Summary: Citrus crops play an important role in the economies of many countries, including the United States and India. However, yields are threatened by a variety of insects, mites, and plant diseases. Biological control methods, like pheromones and beneficial bacterial species, help to combat these threats.
Overview:
- What pests affect citrus plants?
- Asian citrus psyllid
- Alternaria brown spot
- Brown soft scale
- Brown citrus aphid
- Citrus red mite
- Citrus rust mite
- Citrus canker
- Citrus bud mites
- Citrus leafminer
- How do I manage citrus pests?
- Summary
- FAQ
Citrus fruits such as oranges and lemons are key crops that support the economies of major producers like the United States and India. The United States produces about 2.75 billion kg of oranges per year, while India produces roughly 3.8 billion kg of lemons. However, citrus fruits are vulnerable to pests and diseases, which can reduce yields. This article focuses on identifying and controlling common citrus pests and diseases, highlighting the use of biological methods.
What pests affect citrus plants?
Citrus plants face threats from numerous pests and diseases, including mites, insects such as aphids and scale, as well as viral, bacterial, and fungal infections. Citrus pests and diseases can significantly impact citrus-growing industries. For instance, Florida’s orange production has dropped by over 90% in the last twenty years due in part to citrus plant diseases.
Asian citrus psyllid (Diaphorina citri)
This pest damages citrus plants in its adult and nymph phases by sucking fluids from plants, injecting toxic saliva, and transmitting the citrus greening disease. Adults are brown with a dark band on their wings and grow to about 4 mm. Nymphs are flat, oval-shaped, and yellow-orange in colour. Damage appears as drying and dead plant parts and the appearance of sooty mould, which grows in the honeydew secreted by this pest. Citrus greening disease causes plants to produce small, misshapen fruit.
Alternaria brown spot (Alternaria alternata)
This is a disease that affects citrus plants, and it is caused by a fungus called Alternaria alternata. Damage can appear as small circular rings on immature leaves, but can also cause large dead (necrotic) spots. Younger plants are particularly vulnerable, and the infection can cause premature fruit drop. Mature fruit is more resilient but can develop a corky appearance on the rind. Damage can be particularly severe in wet and warm environments.
Brown soft scale (Coccus hesperidum)
This insect pest causes damage to many plants, including citrus crops, by piercing plant tissue with specialised mouthparts and sucking plant fluids. Adults are flat and oval-shaped and grow to about 5 mm and appear in shades of yellow and brown. This pest feeds on stems and leaves, and damage can appear as leaf wilting, fewer flowers, and reduced fruit yield. Brown soft scale insects produce large amounts of honeydew, which can cover the entire surface of leaves when infestations are severe.
Brown citrus aphid (Aphis citricidus)
This pest, also known as the black citrus aphid or citrus aphid, harms citrus plants by feeding on plant tissues, but more importantly, by transmitting the citrus tristeza virus (CTV). Adults are brown or black and can grow up to 2.5 mm. Symptoms of adult and nymph feeding on leaves include flower wilting and dropping, stunted plant growth, and crinkled leaves. Symptoms of CTV include stem pitting and rapid plant decline.
Citrus red mite (Panonychus citri)
This pest damages plants by sucking plant fluids from leaves and fruit. Adults and nymphs are red-purple, and females can grow to 0.5 mm long. Eggs are red with a curved stalk on top. Damage can appear as leaf stippling, which can progress to large necrotic areas with more severe infestations. Leaf drop may occur, which exposes fruit to sunburn. Effects of the citrus red mite are often worse in hot, dry conditions where trees are already stressed.
Citrus rust mite (Phyllocoptruta oleivora)
This pest, also known as the silver mite, damages citrus plants by feeding on the fruit rind, though they can be found on other areas of the plant. Adults grow to about 0.25 mm, are yellow or pale brown, and can only be seen with a 20X lens. Damage appears as fruit discolouration: silver on lemons and rust colour on oranges. Citrus rust mite activity can cause fruits to lose fluids more rapidly and appear shrunken.
Citrus canker (Xanthomonas citri)
This is a plant disease caused by the bacterial species Xanthomonas citri. Various citrus plants are affected by this disease, which causes lesions on leaves, fruit, and stems. These lesions are the clearest way to know if an infection is present, and can develop a corky appearance over time. Lesions are typically raised and brown with a yellow ring around the outside. Xanthomonas citri can spread by rain splashing and by wind, and can quickly infect entire growing areas.
Citrus bud mites (Aceria sheldoni)
This pest damages citrus plants by feeding inside flower buds. It primarily attacks lemon fruits but can damage many types of citrus plants. Adults grow to 0.25 mm long and can only be viewed with a 20X lens. They are wedge-shaped and typically appear in shades of green, yellow, and pink. Damage appears most clearly as misshapen fruit, but deformed leaves and flowers are also common symptoms.
