Aphids are small sap-sucking insects (1 to 10 mm long): they feed on plant sap by piercing the tissue. These pests are formidable because they reproduce very quickly. They are viviparous and can reproduce parthenogenetically (asexually), so a single female can produce 40 to 100 clones in her lifetime. They also carry viruses, which they transmit through their rostrum (mouthparts) as they feed. They are also polyphagous, meaning they feed on many different plants. There are many species of aphid, in a range of colours (green, orange, black…), and they belong to the Aphididae family. The species most often found on begonias is Aulacorthum solani but other species can appear o
n this plant.
Most aphids are apterous, meaning they have no wings. But when the colony becomes too large, a winged sexual form appears, allowing the insects to spread to other food sources. This phenomenon is called polymorphism and allows them to adapt to their environment.
Symptoms
The damage caused by aphids is leaf deformation (puckering, curling…), virus transmission and a general weakening of the plant. If the infestation is too heavy, it can lead to the death of
the plant. The honeydew secreted by aphids can lead to a black fungus on the leaves, which reduces photosynthesis: this is sooty mould (Cladosporium).
One of the first visible signs of an aphid colony is the presence of white moulted skins (exuviae), which stay on the plant or fall to the ground. Ants on and around the plants are also a good clue.
Aphids are scattered over the plant at first, but soon gather in clusters on young shoots and flower buds.
Treatments
Many natural solutions are available to control aphids effectively. First, we will look at the possibilities offered by biological control, that is, using the insects that are natural enemies of aphids. Then, in a second part, we will cover natural products that are easy to find and use.
Biological control offers several natural enemies of aphids, the most widely used being parasitoid flies, gall midges and lacewings. But European ladybirds are also a good solution. It is important to avoid chemical treatments and keep interventions to a minimum, to encourage the natural arrival of these beneficial insects. Indeed, many of the insects used in biological control live naturally in France.
Parasitoid wasps are insects (Aphidius ervi / Aphidius colemani) that look like small flies but actually belong to the order Hymenoptera, and can appear spontaneously in greenhouses or in the garden. They are tiny and very short-lived, but they are highly effective at controlling aphids. The female lays an egg inside the aphid’s body, and the larva develops within its host, turning it into a “mummy” before emerging through a small round lid. These mummies are quite easy to spot on the leaves within aphid colonies.
Lacewings are insects (Chrysoperla carnea) are very common in our gardens and are wrongly called mayflies. They are light green with golden eyes; it is the larvae that are predators, the adults feeding only on pollen and flower nectar. The larvae are rather hard to spot, as they are mainly active at night.
Gall midges (Aphidoletes aphidimyza) belong to the Diptera family and look like small mosquitoes. It is the larvae that are predators. They appear in spring after overwintering in the soil. They are orange, translucent and elongated. The adults lay their eggs directly in aphid colonies.
The ladybird is a small, rounded insect, red with black spots. But its colours can vary a great deal (orange, yellow, black…), even within the same species. It belongs to the beetle family (Coleoptera). The larva, easily recognised by its tapered grey body dotted with small orange spots, is the most voracious, although the adults also eat aphids. Many ladybird species are useful against aphids. The most useful are the two-spot ladybird (Adalia bipunctata) and the seven-spot ladybird (Coccinella septempunctata) because they occur naturally in France. Beware: the harlequin ladybird (Harmonia axyridis), introduced in the 1980s for biological control, is now considered a pest. It causes many problems because it multiplies excessively and competes directly with native ladybirds.
As biological control can be tricky to set up, you can also use natural products, which may be enough to control aphids.
More and more “organic” treatment products are on sale. It is nevertheless advisable to avoid them, as they are not entirely harmless. For example, rotenone, an ingredient in many insecticides, is reportedly not as completely safe as was claimed when it first came on the mar
ket.
It is much simpler to use black soap or washing-up liquid diluted in water, sprayed directly onto the aphids. Use 15 to 30 g per litre of water and repeat every week until the pests have gone. However, it is best to remove the most heavily infested parts of the plant before spraying.
Other solutions include garlic infusion, wormwood decoction or peppermint liquid manure.
It is also very important to check your plants daily so that pests are spotted as early as possible. If only a f
ew aphids are present, simply crush them by hand to stop them multiplying.
Ants play a significant role in the spread of aphids, as they protect and disperse them in exchange for the honeydew the aphids produce. It is therefore essential to get rid of nearby ant colonies, so that they do not attack the beneficial insects and do not keep spreading the aphids. Traps using non-toxic substances are available in shops.