Overview
What are aphids?
What are the different types of aphids?
What is the impact of aphids?
How do I know if I have an aphid problem?
How to get rid of aphids
Conclusions and future directions
Aphids are small insects that feed on sap from crops and other plants. Globally, there are around 5,000 species of aphids with roughly 1,350 in North America alone. The different aphid species vary both in their appearance and in the type of plants they attack. Aphids are soft-bodied insects and many species are harmless to plants. Problematic species only cause damage to plants once their number reaches a certain level. In Canada alone, aphids attack several important crops such as apples, wheat, potatoes, and cabbage. This can lead to significant loss of yield and aphids can also transmit diseases that cause further damage to plants.
In this article, we will provide an overview of what aphids are and how to identify them, before providing instructions on how to get rid of aphids.
What are aphids?
Aphids are small soft-bodied pear-shaped insects that are usually green but can also appear in other colours such as yellow, pink, purple, and reddish-brown. Some aphid species can have wings, particularly in spring and autumn and when the colony is large. Aphids are generally distinguished from other insects by the presence of cornicles. Cornicles are tube-like structures that point outwards from the body. Aphids can be found as lone insects but only cause damage to plants in higher numbers.
Aphids can reproduce both sexually and asexually (with and without a mate). This means that their numbers can grow rapidly, and 12 generations can be produced annually. Juvenile aphids are called nymphs which go through several distinct stages of growth before maturing into adults. This process takes about one week.
Aphids can be protected by other insects such as ants. Ants effectively “farm” aphids and use their honeydew as a food source. In return, the ants keep aphid predators away. Aphids also share a symbiotic relationship with a bacteria called Buchnera aphidicola which produces essential amino acids for aphids in return for protection and nutrients.
What are the different types of aphids?
There is a huge variety of aphid species that differ in their appearance and crops they affect. Some of the most common or problematic species are:
Apple aphid (Aphis pomi)
These pests are also known as green apple aphids. They feed on leaves and young shoots but do not cause significant damage in low numbers. In high numbers, they are capable of stunting plant growth and damaging fruit quality. The honeydew they secrete can cause mold growth on plants.
Cabbage aphid (Brevicoryne brassicae)
As the name suggests these aphids attack cabbage and similar plants like broccoli and kale. They are typically grey-green and secrete honeydew. Depending on their number, cabbage aphids can cause yellowing of leaves and even result in complete crop loss.
Green peach aphid (Myzus persicae)
These insects are normally yellowish or green, but pink varieties are also observed. They feed on peaches and other crops like potatoes and can cause significant loss of crops. In addition to direct damage caused by feeding, this species also spreads many damaging plant viruses to the crops it attacks.
Pea aphid (Acyrthosiphon pisum)
These pests target legume crops including peas and are found in green and pink varieties. Like other aphids, they spread plant diseases and promote mold growth by secreting honeydew.
Woolly apple aphid (Eriosoma lanigerum)
These pests attack apple trees and similar plants and are normally reddish-brown. They secrete a waxy substance that covers them in what appears like a layer of cotton. They can attack the bark of trees which can become a site of bacterial and fungal infection.
Rosy apple aphid (Dysaphis plantaginea)
These insects attack apple trees and are purple or pink in appearance. Rosy apple aphids attack leaves, and fruits, in addition to consuming sap. Apple trees infested with these pests produce smaller fruits that do not drop. Like some other aphid species, these pests promote mold growth and are responsible for transferring plant diseases.
For more species or additional information, The University of California, the University of Wisconsin-Madison, and the state university Bugwood Network offer a wealth of research and resources on aphid identification, biology, and management.
What is the impact of aphids?
Aphids have long specialized mouthparts that allow them to puncture plant tissue and gain access to sap from leaves or other parts of the plant. Large infestations can cause stunted growth in the plant and can result in fruits not maturing properly. As we’ve mentioned, aphids attack a huge variety of host plants. It’s probably more common to find any given plant with an aphid feeding on it than without. Many aphid species produce honeydew, a sugary substance that encourages the growth of a mold called sooty mold which can inhibit the growth of plants.
