It’s likely that most gardeners in Britain have, at some point, tried to grow True Lilies or Fritillaria, and many will have encountered a problem—not due to a lack of gardening expertise, but because of an infestation of the Chrysomelid leaf beetle Lilioceris lilii.
Commonly known as Red, Scarlet, or simply Lily beetles, these insects are easily recognised by their black bodies and bright red, glossy wing cases. Lily beetles are an invasive species originating from the Mediterranean region. Until the 1940s, they were only occasionally seen in Britain, limited to locations where their only known food plants, True Lilies and Fritillaria, were grown.
After the Second World War, however, True Lilies and Fritillaria became increasingly popular in-home gardens. Consequently, Lily beetles spread rapidly across Britain and are now recognised as an established species.
Each spring, in areas where their host plants are regularly grown, adult Lily beetles emerge from the soil, within pots and flower borders, where they have hibernated over winter.
Attracted to the fresh new leaves, the beetles feed and mate. Females lay small orange eggs in neat lines on the undersides of leaves, which hatch after a few days into tiny grubs.
The grubs feed from the leaf tip toward the stem, covering themselves in their own excrement as a deterrent to predators. Over the following three to four weeks, they voraciously consume the leaves, often defoliating the plants before descending into the soil to pupate. By mid-summer, the next generation of adult Lily beetles emerges from the pupae to continue feeding on any remaining foliage before returning to the soil to hibernate for winter.
Unfortunately, adult beetles don’t limit their feeding to stems and leaves, they also attack buds and flowers. As a result, infestations can cause severe, and sometimes fatal, damage to host plants.
Understandably, gardeners want to control Lily beetles effectively. However, conventional chemical insecticides, including organic-approved pyrethrum-based products, can also be lethal to beneficial insects such as bees and hoverflies, which are naturally attracted to Lily flowers.
For an environmentally safer approach, consider manually removing beetles, grubs, and eggs. Follow this with a spray of a non-toxic plant strengthener and Lily beetle deterrent, such as Grazers G4. If your lilies are in pots, it’s also worthwhile to repot them during winter, disposing of any hibernating beetles that might be present in the soil.
About Dr Ian Bedford
Ian has been fascinated by the bug world for as long as he can remember. From studying butterflies on the South Downs as a youngster, he went on to pursue a career in Research Entomology and ran the Entomology Dept at the John Innes Centre in Norwich up until his recent retirement.
'Bug of the Month'
Visit our 'bug of the month' archive.
Every month Ian will share his knowledge on how to protect your plants and gardens from preventable pest invasions while providing valuable insights into the insects regularly found in our gardens.