Small numbers of silver Y moths arrive from the Continent every year, but damage normally only becomes noticeable on crops – such as lettuce, peas and beans and sometimes potatoes – after populations have built up during the summer.

“A lot of low-growing crops are susceptible,” said ADAS entomologist Mike Lole.

This year the moths have been seen earlier and in higher numbers, with some growers on the south coast already having to manage the pest. “Growers should be looking out for first signs of damage now, which should be easy to spot while the caterpillars are still small,” Lole said. “Ignored, they can defoliate crops.”

Migrations of painted lady butterflies, from North Africa, are more sporadic, depending on the size of populations that develop in that region and flights coinciding with strong northerly winds. “It doesn’t happen every year but this year it looks like an exceptional migration,” said Lole.

The butterfly’s caterpillars feed mainly on thistles and other weeds but damage has been recorded on French and runner beans and, if left unchecked, plants can suffer defoliation as a consequence