Plant diseases transmitted by aphids
Aphids carry a huge variety of plant diseases (primarily viruses) which can cause complete loss of crops. These include:
- Potato Virus Y (PVY) primarily affects potatoes but also damages other plants. Symptoms of PVY include mosaic and mottling patterns on leaves, leaf drop-off, tuber malformations (in potatoes), and loss of crop quality and yield.
- Bean Common Mosaic Virus (BCMV) affects common beans and other legumes like peas. Symptoms of BCMV include mosaic and mottling patterns on leaves, stunted growth, and reduced pod size.
- Barley Yellow Dwarf Virus (BYDV) primarily affects barley and wheat but also oats and rye. Symptoms of BYDV include yellowing and reddening of leaves, stunted growth, reduced grain quality and yield, and increased susceptibility to other stresses.
Controlling aphid populations is essential for preventing the spread of harmful plant viruses, which can significantly reduce crop yield and quality.
Are aphids dangerous to humans?
Aphids are not dangerous to humans. They don’t bite or sting and are not poisonous.
How do I know if I have an aphid problem?
Aphids typically do not cause damage until there is a significant infestation. The first signs you are likely to notice include a curling or yellowing of leaves. You may also notice that fruits are not growing to the expected size. Other signs include the appearance of black sooty mold or a woolly cotton-like material that appears with some species like the woolly apple aphid.
Aphids are visible to the naked eye so a large infestation may be easily observable upon inspection. If you notice an increased number of ants around plants or trees it may be a sign that there is an aphid infestation. This can usually be confirmed by checking higher up on the plant.
How to get rid of aphids
Aphid control benefits significantly from integrated pest management, which combines cultural and biological methods to achieve sustainable solutions.
Cultural control
Aphids can be particularly damaging to young seedlings, therefore checking the planting area for aphids before planting can help prevent damage. Similarly, seeding early in the season can help plants reach maturity before the aphid population becomes damaging in the warmer months. Monitoring young plants regularly to see if they are doing well can help you catch infestations before they become damaging. If you spot an infestation, you can cut away the infected part of the plant and dispose of it safely without bringing it into contact with other plants. Controlling ant populations can also help reduce aphid numbers. Clearing away debris from crops and practising crop rotation methods can help disrupt nests and reduce ant numbers.
Biological control
Biological control methods offer a safer alternative to chemical pesticides. Biological control methods to target aphids include applying natural substances to plants, encouraging the growth of native natural enemies or releasing biocontrol agents, which include predators and parasites. These last two examples are known as macrobials.
Natural substances can be effective biological control methods against aphids. Mineral oil is effective at killing aphids and works by preventing the pests from respiring (breathing). Pyrethrin is a compound found in the seed cases of flowering pyrethrum plants which has insecticidal effects against aphids. It works by attacking the nervous system of insects.
There are many natural predators of aphids including lady beetles, lacewings, and minute pirate bugs. Typically, predator numbers do not increase significantly until there is already a large population of aphids, which might be too late, thus, it can be beneficial to release native predators of aphids earlier in the season to help control their numbers.
Using parasitic wasps that lay eggs in aphids and use them as hosts is another option for control.
Chemical control
Chemical pesticides have been used to control aphid numbers, but they cause significant damage to the environment. Furthermore, chemical control methods deplete natural predators of aphids and can create a niche for aphid levels to grow. Leaf curling can protect aphids from chemical pesticides.
It’s important to remember that on some mature trees, the honeydew produced by aphids can attract beneficial insects that can keep other pests at bay. So, it’s important to ensure only to employ anti-aphid measures when you are sure they are causing a problem.
Conclusion and future directions
Aphids are a diverse group of pests that can significantly impact a wide range of crops. Their sexual and asexual reproductive capacity means their numbers can increase rapidly in different seasons. Aphids feed on sap from host plants, encourage mold growth, and transmit plant diseases. Identifying aphids early and understanding the specific species involved are crucial steps in managing their populations. Cultural and biological control methods are important for controlling aphid levels while minimizing damage and reducing environmental impact.
Discover more solutions to your pest problems by consulting the CABI BioProtection Portal. To learn more, visit our dedicated article about different types of biological control.
