Aphid pests in vegetable crops

Aphids – an overview:

This page provides an overview of aphid pests in vegetable crops. The related tools listed at the end of the page provide detailed information about their identification, damage and management. It is important to be able to identify insect pests such as aphids, and to have unknown species expertly identified so that they can be appropriately managed.

Aphids are a major pest of vegetable crops. They are sap-sucking insects and have piercing, sucking mouthparts, and are the most common group of virus vectors or carriers. All potyviruses (the largest group of viruses) are transmitted by aphids. They are commonly seen in spring and autumn when the weather is mild and humid. Aphid colonies produce honeydew, encouraging sooty mould growth and reducing plant vigour.

Aphids are generally found on the underside of leaves and also attack soft growing tips. In lettuce winged aphids are usually found on the outer leaves, whereas aphid colonies are usually found on the undersides of wrapper leaves. Currant lettuce aphid (CLA) prefers to feed on new leaves inside the heart.

Aphids have winged and non-winged forms. The winged form is primarily a dispersal form, while the non-winged form is primarily a 'brood mother'. Aphid colour varies with species and what they have been feeding on.

Aphids as sap suckers

The mouthparts of aphids have a needle-like stylet that allows the aphid to access and feed on the contents of plant cells. During feeding, aphids simultaneously suck on and ingest sap contents and inject saliva, which can contain viruses if the aphid has previously fed on an infected plant. Large numbers can cause leaf distortion and wilting and malformation of new growth including flower buds as a result of their direct feeding.

Aphids as virus vectors

Most importantly aphids transmit plant viruses while they feed. Particular species of aphids can transmit viruses like necrotic yellows, potato virus Y, watermelon mosaic virus types I and II, lettuce mosaic virus, and other similar mosaic viruses. A virus can only be transmitted by an aphid in one of two ways:

Non-persistent transmission – This takes less than one minute of feeding for an aphid to acquire the virus and the same time to infect another plant when feeding. Virus remains viable on aphids’ mouthparts for few hours only.
Persistent transmission – It takes several hours of feeding for an aphid to acquire a virus, which then circulates through the aphid’s body to the salivary glands before transmission occurs. This takes at least 12 hours. The aphid can also transmit the virus for many weeks or the rest of its life without needing to obtain more virus from an infected plant.

Types of Aphids

Currant lettuce aphid (Nasonovia ribis-nigri)

Also known as the lettuce aphid, this is a serious pest of lettuce, endive, radicchio and chicory. It is primarily a contamination pest, colonising lettuce hearts and rosettes, making them unsaleable. Under heavy insect loads, lettuce hearts fail to form or die. Adults can be winged or wingless and are greenish to yellow-green, with irregular narrow dark bands on the abdomen.

Potato aphid (Macrosiphum euphorbiae)

Usually green, but sometimes yellowish or pink, the potato aphid is a pest of tomato, eggplant, capsicum, potato, beetroot, and sweetpotato. It causes direct damage to leaves, stems and flowers by sucking sap and indirectly through the transmission of many viruses.

Melon aphid (Aphis gossypii)

Also known as the cotton aphid, it can be a major pest of cucurbits. Wingless adults are light to dark green. The winged form varies in colour from green to almost black. Large populations in a seedling crop can cause leaves and growing tips to die. They also transmit plant viruses while they feed.

Green peach aphid (Myzus persicae)

A major pest of crops such as tomatoes and capsicum, it can be found on cucurbit, solanaceous, brassica and leafy vegetables. They have a small body and are light to dark green. Large numbers cause malformation of new growth including flower buds. They transmit a large number of viruses.

Other types of aphids

include Sowthistle aphid (Hyperomyzus lactucae), Brown sowthistle aphid (Uroleucon sonchi), Turnip aphid (Lipaphis erysimi), Cabbage aphid (Brevicoryne brassicae), and Foxglove aphid (Aulacorthum solani).

Pest management

Integrated Pest Management (IPM) can provide effective control of aphid populations. Aphids have many predators, including ladybird beetles, lacewings, hover flies and parasitic wasps. The parasitic wasp Aphidius colemani effectively controls aphids and is commercially available.

When monitoring for the presence of aphids check underneath leaves and on all stage of plant growth. Sticky cards are a useful monitoring tool for aphids. The presence of honeydew, or black from sooty mould growing on the honeydew is a sign of aphid activity.

Ensure weed populations that are virus hosts (e.g. sow thistles) are controlled around crops.

Insecticides can be used if high numbers of aphids are present, but resistance can develop. Control of CLA with insecticides can be difficult as they prefer to colonise lettuce hearts.

Plants cannot be cured once infected by a virus. Take the following measures to protect your crop from infection.

Exclude or avoid the virus

Plant virus-free seed and healthy seedling transplants, cuttings, and tubers; grow crops in regions where the virus seldom occurs; and avoid movement of transplants with possible virus infection or aphid infestation between regions.

Reduce virus levels

Control weeds and other hosts of viruses and aphids around crops, headlands and farm buildings; destroy old crops promptly; do not plant new crops next to old, diseased plants; and plant upwind of potential virus sources to reduce the number of aphids moving from sources of infection to young crops.

Protect the crop

Use virus-resistant varieties; use highly effective mulches and oil sprays to deter aphids from landing and feeding; use tall barrier crops, windbreaks and bare land to reduce the numbers of aphids entering the crop; and use row covers to protect plants until flowering.

Chemical control

Appropriate insecticides are usually effective in killing aphids breeding on plants and can provide good control of viruses transmitted in a persistent manner. However, they are usually ineffective for preventing spread of the non-persistent viruses, due to the extremely short feeding times of this category of transmission.