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  • Writer's pictureDave Aplin

Of aphids and Russian dolls

I purchased a pepper plant the other day. On getting it home I inspected it closely and noticed greenfly on its growing tip. Also known as aphids (along with blackfly), they are annoying garden pests capable of significantly halting plant growth, distorting foliage and spreading viruses. Although tiny and inconspicuous, they can quickly become a major sap-sucking problem, capable of quickly colonising other plants. Since prevention is always better than cure, the plant remained in isolation for a couple days, as I frequently inspected them and removed more unwanted guests.

At university, I was taught a great appreciation for the private life of insects. While we studied many exotic plant insect associations, the most memorable were the aphids. There are over 500 types in Britain, most go unnoticed, many confined to just a few host plant species while others demonstrate less loyalty and can become pests. They have interesting lifecycles. During favourable conditions, when food is plentiful, aphids undergo asexual reproduction. Not requiring ‘two to tango’, means a solitary aphid can produce a large, sap-sucking colony within days. At university we investigated aphids under binocular microscopes, told to squeeze them and observe as many as a dozen unborn aphids shoot out from a single specimen. Then, with the microscope’s magnification ramped up, asked to do the same to the unborn aphids. Incredibly the same thing occurs, unborn aphids that are themselves pregnant! It was like opening a Russian doll for the first time. Undertaking this squeamish act allowed me to appreciate how they can multiply incredibly quickly.

Aphids colonise very quickly, pregnant aphids have unborn babies that are themselves pregnant.

Aphids feed by inserting their mouth-parts through the surface of the leaf or stem literally tapping into the plant’s sap. The sap then flows freely like putting a hosepipe onto a tap and turning it on. Sap flows in one end and of the aphid and out the other with it extracting what it needs. The unwanted sap drips from the aphid’s bottom as honey dew, a sticky substance, normally falling onto lower leaves. Ants can often be seen ‘milking’ the aphids for this sugary substance and to a certain degree protect the aphids from their predators. The clear honey dew is often colonised by a black fungus called sooty mould.

Sooty mould, the black fungus living off honey dew.

Sooty mould is unsightly and often the first sign people notice that aphids have arrived. Incidentally, sycamore trees host large colonies of aphids, so if you are unfortunate to park under one then you are likely to have it covered with sticky aphid ‘poo’ before it gradually turns black as sooty mould arrives. Fortunately, it washes off easily.

Inevitably, the aphid’s food supply diminishes heralding their departure to pastures new. However, with no wings finding new pastures could be a real challenge. Ingeniously, however, low food quality initiates aphids to produce winged babies that allow them to disperse and make colonies elsewhere. In autumn eggs are laid, overwintering until the first flush of spring growth.

Vigilance is certainly key to prevention, but we are also fortunate to have natural garden allies. Perhaps the most interesting are minute parasitic wasps that lay their eggs directly into the aphids. This makes the aphid grow abnormally into a ball-like structure where the eggs hatch and larvae devour the aphid from the inside out.

The swollen, balloon-like aphid has been infected by a parasitic wasp, a natural predator.

Ladybird larvae are also ferocious predators along with the larvae of hoverflies, lacewings, earwigs, beetles and birds. Predatory insects can be purchased by mail order to boost natural populations. Spraying diluted washing-up liquid can be effective as it suffocates the aphids, although in my opinion, chemicals should only be considered a last resort as they often kill beneficial insects working on our behalf.

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