Two leaf beetles cause a disproportionate number of calls to gardening helplines because they are bright and easy to see. The red lily beetle attacks lilies and fritillaries, and the rosemary beetle feeds on rosemary, lavender, sage and thyme. This article summarises the Royal Horticultural Society (RHS) advice pages “Red lily beetle” and “Rosemary beetle”. It covers gardens in the UK, and the RHS says the rosemary beetle is now widespread in England and Wales, established in Scotland and present in Northern Ireland. This is general gardening information rather than a recommendation of any product.
Red lily beetle: recognising it
The RHS says adult lily beetles are 8mm long with bright red wing cases and thorax and a black head and legs. Clusters of orange-red, sausage-shaped eggs appear on the undersides of leaves, and the larvae reach 6 to 8mm, rotund and reddish brown with black heads, usually hidden under their own wet black excrement (frass). Young larvae graze the underside of leaves, leaving white or brown dried-up patches, and older larvae eat entire leaves from the tips. Adults make rounded holes in leaves and can also feed on petals and seed pods. The RHS says lilies and fritillaries are the only plants on which eggs are laid and the grubs develop. It warns that other red beetles, such as cardinal beetles and red soldier beetles, can be confused with the lily beetle, but these do not damage lilies.
Red lily beetle: season and impact
According to the RHS, lily beetles are active from late March to October. The adults overwinter as adults in soil, leaf litter and other sheltered places, not necessarily near lilies, and begin emerging on sunny days in late March and April. Eggs are laid in small batches from April to mid-summer, larvae go into the soil to pupate when fully fed, and the next generation of adults emerges from mid-summer onwards. There is only one generation a year, and the RHS says the late-summer adults will not mate and lay eggs until the following year. It says plants can still flower well despite leaf damage, but that heavy defoliation in early summer can result in undersized bulbs that may not flower the next year, and that lily beetle has become widespread in Britain and Ireland since the early 1990s.
Red lily beetle: what the RHS advises
The RHS advice is to tolerate populations of plant-feeding beetles where possible, since lilies and fritillaries can tolerate some damage, and to encourage wildlife such as birds, frogs, wasps and predatory ground beetles that eat the larvae and sometimes the adults. It advises removing beetles by hand where practical. It explains that beetles find lilies partly through volatile chemicals from the leaves and produce aggregation pheromones that draw more beetles, and concludes that there is no advantage in attempting to treat the soil below lily plants.
Rosemary beetle: recognising it
The RHS describes adults as shiny, 6 to 7mm long, with metallic purple and green stripes on the wing cases and thorax, and larvae as greyish-white with darker stripes, 8mm when fully grown. Both feed on foliage and flowers of rosemary, lavender, sage, thyme and some related plants, and leaves can be reduced to short stumps with greyish-brown discolouration. It says that in most cases the overall appearance and health of the plant remain unaffected.
Rosemary beetle: season and impact
The RHS says the beetle is most active from August to April, though adults are present throughout the year. In mid-summer adults are on host plants but usually do little or no feeding; in late summer they start feeding, mating and laying eggs, which hatch after about ten days. Adults and larvae then feed on the foliage from autumn to spring in mild weather. There is one generation a year, but because the adults are long-lived, old and new generations can overlap. The RHS says it was first found breeding in central London in the late 1990s and has since spread rapidly.
Rosemary beetle: what the RHS advises
The RHS says host plants often survive populations of this beetle without noticeable adverse effects, so control is not usually necessary and the beetle can be accepted as part of the biodiversity a healthy garden supports, or treated as a colourful addition. It recommends encouraging birds, frogs and predatory ground beetles and, where practical, removing beetles by hand, which can help keep numbers below the level at which serious damage occurs. For taller rosemary and lavender it suggests tapping or shaking branches over newspaper spread underneath to collect adults and larvae.
Where a pest controller fits in
Neither page recommends professional treatment. The legal position on pest products in Great Britain is set out in the site’s article on pest products as biocides.
Common questions
Should I treat the soil for lily beetle?
The RHS says there is no advantage in attempting to treat the soil below lily plants.
Is the rosemary beetle harmful to plants?
The RHS says the beetle does not eat enough to harm healthy plants and control is not usually necessary.
The bottom line
The RHS treats the red lily beetle as tolerable and manageable by hand, with natural enemies helping, and treats the rosemary beetle as something to accept or pick off, not to spray. For both, hand removal, patience and encouraging predators are the RHS’s recommended approach.
Sources
- Royal Horticultural Society, “Red lily beetle” (rhs.org.uk)
- Royal Horticultural Society, “Rosemary beetle” (rhs.org.uk)