Plant aphids are among the most common parasites that can be found on garden trees or vegetable crops. There are countless species of these lice, and in this article, we will explore the most common ones in Italy, how they behave, and the types of damage they can cause to our plants. Fortunately, there are many biological remedies available to eliminate these aphids without resorting to chemical pesticides. Let’s start by understanding their natural predators and explore a range of other eco-friendly solutions.
But first, let’s get to know aphids better.
The Vast Family of Plant Aphids
Aphids, also known as plant lice, belong to a superfamily with around 4,800 different species worldwide, of which slightly over 1,300 exist in Europe. Approximately 250 of these species are highly damaging. As a result, correct entomological identification is far from simple. Here is the generally accepted taxonomic classification in the scientific community:
- Domain: Eukaryota
- Kingdom: Animalia
- Subkingdom: Eumetazoa
- Division: Bilateria
- Subdivision: Protostomia – Phylum Arthropoda – Subphylum Hexapoda
- Class: Insecta
- Subclass: Pterygota
- Infraclass: Neoptera – Cohort Paraneoptera
- Order: Hemiptera (synonyms Rhyncota, Cimicida, Homoptera + Heteroptera)
- Suborders: Heteroptera, Sternorrhynca, Coleorrhynca, Auchenorrhynca.
Aphids belong to the suborder Sternorrhynca, further divided into:
- Infraorder: Aphidomorpha
- Superfamilies: Aphidoidea, Phylloxeroidea, Aleyroidoidea, Coccoidea, Psylloidea.
The Aphidoidea superfamily is divided into eight families, among which is Aphididae, the family of interest to us.
The Most Troublesome Aphid Species for Vegetables, Fruit Trees, and Plants
As it is evident, listing all the plant aphid species present in our territory is not possible. Let’s limit ourselves to the main ones that we often encounter and that can cause severe damage to our crops:
- Aphis gossypii, which particularly attacks plants belonging to the cucurbit family (zucchini, cucumbers, melons, pumpkins)
- Aphis fabae, better known as black aphid, a parasite of crops such as broad beans, chard, tomatoes, green beans
- Myzus persicae, highly problematic for peach trees and other stone fruits
- Macrosiphum rosae, or rose aphid
- Myzus cerasi, or black cherry aphid
- Brachycaudus cardui, or brownish-green aphid of the artichoke, which typically attacks plants belonging to the Asteraceae family
- Aphis citricola, better known as green aphid of citrus. Besides citrus, it also attacks grapevines, roses, and various vegetable species
- Brevicoryne brassicae, or cauliflower aphid, a species that primarily attacks cruciferous plants
We could continue with more species, but these are the main ones. As you may have noticed, there is a close connection between the species’ names and the plants they affect, even though most aphids have some degree of mobility.
Damages to Crops
Aphids are phytophagous and polyphagous insects that attack plants by piercing the surface of leaves, small branches, and buds to suck the sap and feed on it. Infestations on plant organs cause deformations, necrosis, curling, and yellowing of leaves, resulting in significant productivity losses depending on the severity and timing of attacks.
Additionally, aphids produce secondary damages, such as honeydew, which attracts ants and leads to further problems for plants. Another issue is the sooty mold, a black patina that forms on the upper surface of leaves after prolonged infestations.
Besides these problems, aphids also cause indirect damage by transmitting viral diseases, which can be severe for certain crops. Through their saliva, aphids inject various types of viruses into plants. An example is the famous citrus tristeza virus transmitted by the green aphid, Aphis citricola.
Lastly, there is a significant aesthetic impact, especially in leafy vegetable infestations, which become unfit for commercial purposes.
In conventional agriculture, aphids are usually eliminated using pesticides, harmful to humans and the entire ecosystem.
Biological Defense: Predatory Insects
There are several biological remedies against plant aphids. Employing them together creates an integrated defense strategy that avoids the use of harmful pesticides in the environment. The first natural remedy against aphids is the use of predatory insects that are harmless to plants but feed on aphids. The presence of these natural predators is possible only in biodiverse environments free from pesticides. Conventional treatments do not distinguish between good and harmful insects, leading to an imbalance in the ecosystem.
Among the aphids’ voracious predators are ladybugs, especially the common ladybug (Coccinella Septempunctata) and the two-spotted ladybug (Adalia Bipunctata). A single ladybug can devour 100 aphids in a day. Other natural aphid predators include certain species of wasps, lacewings, and hoverflies.
