Rosemary Beetle, Chrysolina americana, is a brilliantly coloured beetle in metallic hues of green and purple. A stunning addition to the garden perhaps, but if you have too many, they will decimate your plants.
What is Rosemary Beetle?
Beetles are the largest insect group with around 4,000 species in the United Kingdom alone. They’re easy to recognise as their front wings form a hard case, covering the second pair of wings and the abdomen. All beetles have biting mouthparts – which is what can do damage to our garden plants.
Chrysolina americana is a species of beetle belonging to the family Chrysomelidae. However, it is not one of our native UK species, having imported itself, presumably on plant material, sometime in the late 1980s or early 1990s. Despite the name Chrysolina americana, the beetle comes from southern Europe.
Other family members which are natives to the UK include Chrysolina herbacea, the mint leaf beetle. Many of these beetles are stunning to look at,. And the rosemary beetle is no exception, with metallic green and purple longitudinal stripes along its body. It was first found outdoors at RHS Wisley in 1994. In less than ten years it had become widespread throughout the Greater London area. Milder winters have no doubt allowed it to establish in many part soft h UK.
Which garden plants does it affect?
As you would expect, Rosemary, Salvia rosmarinus, previously Rosmarinus officinalis, is a favourite host plant. However, Sage, Salvia officinalis and Thyme are also affected, as is Lavender, Lavandula species. Indeed, rosemary beetle is sometimes called lavender beetle.
Other plants which are affected include Salvia which are becoming popular garden plants. For example, Salvia x jamensis hotlips and Salvia x jamensis Nachtvlinder and the commonly named Russian sage, Perovskia atriplicifolia, now Salvia yangii.
These herbs and ornamental garden plants are all members of the Lamiaceae family, the Mint family. Which is why cotton Lavender, a much loved small hedge plant does not suffer from rosemary beetle. Although known as Lavender, this is Santolina chamaecyparissus, not a member of the Lamiaceae. (Botanical Latin has its uses, you see!)
How to reduce the impact of Rosemary Beetle
The simplest method is to frequently check on susceptible plants, particularly in the late spring when the young adults emerge and in mid to late August when they start to feed again and to mate. This way action can be taken before a damaging population has developed.
However, whilst the adult beetles are not eating your plants during June and July, they are likely to still be present on and around their host plants. If you have a large population it would seem a good plan to undertake control throughout this period. That way there will be fewer adults to mate in the late summer.
How to control Rosemary Beetle
This is one of those times when I tend to sound very ‘teachery’. As far as I have seen there is no use, or very little, in trying to control Chrysolina americana by spraying both beetles and plants with a pesticide. And I include organic pesticides in that statement.
For a start plants should not be sprayed when in flower because of killing the bees and other pollinating insects. So a pesticide which works by making the plants indigestible is therefore a to be avoided. Secondly, these are beetles, they a have a hard carapace, body armour if you will, which makes it more difficult for a pest related spray to be effective.
The best way to be rid of a large number of these pests is by using cultural methods. In this case, pick and squish. Now the beetles, for all they are highly visible, are expert at hiding in leaf joints. This makes them tricky to knock off easily, so keep one hand underneath and use the other to nudge them off the plant. I then drop them onto the path and squish with my boot. If you’re a trifle more squeamish, then drop the rosemary beetles into a bucket of water. Remember to pour this down the drain not use it to water the garden!
Or perhaps you fancy an easier option?
If you’re too pressed time to be checking for lavender beetles, or are still feeling squeamish, but do not want to use pesticides, you could try this: –
Encourage wildlife in the garden, in particular predators such as birds, frogs and ground beetles who will eat the larvae and sometimes the adult beetles too.
In fact, why not encourage the wildlife anyway? If they can keep rosemary beetles and other unwanted inhabitants down to levels where your plants can thrive, then you have yourself a successfully biodiverse garden.
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