Dear Readers, today I decided to do a bit of pruning on my two front-garden buddleias. I know it’s not quite the right time of year, but the wind had snapped a branch off of one of them, and the other was starting to hang over the pavement, making it difficult for people to walk past. However, while one of them was in perfect health, the other was not.
My left-hand buddleia has leaves that are sticky and wet with honeydew, the result of a positive plague of aphids, both green and black. I had noticed that the lid of the wheelie bin underneath the shrub was tacky, but I hadn’t really noticed the degree to which the plant had been attacked.
So, what’s going on? It all starts when a winged greenfly lands on a plant. First, it tests the sap of the plant by boring a hole – it has sharp mouthparts but, much as a mosquito injects an anti-coagulant so that our blood doesn’t clot while she is feeding on us, so an aphid injects a substance that stops the sap from coagulating.
Aphids are true bugs, and plant sap is their only food. Furthermore, they feed ‘passively’ – once they’ve tapped into the plant’s juices, sap is forced into the animal’s stomach purely by the higher pressure in the veins of the plant. Plant sap is not a complete food, because it has no protein, but the aphid has gut bacteria to help with that problem. To regulate pressure, the aphid excretes honeydew, which is what is causing the stickiness.
The honeydew is attactive to many sugar-starved insects. Ants will ‘farm’ aphids, drinking the honeydew and carrying the bugs from one plant to another. Some butterflies, such as the hairstreaks, the holly blue, the speckled wood and the white admiral , use honeydew as part of their adult diet. I have seen honeybees licking leaves that are splattered with the sugary secretion, and many solitary bees are also attracted to the stuff. Sadly, if you have ever parked your car under a lime or sycamore tree in summer and have gone away on holiday, you will be giving it a good scrub when you come back. Furthermore, there are types of fungus that grow on the honeydew, disfiguring the plant further. Finally, aphids may introduce plant viruses into a plant with their first bite, even if they then don’t settle and start to feed.
As you probably know, aphids can perform parthenogenesis – that is, they can have young without mating. Each new aphid is a clone of her mother, and already has the embryo of the next generation maturing within her. In the course of her lifetime, an aphid born in spring could have twenty to forty generations of young in a single season, literally billions of offspring. No wonder my poor buddleia is looking a little overwhelmed.
Fortunately, aphids are the hamburgers of the insect world, preyed upon by pretty much everybody. I found a couple of ladybirds on the buddleia and a small brown hoverfly larvae. There are also twelve species of parasitoid wasps, tiny creatures only a tenth of an inch long, who feed exclusively on aphids – in fact, some of them are used in commercial greenhouses to control greenfly and blackfly numbers without having to resort to chemicals.
I wondered why the goldfinches that I saw a few days ago were returning to my buddleia over and over again, and now I suspect that they were munching away on the aphids. 83% of the diet of the American goldfinch is apparently comprised of aphids, so why should the UK one be any different? Blue tits will also work over a shrub in search of small, squidgy creatures to feed to their offspring – caterpillars are the favourite, but a beakful of aphids will do at a pinch.
Still, I do wonder why one of my buddleia is a sticky mess, and the other is pristine. One is in full sun for most of the day, so maybe it’s healthier anyway? Who knows. What I will probably do is get my long-suffering husband to give the shrub a good burst with the hose pipe next time he’s doing the watering – hopefully that will at least give the aphids a shock. Lots of other methods have been tried: various chemicals (which I won’t entertain, of course – I don’t want to kill off everything that feeds on the greenfly), the insect predators already mentioned, and traps – apparently aphids are attracted to the colour green (no surprise there) so I imagine these traps are about as green as you can get. I think my main aim will be to see if I can bolster the health of my honeydew-covered buddleia – maybe it isn’t getting enough water, especially during the current semi-drought conditions. I will let you know how I get on!
Interestingly, humans have also been fed on honeydew, both in the form of honeydew honey, from bees who feed on the stuff, and in the form of manna – the solidified honeydew found on tamarisk and some other trees. What effect it has on humans is unknown: I just had a tiny taste from the buddleia, and the honeydew tasted, well, sweetish, as you might expect, but not as delicious as maple syrup. But see what Samuel Taylor Coleridge has to say about it, in Kubla Khan. If I can get hold of some Milk of Paradise, I could be flashing and floating all over the place.
And all who heard should see them there,
And all should cry, Beware! Beware!
His flashing eyes, his floating hair!
Weave a circle round him thrice,
And close your eyes with holy dread,
For he on honey-dew hath fed,
‘And drunk the milk of Paradise.