The RHS is conducting a nationwide survey to plot the spread of four of the UK's most troublesome, non-native, invasive garden pests.
The RHS survey aims to record the distribution of four species of garden pests, and determine their rate of expansion across the UK.
The four pests under investigation are:
Lily beetle (Lilioceris lilii) - the third most troublesome pest in the UK in 2009. Lily beetle adults and larvae defoliate lilies and fritillaries. It became established in Surrey in the late 1930s and until the 1980s remained confined to South East England. Over the past two decades the beetle has spread rapidly and is now found throughout much of England, with more scattered records in Wales, Ireland and Scotland.
Rosemary beetle (Chrysolina americana) - The Rosemary beetle was first reported outdoors in Surrey in 1994. It is now common in parts of England, with scattered reports from the rest of the UK and Ireland. The adults and their grey grubs defoliate rosemary, lavender and some other related plants.
Berberis sawfly (Arge berberidis) - The caterpillar-like larvae of Berberis sawfly defoliate some Berberis, especially Berberis thunbergii, and Mahonia species and hybrids. This insect was first reported from Essex in 2000, and has spread rapidly across much of England although it's not yet known to be in Wales.
Hemerocallis gall midge (Contarinia quinquenotata) - Hemerocallis gall midge was first reported in Surrey in 1989. It has spread throughout England and into parts of Wales and Scotland. Larvae of the 2mm long midge develop within the flower buds, causing them to become abnormally squat and swollen. Damaged flower buds dry up and fail to open.
Andrew Halstead, RHS Principal Entomologist, says: "Wales is an under-recorded area of Britain and the support of Welsh gardeners is vital for us to get a true understanding of the distribution of these pests."
"Adult Berberis sawflies start emerging in late April and both adult rosemary beetles and lily beetles should be visible on sunny days throughout that month. Hemerocallis gall midge is a tiny fly that is likely to be missed by most people, but the abnormally swollen flower buds are easy to spot, although not until late May to early July."
Gardeners can find out more about the survey and submit their records at www.rhs.org.uk/science/plant-pests.