut open their fruit to see a big pink maggot inside."
Horticulture consultant John Adlam said: "Your typical gardener will not be aware of it until it's too late because the caterpillar is right inside the fruit. But commercial plum growers will know to look out for it and will spray accordingly."
A pheromone specific to the plum moth is available and a range of crop-protection products is indicated for its control, he added.
"They tend to be broad-spectrum but targeted in their mode of action so as not to affect ladybirds and other predators. It's all to do with weather conditions.
"It's a pest that comes and goes. We don't see it every year but the weather seems to have suited it at key times last year."
Slugs and snails - Number-one garden pests
Based on numbers of enquiries to the RHS's insect specialists, the list was once again topped by slugs and snails, which benefited from prolonged wet conditions last year.
Capsid bugs were another new entry in the listing at number three, with the RHS describing the sap-sucking pests as being at their highest levels for 25 years.