When we put a new plant in our gardens we also invite the right kind of insects to go with it. Some plants attracts bees and beautiful butterflies, other plants less attractive things like vine weevils and if you grow lilies I am sure you are familiar with the red lily beetle. Aphids is a familiar insect to most gardeners but did you know that there are more than 4,000 species of aphids that have been described, some 250 of which are pests of crops and ornamental plants? There are more than 500 aphid species in Britain! One less common aphid is Aphis nerii, the oleander aphid, also called milkweed aphid, sweet pepper aphid, or nerium aphid.
Every late autumn I have a bit of an invasion in my garden. Small, yellow bugs that have only one thing in mind, attacking my highly precious Dregea sinensis – the wine covering my arch. If I keep up with the general spraying against aphids I can keep the numbers down, but sooner or later the war against these particular bugs gets lost - and they win. I use a combination of soil drenching and spraying with a liquid mix of herbs and fermented soy oil, an amazing product against all kind of aphids, including these ones. In fact, I never see any Aphis nerii until the autumn, but when they do turn up, it doesn’t take long before the population just explode in numbers. My lovely Dregea looks just fine on a distance, and although some of the leaves are getting yellow and dropping off, new shoots are still being produced. A couple of years ago I wrote a post on my blog about these bright yellow bugs and did some research online – so now I know a lot more about them then before. They are wicked creatures!
The Latin name for them is Aphis nerii Boyer de Fonscolombe (now that’s a bit of a tongue twister!) Aphis nerii is a common pest of several important ornamental plants in the families Apocynaceae and Asclepiadaceae. This bright yellow aphid with black appendages is commonly found in Florida feeding on oleander, Nerium oleander, milkweeds, such as butterfly weed, Asclepias tuberosa, and scarlet milkweed, Asclepias curassavica, and wax plant, Hoya carnosa. The Aphis nerii is found in tropical to warm temperate regions throughout the world. This species probably originated in the Mediterranean region, the origin of its principal host plant, oleander. So what on earth are they doing in my garden in London??
Well, going back to what I started to say - I probably invited them....by having the right plant. Dregea sinensis is a plant similar to milkweed, with sticky white sap and highly scented flowers. Aphis nerii is a bright orange-yellow insect with black legs, and stalks known as cornicles on the back of the abdomen. The aphids' bright colouring is a warning to predators. The aphids take up the cardenolides (cardiac glycosides) produced by the plants. When a predator disturbs the aphids, they exude the cardenolides in a waxy compound through the cornicles. Predators usually back away to clean the defensive compound from their mouthparts. Aphis nerii has another amazing protection called phenotypic plasticity: the ability of an organism to change its phenotype, or body form, in response to changes in the environment. They get a bit edgy whenever they feel that something is approaching them too closely. When getting close to a colony, the whole colony can in perfect harmony, ‘kick’ their tiny black legs up into the air much like a herd of miniscule ‘bucking broncos’!
When the Aphis nerii finds its preferred hosts, the population explodes. All of the aphids are females; they reproduce by parthenogenesis (clones of the mother) and they bear live young (nymphs). If conditions become too crowded on a plant or the plant declines in health, some of the aphids develop wings and will colonize new parts of the plants – or new plants if they can find any suitable.
Like other aphids the Aphis nerii secretes a viscous sugary substance known as honeydew. This secretion is greedily sought after by other insects, especially ants. Some ants live in close proximity to, and tend to aphids, although I have never seen any ants on my Dregea. As honeydew accumulates on the leaves, a black sooty mould often follows and is rather unsightly. This leaves a mix of dead aphids, honey dew and black mould. Uhg!
The Aphis nerii tend to colonise the new tender shoots of my Dregea, especially those that have twined themselves into thick clusters of stalks sticking right out from the arch. Brushing the aphids off is impossible, firstly because squashing them leads to completely orange coloured fingers which is hard to wash off. Secondly, the soft shoots are difficult to handle without breaking them, opening up a dripping tap of white sticky sap that itches with skin contact and leaving marks on clothes that doesn’t go away in the wash. Spraying and soil treatment is so far the only solution I have found, until the leaves drop off in the winter and the aphids die of starvation. Yeah!
Aphis nerii can be attacked by parasitic wasps. The wasp lays its egg by inserting its ovipositor within the aphid nymph; the wasp larva then utilizes the aphid’s internal organs as its food source. After the parasite goes through metamorphosis and becomes a wasp inside the body of the aphid, it cuts a hole in the back of the aphid’s abdomen and emerges, leaving the aphid’s empty body, referred to as a “mummy”, as a hollowed-out dry shell with an open door in its back. Not sure if that’s happening in my garden, I might not have the correct wasps present, or the correct climate for the correct wasps, in any case, if the wasps are here they are not doing a particularly great job! And generalist predators that survive eating the Aphis nerii suffer the effects of the cardenolides. Ladybirds develop deformed wings and spiders weave strange disrupted webs. I saw three Harlequin ladybirds on the Dregea today when taking my photos but none of them seemed particularly interested in munching any yellow aphids. Perhaps they know what’s in for them if they try to make a meal of them!
But it is not all doom and gloom on my arch, despite all the yellow aphids. The plants are still alive and we are already in October so it won’t be more than a few weeks or so until the plant starts to drop its leaves and the sap go slower, and the aphids will lose interest. When the temperature gets low enough the aphids will all die, but a few must survive and hibernate somewhere in order to continue the cycle – exactly where and how I am not sure.
I have been contacted by an Entomologist and Invertebrate Collections Curator at the Plant Pests and Diseases Programme at The Animal and Plant Health Agency (Part of DEFRA), and have been asked to send in samples of my aphids so they can study them. Apparently very little is known and understood about Aphis nerii in UK, and how widespread it is. There is much knowledge from warmer climates, but Southern England is not the same as Florida and Southern Spain, even though we would like to think so at times! The curator found me through the blog post I wrote two years ago and we have been emailing each other, and I have also been contacted by a colleague of the curator working in Cardiff who would like me to look out for Aphis nerii mummies! I have been out looking, but I suspect it is too late in the season for any mummies left. I have however ample samples of live aphids to send off.
So the question goes out to all of you now, do any of you have these aphids in your garden? They are very distinct so should be easy to spot, and they only go for plants with white sticky sap like milkveeds, oleanders and sometimes euphorbias. It would be great if you could have a look and report back to me. If you find any, please let me know which plant you found them on. I will report back to the curator and hopefully this will lead to better understanding of this rather exotic aphid.
To see these wacky aphids up-close and personal I have made a video of them, you can watch it on YouTube here: http://youtu.be/jmpxKyIahrA
A little warning though, this is a very up-close video, don’t eat your lunch whilst watching it :-)