Citrus leafminer (Phyllocnistis citrella)
This pest is a species of moth that damages citrus plants during its larval (caterpillar) phase. Adults are small and grow to around 6 mm. They have white bodies and a distinctive black spot on each wing. Larvae primarily attack young leaves and feed in a characteristic waving tunnel pattern within the leaf. Leafminer excrement makes these tunnels more visible and is a clear indication of an active infestation. The growth of young plants may be stunted as a result of citrus leafminer feeding.
How do I manage citrus pests?
There are several methods available for managing citrus plant pests. A combination of approaches using Integrated Pest Management (IPM) practices and biological control methods works well in many cases.
Monitoring
Keep a lookout for the symptoms described above. The most common symptoms of damage caused by citrus tree pests and diseases are leaf discolouration and distorted fruit shape and size. The mite species described above are too small to see with the naked eye, but larger pests may be visible on plants, especially with large infestations. Some pests and diseases cause characteristic damage, such as the raised brown lesions caused by Xanthomonas citri bacteria and the tunnelling pattern of citrus leafminer larvae when feeding.
Cultural control
Cultural control involves using specific farming or gardening practices to lower the risk of pest infestations. This method of crop management depends on correctly identifying the pest. For instance, when managing the citrus red mite, keeping plants well watered can protect crops from damage in hot and dry conditions. However, overwatering can encourage the growth of fungal species like Alternaria alternata, which causes Alternaria brown spot. Removing affected leaves can be helpful in some instances, but leaves damaged by citrus leafminers should not be removed, as their healthy portions are still essential for the tree’s vitality.
Biological control
- Natural Substances: These are typically derived from plants and can be used in sprays to repel or kill pests. For example, a soybean oil spray can help combat Asian citrus psyllids by preventing growth in its developmental phases and repelling it from plant tissues.
- Semiochemicals: These are messaging compounds that can be used to disrupt pest behaviour. For example, specific pheromones can disrupt the mating of California red scale insects and help protect orange crops in this crucial growing region.
- Microbials: These are microorganisms like bacteria, fungi, and viruses that harm pests and disease-causing microbes but not crops. For example, Pseudomonas chlororaphis is a bacterial species that combats the fungal species that causes citrus canker by releasing antibiotics into the soil.
- Macrobials: These are larger animals, like certain insects, that feed on or parasitise pests. For example, a species of ladybird beetle called Cryptolaemus montrouzieri feeds on brown soft scale insects, helping to control their numbers.
Chemical Pesticides
As a world leader in nature-based pest management knowledge implementation, CABI encourages IPM as the preferred, ecologically-based approach to producing healthy crops, which allows for chemical pesticide use only as needed, and when adhering to measures that limit the exposure of people and the environment to them (see FAO, International Code of Conduct on Pesticide Management).
Before considering the use of chemical pesticides, such as liquid copper fungicide, farmers should explore all available non-chemical solutions to control citrus pests, including the ones listed above. For tailored pest management advice, visit the CABI BioProtection Portal, where you can enter your location and pest problem to explore customised solutions.
In case chemical pesticides are considered for use, farmers should look to select lower-risk chemical pesticides which, when used as part of an IPM strategy, help manage pest problems while minimising harmful effects on human health and the environment. Agricultural advisory service providers can provide information on lower-risk chemical pesticides that are locally available and are compatible within an IPM strategy. These experts can also advise on the required personal protective equipment.
Summary
Citrus crops play a vital role in global agriculture, but yields are threatened by a wide range of pests and diseases. Insects, mites, fungi, bacteria, and viruses can all severely reduce yields and fruit quality. Effective management requires accurate pest identification and the use of IPM strategies. Within the Florida Department of Agriculture and Consumer Services, the Florida Division of Plant Industry plays a central role in safeguarding the state’s agricultural resources and supporting the broader agriculture and consumer services mission. By combining cultural practices, biological controls, like spraying horticultural oil, and careful pesticide use when necessary, growers can protect citrus plants while promoting sustainable and resilient production systems.
We have prepared extensive guides on protecting other crops, including tomato and mango, and dealing with pests such as thrips and aphids.
FAQ
What is the major pest of citrus?
The pests that affect citrus plants can vary by region, but many have a global distribution. For instance, Asian citrus psyllids and citrus leafminers are significant pests in India and the United States.
How do I get rid of bugs on my citrus tree?
Before choosing a control method, it’s essential to identify the pest responsible. In some cases, bugs on citrus trees are harmless and require no action. However, if you notice symptoms described above and identify the pest, then visiting the CABI BioProtection Portal can help you find a specific solution.
What do citrus aphids look like?
Citrus aphids are small insects that grow up to 2.5 mm in their adult phase. Aphid species that affect citrus plants are typically dark brown or black. Immature aphids may have a reddish colour. Aphids secrete honeydew that attracts ants, and the presence of ants can be a useful sign of a citrus aphid infestation.
What do citrus thrips look like?
Adult citrus thrips are tiny, yellow/orange-winged insects that grow up to 0.9 mm in length.