Natural Macerates as Preventive Measures
In organic agriculture, prevention is the best defense. Regardless of the problem, critical conditions should be prevented, or immediate solutions should be implemented. As mentioned earlier, natural macerates and preparations such as nettle macerate, garlic infusion, tomato leaf and female macerate, fern macerate, and horsetail decoction and macerate play a crucial role.
These natural remedies are easy to produce and effective in the biological defense against plant aphids. To be clear, the mentioned remedies are not insecticides but rather antiparasitic repellents. When sprayed on aphids, they don’t kill the insects directly but rather irritate and drive them away. Therefore, it is ideal to use them even before an infestation occurs to prevent aphids from attacking and damaging plants. For example, spraying a bit of nettle macerate when the plant is clean can prevent the problem from arising. If the infestation has already started with the first appearances on the leaves, the natural preparation is still effective. However, when the aphid colonies become too strong and no longer affected by the repellent action, these remedies may become less effective.
Pure Marseille Soap
If the aphid infestation has exploded, more energetic intervention is required. In such cases, Marseille soap comes to the rescue. Aphids are insects with soft bodies, making them more susceptible to the efficacy of soap. Use a solution of water and soap during the early hours of the morning, directly targeting and wetting the insects. Once the sun comes out, the soap film will dry on the aphids’ bodies, suffocating them by blocking their breathing and leading to their demise.
Another advantage of soap is its effectiveness in resolving secondary problems resulting from aphid infestations, such as honeydew and sooty mold. A well-formulated solution of water and soap can completely clean the plant.
A piece of advice is to always add a bit of soap to the natural macerates for a dual effect. You can find a suitable product for agriculture here.
An even more effective solution for combating plant aphids, especially in professional agriculture, is the use of neem oil or azadirachtin.
The main effect of this product on aphids is to block their development in their juvenile stages (juvenile hormone action). It interferes with their hormonal system, causing a chitin-inhibiting effect based on the blockage of ecdysone.
The greatest efficacy is observed on the juvenile stages because it causes moulting alterations, preventing larvae from forming their outer cuticle. You can find a suitable product for agriculture here.
Note that neem oil and azadirachtin (its active ingredient) are permitted in organic agriculture because they are natural molecules that do not accumulate in the environment. They are also highly photodegradable, meaning they degrade quickly when exposed to sunlight. These substances are non-toxic to vertebrate organisms and have low toxicity to beneficial insects like bees and other pollinators. However, it is still good practice to use them in the evening when these insects are not active.
- Nat Prod Rep – The semiochemistry of aphids – Discusses chemical signaling between aphids for mating and avoidance of antagonistic organisms, and between aphids and plants for host location or avoidance.
- Plant Sci – Plant defense against aphids, the pest extraordinaire – Reviews plant defenses targeting aphids, including antibiosis and antixenosis, which interfere with aphid growth, survival, fecundity, and behavior.
- C R Biol – Aphids as transport devices for plant viruses – Presents an overview of the different types of virus-aphid interactions and the underlying mechanisms.
- J Plant Physiol – Phloem: At the center of action in plant defense against aphids – Reviews how plants utilize their ability to combat aphids, one of the most damaging pests of plants.
- Am Nat – Context-Dependent Benefits of Aphids for Bacteria in the Phyllosphere – Studies how aphid honeydew can sometimes increase P. syringae epiphytic growth but that the bacteria may not always benefit.
- C R Biol – Evolutionary ecology of the interactions between aphids and their parasitoids – Reviews the interactions between aphids and their parasites, including endoparasitoid insects.
- Heredity (Edinb) – Wing dimorphism in aphids – Provides an overview of the major features of wing dimorphism in aphids, focusing on dispersal versus reproduction.
- Curr Opin Insect Sci – Factors driving susceptibility and resistance in aphids that share specialist fungal pathogens – Compares factors influencing susceptibility and resistance in aphids that share specialist fungal pathogens.
- Plant Signal Behav – Early interactions during the encounter of plants, aphids, and arboviruses – Discusses how aphids infest plants and transmit viral diseases, focusing on the early moments of the plant-aphid encounter.
- Insect Sci – The evolutionary ecology of symbiont-conferred resistance to parasitoids in aphids – Explores the evolutionary ecology of symbiont-conferred resistance to parasitoids in aphids, focusing on defensive symbiont frequencies.
- University of Wisconsin – Madison: “Aphids, in-depth” – Aphids are common soft-bodied insects that can be many different colors. They are primarily northern temperate zone insects with about 1,350 species in North America. Aphids feed on stems, leaves, and even roots, reproducing rapidly to form colonies. They can cause various symptoms in plants, but natural controls often bring aphid populations under control.