As gardeners, we have many pests to contend with, and whilst it would be lovely to be completely at one with nature, months spent nurturing beautiful blooms can challenge that when the sight of a black powdery mass smothering buds, shoot tips and the underside of tender leaves greets us. It’s not just the blemish on an otherwise magnificent plant that tests our tolerance, but left untreated, what creates that dusty mould can wreak havoc with our hard work, leaving a trail of curled and distorted leaves, potentially weakening plants and transmitting viruses, which will force us to destroy the plant to prevent the spread of disease. Welcome to the life of the not-so-humble aphid.
These miniscule creatures can vary between 1 and 7mm in length and are sap-sucking insects, often called blackfly and greenfly, although they are available in a variety of colours. With over 500 species in the UK alone, they excrete a sticky honeydew that attracts mould, but it also attracts predators, and so aphids form the basis for many food chains for the likes of ants, earwigs, ground beetles, hoverflies, ladybirds and parasitoid wasps; these saviours can serve as a natural control in a garden with a healthy balanced ecosystem, but as many are not prevalent until the summer, infestations of aphids can easily take over in spring if left to their own devices, and although each species will have its favourite species of plant, they’re not too fussy, so just about any plant in our gardens is at risk. Whilst most aphids devour the sap found in leaves, stems and flowers, some species will feed on the plant roots, so nothing and nowhere is safe.
So how can something so tiny cause so much damage? Because they are masters of reproduction! No matter what we feel about the destruction they cause, the aphid is a phenomenon that has adapted to ensure that despite being the foundation of many food chains, it prevails, and with style…
There’s safety in numbers, and that’s how aphids ensure the numerous predators eyeing them up for breakfast don’t deplete numbers entirely; in fact, in the unlikely absence of disease, parasites and predators, 600 billion aphids could potentially descend from just one – 600 billion… from one! It seems that aside from fully protecting themselves from being eaten, they have just about covered every angle when it comes to survival, and here’s how:
When the over-wintering eggs hatch in spring they are predominantly – if not exclusively – female, and these females will produce live young without the need for males. This parthenogenesis continues throughout spring and summer, producing more and more females who produce more and more females for as many as 30 generations – sorry chaps! And just to make the males feel even more inadequate and increase the aphid population quicker, the females are born pregnant! Could there be a more efficient reproduction cycle? It would seem males are only necessary later in the year, so when the temperature starts to drop, male aphids are produced and all offspring are then born capable of mating; a mate is found, and the resultant eggs will once again over-winter to produce an all-female squad the following year.
Now the family tree’s taken care of, aphids have other weapons at their disposal to prevent their early demise. Although these creatures may be tiny, too many of them on one plant, and their food source will not go the distance, so it’s up to the aphids to do just that. As the conditions change, they will produce a generation capable of growing wings; that generation will then migrate to a new plant to start a new colony and continue the work of the one it’s left behind – genius!
Large numbers are obviously not the only threat to these minute, irritating – yet curious – pests. A tasty treat for many an insect, they need to protect themselves as much as possible, and they have a number of tricks up their imaginary sleeves. First, the majority of aphids have exhaust pipe-like structures on their rear ends, and when threatened by parasitoids, they’re able to release a sticky substance that essentially glues the mouth of their predator shut; the same exhaust pipes can release pheromones to warn fellow aphids of an attack, allowing them to take shelter; ladybirds have learned the technique, unfortunately, and are able to follow the warning pheromones and help themselves to the hiding bugs. There are some aphid-loving ants, however, keen for an endless fix of the honeydew aphids excrete, that will nurture and protect the source to ensure there’s no threat to their supply, fighting off predators, carrying their suppliers between plants and even taking them back to their nests for over-wintering, safeguarding them until spring!
But when there are no ants to come to the rescue, aphids will employ a number of other tactics to protect themselves. They give a pretty mean kick when they need to – relatively speaking – and will pound attackers mercilessly. Some have spines that will potentially make them harder to eat, others have thick skin that has the same effect, and they’ve even been known to seek out eggs of their enemies to stab and kill them before they hatch. If all else fails, aphids can always roll off their food source and hide until the danger has passed. But some species will produce young whose sole aim is protecting the collective – soldier aphids – and whilst they’re not averse to sacrificing themselves for the greater good, they have particularly strong legs to enable them to restrain and squeeze attackers; interrogation techniques have yet to be observed, but given these creatures’ ability to adapt, it surely won’t be too long!
So what can we do to minimise the damage they cause to our gardens? The RHS recommends leaving them in the main to nature, allowing predators to feast on them to bring their numbers down by late summer, but if their exponential expansion gets out of control before that time, organic pesticides are the better option, and though they may not be as potent, needing more applications than non-organic products, they’re less likely to harm the very predators you need to complete the task. If you can tolerate aphids until the cavalry arrives, do, but you can do some of the work yourself, if you’re happy to get your hands dirty; aphid colonies can be squashed between fingers and thumbs. It is possible to purchase natural enemies and introduce them to your greenhouse if you spot a problem there, but again, they won’t be effective until the warmer months.
The use of chemicals should be a last resort and localised, concentrating only on the areas that are affected. Tall trees will not be impacted unless the entire tree is sprayed, but over-wintering eggs on fruit trees and shrubs will be eradicated by using a plant oil winter wash applied on a dry, frost-free day, when buds are dormant, and there’ll be less damaging to the enemies you need to encourage. If your greenhouse becomes infested before you have the chance to allow nature to intervene, there are an array of fumigants that can help. Whatever method you use, caution is necessary when it comes to flowering plants as pesticides are completely indiscriminate and a hazard to bees.
It appears that nature is the best solution, and it always seems to provide the answers we need, but it also helps the causes that lead us to ask questions in the first place; aphids are the perfect example of that. Maybe learning to work together is the ultimate message… but wouldn’t that be a whole lot easier if it didn’t cost us our favourite blooms? However we feel about these diminutive pests, there’s no denying they’re a fascinating subject.