Q What is rosemary beetle?
A Rosemary beetle (Chrysolina americana) is a small, pretty beetle that feeds both as an adult and a grub on rosemary and related plants. It is closely related to the Colorado beetle that devastates potato crops in Europe.
Caption: Rosemary beetle is a relatively new pest in the UK
Q How do I recognise it?
A The adult beetle is 6mm to 8mm long, metallic green, with longitudinal bronze stripes. The larva is greyish, with darker stripes running the length of its body, which can be up to 8mm long.
Q Where has it come from?
A Rosemary beetle originates in southern Europe. It has probably become established here as a result of climate change bringing warmer winters. The beetle was first spotted living outdoors in Britain at the Royal Horticultural Society (RHS) garden at Wisley in 1994, though this population subsequently died out. By 1998 it had become established in several places in southern England, and was back at Wisley by 2003.
Q What plants does it attack?
A In addition to rosemary, the beetles are commonly found on lavender and will also attack sage and thyme. All these herbs are closely related and belong to the mint family (labiatae). So far this pest has not been recorded feeding on mint, but there is a closely related mint beetle (C. menthastri) found in the midlands and south of England. It is slightly larger and a dark, iridescent blue-green with no stripes. It is a native and feeds on wild and cultivated mint.
Q What damage does it do?
A Both adults and larvae eat the leaves and flowers, leaving a ragged appearance Q Could I mistake it for anything else? A The beetles are very distinctive, though they do have a similar British relation. This is the rainbow leaf beetle (C. cerealis), which is about the same size but even more striking with iridescent red, gold, green and blue stripes. You are unlikely to see it, however, as it is only found in Snowdonia where it feeds on wild thyme. Because of its rarity, it is a protected species.
Similar damage to that caused by the rosemary beetle could be caused by slugs, especially on thyme plants. Small moth caterpillars may also attack shoot tips, but tend to hide away by binding leaves together with silk, whereas the beetle larvae remain visible.
Q How serious is it?
A Beetles and larvae can occur in large enough numbers to seriously damage plants, and possibly kill young specimens. They are also active over a long period, so attacks should be controlled where possible.
Q What is its life cycle?
A From September to May there are likely to be beetles at all stages of their life cycle. The adults start to lay eggs in late August, and continue in mild spells through to spring. The eggs are sausage shaped, about 2mm long, and found on the underside of the leaves. These hatch after about 10 days, the larvae feed for three weeks or so, then crawl down into the soil to pupate, emerging as adults after another two or three weeks. Egg-laying stops in late spring, and by early summer only adults are seen. They spend June and July in a dormant state, not feeding, then become active again in August.
Q When do attacks occur?
A Almost all year round, though they are likely to be worst in spring and autumn.
Q How do I control an attack?
A Adults and larvae can be picked off by hand, or shaken onto a sheet spread out under the plant, and destroyed. On purely ornamental plants you can also use any insecticide recommended for a wide range of plant pests. On herbs for culinary use, use pyrethrum-based products such as Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg or Westland Earth Matters Natural Insecticide.
The best time to spray is late summer/ early autumn or spring when adults and larvae are most active. However, you should not spray when the plants are in flower, to avoid harming bees.
Q What should I do with affected plants?
A Badly infested shoots and other areas of damage can be snipped off to improve the plant's appearance,