The varied damage caused by leaf aphids
Leaf aphids mainly damage young plants by sucking out cell sap. However, at the same time they emit a poison that curls the leaves. Due to the resulting formation of leaf "shelters" as can easily be seen at the tips of branches, the aphids are more difficult for garden birds and other predators to reach and often at least one pair remains to maintain the species.
As the leaf aphids weaken the plant by sucking it, the curling branch tips result in deformed branches. In decorative and fruit gardens this is certainly not desirable, since correctly formed trees are essential - either for aesthetic reasons or for the fruit yield.
Leaf aphids also have consequences if their spit transfers damaging viruses to plants. This can cause widespread damage - especially in fruit orchards or professional rose gardens.
Leaf aphids and "discarded food"
Lastly, the honeydew created by the leaf aphids plays a significant role: Since leaf aphids reproduce very quickly, especially in favourable conditions, they need protein most of all.
They take this from the plant's juices and excrete the carbohydrate (sugars) it contains as honeydew. This is all the more amazing since plants are the epitome of global sugar producers, and many animals - including we humans - use sugar as a food to build up proteins in our bodies. This is why we tend to eat plants - but leaf aphids throw them away for the most part!
The honeydew is now used by ants (and also flies) as food. So ants act towards leaf aphids in a relatively similar way as we do with cows: They "milk" leaf aphids for the honeydew. To do so, the ants deliberately position leaf aphids onto plants, distributing them around the garden, caring for them, and even defending them.
If ants do not consume the honeydew, it spreads over the plants affected by the leaf aphids and it is then colonised by sooty mould, which consumes it.
This is why you frequently only discover aphids on plants in the later stages when the black mould covering the plant becomes obvious.
Leaf aphids - fascinatingly fruitful
Before learning about how to remove leaf aphids, let us first take a brief look at their thoroughly astounding and fascinating life cycle: Leaf aphids spend the winter as eggs, which is why they are combatted at this stage in commercial fruit-growing enterprises. A so-called matriarch emerges from the egg and procreates asexually! This creates leaf aphid "virgins", usually without wings - although some do have wings and can switch plant hosts more easily and thus ensure faster dissemination of the creatures in the wild, on agricultural land, in forests, and in gardens. These virgins also reproduce asexually. Only towards autumn do these exclusively female animals produce male aphids, again by virgin birth. The aphids pair up and the eggs laid by the last female generation of the year, at the time when the sexual generation actually changes, allow the leaf aphids to spread again the following year.
With up to 50 leaf aphid generations per summer and up to six live-born young aphids per virgin per day, it is easy to see how one matriarch can produce around 1027 leaf aphids per year - that's why the number of aphids born is almost countless. But each leaf aphid only lives for around five or six days. When you consider how many young birds, ants, common lacewing, gall midge, hover fly and ichneumon fly larvae grow to maturity by feeding on leaf aphids, it is clear that the leaf aphid requires a massive reproduction rate to survive as a species. It is also obvious why, in the northern hemisphere, the around 450 species of leaf aphid are among the most significant agricultural, forestry and garden pests.
What to do about leaf aphids?
Looking at the biology of the leaf aphid makes it clear that it is a thankless task trying to exterminate every last leaf aphid from a garden. Once the last one has gone the next one is sure to fly in from somewhere - or is brought in by ants.
Prevent leaf aphids by excluding them wherever possible. The ventilation openings on greenhouses and conservatories can be closed with gauze, thus ensuring good ventilation but preventing leaf aphids from entering. Apply the same principle to vegetable gardens by using fleece to cover the vegetables. The fleece not only creates a micro-climate that encourages growth, it also provides effective aphid protection.
Some varieties of plant for home gardens are even resistant to leaf aphids, such as certain salad plants. You can deliberately choose to grow such varieties. These include the following varieties, listed by lettuce type (source: Bavarian State Institute for Wine-Growing and Gardening, Veitshöchheim):
Head lettuce: 'Casanova', 'Fiorella', 'Irina', 'Jiska', 'Osaka', 'Estelle'
Iceberg lettuce: 'Barcelona', 'Bennie', 'Fortunas'
Mini iceberg lettuce: 'Minas'
Oakleaf lettuce: 'Sirmai', 'Smile'
Butterhead lettuce: 'Leny'
Lollo Bionda: 'Barman', 'Lorenzo'
Lollo Rossa: 'Solsun'
In the case of raspberries, which are also popular leaf aphid victims, you can choose varieties which - although not resistant - are not quite so attractive to leaf aphids. For example, the old 'Schönemann' variety suffers from leaf aphids more frequently than the more modern 'Rumiloba' and 'Autumn Bliss' varieties.
How to effectively fight leaf aphids: In addition to prevention measures, you can also fight leaf aphids directly with various compounds. It is important here to select compounds on the basis of whether they protect useful insects and the necessary waiting period, especially in the case of plants you intend to eat. Waiting period: The amount of time you have to wait after using the compound to fight leaf aphids before you can use the treated plant in the kitchen without risk to your health. You will find information on this in the compound's instruction leaflet. You can expect short or no waiting times for natural ingredients (e.g. potash soap or natural oils) used in pesticides such as those produced by Neudorff, including Neudosan, Spruzit and Raptol. You can get more information from specialist suppliers of pesticides.
Granddad's garden tricks - not all of which are suitable in today's world!
In earlier times, when smoking was still prevalent, granddad would take a cigarette butt and place it in water overnight. The resulting brew would be thinned down with a little more water and spread on leaves to prevent leaf aphids. But since nicotine is a highly effective nerve and heart poison, granddad's method is no longer permitted by the German Pesticide Act and was only ever used on decorative plants - never on those that would be eaten.
However, what we can still learn from granddad's garden is how to encourage useful insects. This includes attracting birds by mounting nesting boxes (e.g. for tits) or leaving nesting places (e.g. for redstarts) intact. Even sparrows, which granddad only reluctantly put up with in the vegetable garden since they were believed to damage new plants, are excellent leaf aphid devourers during the hatching and breeding season.
Encouraging useful animals also means encouraging useful insects. You encourage hover flies by planting daisies. These insects feed on daisy pollen and lay their eggs near leaf aphids; when they hatch the hover fly larvae love to gobble up the aphids. Like hover flies, ladybirds, common lacewings, gall midges and ichneumon flies are also useful.
The clever combination of primarily preventative measures and - if necessary - curative measures to fight leaf aphids means that it is now possible to deal with these insects even though they are becoming more prevalent.
With regard to the few leaf aphids that still remain in your garden despite these measures, mischievous folk claim that there are now two reasons for spicing up fresh salad from your garden with a little cut dill: On the one hand it tastes better - and on the other they disguise any leaf aphids which might lurk in the salad...