Tightly curled, yellowing, distorted, and diseased-looking young leaves at the tips of the branches on stone fruit in spring, mainly on cherry and plum trees, is a sign of damage caused by aphids.
What are Aphids?
Aphids are small sap-sucking pests insects around 2-4 mm (1/16-1/8”) long, with soft, pear-shaped bodies. There are around 5,000 different aphid species, they are pests on a wide range of edible crops and ornamental plants. Aphids vary in colour, they can be green, black, red, yellow, brown or gray, and some are even woolly, their bodies are covered with protective white waxy strands. The majority of aphids observed in the garden are wingless, but some mature aphids can have wings, whereas Immature aphid nymphs look just like adults but are smaller in size and don’t have wings.
Leaf-Curling Aphid Pests of Stone Fruit
There are a few species of aphids which cause the curling of leaves on stone fruit. The following aphids are often encountered on stone fruit:
Plum leaf-curling aphid (Brachycaudus helichrysi) – yellow-green in colour, major pests of plums and prunes in spring, then moves to attack ornamental flowers of the Asteraceae (daisy) family in summer.
Black cherry aphid (Myzus cerasi) – also known as cherry blackfly, shiny-black in colour, mainly attacks cherries, particularly sweet cherries.
Green peach aphid (Myzus persicae) – a pest of all stone fruit, particularly peaches, may also attack the flowers and fruit as well as the leaves, causing distortion and discoloration.
Damage Caused by Leaf-Curling Aphids
Leaf-Curling aphids establish large colonies on young leafy shoots in spring, and hide on the undersides of leaves, where they feed by sucking the sap out of leaves and tender young shoots, causing the leaves to curl and yellow, stunting their growth. and reducing their capacity to photosynthesise effectively.
Heavy aphid infestations can weaken mature fruit trees, reducing their yields and fruit quality, and limiting fruiting in the following year. Young fruit trees are less resilient and may be killed in such circumstances. Aphids also excrete sugary honeydew which causes black sooty mould on the leaves and fruit that it falls upon.
When aphids feed, they excrete honeydew, which appears as a shiny, sticky substance covering the leaves below their feeding site. The honeydew encourages the growth of black sooty mould, which turns leaf surface black, blocking out light, impairing the leaf’s ability to photosynthesise. The black sooty mould only sits on the leaf surface and can be washed off..
Since aphids have piecing mouthparts which they use to suck sap from plants, they can spread viruses from one plant to another.
It is important to note that once leaves are curled and distorted, they will stay that way and will not uncurl. Only after these leaves fall in autumn will they be replaced by healthy new leaves in spring.
How to Control Leaf-Curling Aphids
An integrated pest management (IPM) approach is a scientific and strategic way to control pests using a combination of techniques to limit pest populations and the damage they cause, while eliminating or drastically reducing the use of pesticides to minimise risks to people and the environment.
Organically Acceptable Chemical Controls
Horticultural oil, preferably natural plant oil based products (such Eco-oil in Australia), or neem oil, are organically acceptable methods of controlling aphids.
Keep in mind that horticultural oils work by smothering the pest, and must make direct contact with the pest to be effective. That means spraying inside the curled leaves using a spray bottle.
Neem oil works both as an anti-feedant, which causes the pests to stop eating and die after a few days, and also prevents the pests from maturing, disrupting their breeding cycle. Neem oil can be mixed with horticultural oil if desired.
Biological Controls
Leaf curl aphids can be controlled by their natural enemies – ladybirds (lady beetles), green lacewings, brown lacewings, hoverflies, and soldier beetles.
Encourage these beneficial insects by growing companion plants from the Asteraceae (daisy) and Apiaceae (carrot, dill, parsley) family, as well as other companion plants such as sweet alyssum. These all have shallow flowers which provide a nectar source for them.
Grow plenty of perennial plants to provide a home for these beneficial insects to overwinter in.
Cultural Controls
The deformation of new leafy growth occurs in spring, and the distorted leaves will usually change in colour from yellow to brown and fall prematurely.
The best way to manage infestations of leaf-curling aphids is by pruning off the damaged growth which the pests are hiding in, and disposing of it in a sealed plastic bag in the rubbish bin or destroying it by burning. Don’t put the prunings in the compost as the aphids will escape!
Aphid infestations usually only affect certain branches, rather than the whole tree, which makes it easier to control them pruning off affected growth.
Physical Controls
Ants carry aphids onto the new growth of trees in spring, and they also protect them from insects that would eat them. The best way to deny ants access to the tree is by creating a barrier by glue banding or grease banding the tree trunk. Without the protection of ants, aphids are usually eliminated quite quickly by beneficial insects which parasitise them or eat them.
When managing pests naturally, monitoring is very important. Insect trees regularly in early spring for aphid attack, and treat the problem when it first emerges. It’s easier to control aphids when their populations are smaller, as it takes less effort, and far less chemicals.
More articles on Garden Pests, Diseases and Problems
References
- Bulletin of Entomological Research, The Cherry Black Fly (Myzus cerasi), Published online by Cambridge University Press: 10 July 2009, F. M. Wimshurst
- Michigan State University, Department of Entomologu – Stone Fruit IPM for Beginners, Chapter 20, Aphids, Julianna Wilson
- Government of Western Australia, Department of Agriculture and Food, Pestweb, Crop Insects – Lupin Aphids, Aphis craccivora, Myzus persicae, Acyrthosiphon kondoi, Macrosiphum euphorbiae, Brachycaudus helichrysi
- CAB International, Plantwise Knowledge Bank, Species Page – leaf-curling plum aphid, Brachycaudus helichrysi
- University of Minnesota Extension – Aphids in home yards and gardens
- University of California, Agriculture and Natural Resources, UC IPM Pest Management Guidelines: Plum, Leaf Curl Plum Aphid, Scientific name: Brachycaudus helichrysi (Reviewed 5/06, updated 4/09)