The cotton aphid (Aphis gossypii) is a 0.9-1.8 mm small, round aphid with typical black spiracles. The colour varies from light yellow to dark-green, or sometimes almost black. It has a short cauda, no head front knobs, and antennae shorter than the body.

The cotton aphid originates from warmer regions where it is a pest on cotton and Cucurbitaceae. Thanks to the warm climate in greenhouses it can also survive northern winters. The cotton aphid is mainly a pest on greenhouse vegetables such as cucumber and melon, and on ornamentals such as chrysanthemum and hibiscus. Greenhouse strains do not change their host plant. After hibernation in the greenhouse this aphid can become a pest early in spring.

More than other aphid species, populations of cotton aphids can grow very fast. There exist several strains of cotton aphids, each with a specific preference for a certain host and resistance against pesticides.

The green peach aphid (Myzus persicae) is a 1.2-2.6 mm small, oval aphid with conspicuous head front. The colour varies from pale yellow-green to green, and is sometimes red. Spiracles are medium long and the antennae reach till the spiracles. In temperate regions, the green peach aphid usually hibernates as an egg on its winter host (peach, prune or other relatives). After a few generations on its winter host during early spring, it moves back to its summer host.

The aphid may also hibernate in the greenhouse. In this case acquired resistance is better preserved. The green peach aphid can be a pest on greenhouse vegetables (such as sweet pepper, tomato, cucumber, lettuce, eggplant, ...), on ornamentals (such as chrysanthemum, pelargonium, ...) and on open field crops (such as potato, beet, cabbage, tobacco, spinach, ...). The green peach aphid can transmit over 100 virus species.

The tobacco peach aphid (Myzus nicotianae) looks very similar to the green peach aphid. Only some microscopic features distinguish both species. Similar to the green peach aphid, the tobacco peach aphid affects several crops, of which tobacco is preferred. The "red aphid" that has shown up in sweet pepper and eggplant plantations since a few years, appears to be a red form of the tobacco peach aphid. This red aphid is mainly alarming because of its resistance to a lot of pesticides, which emphasizes again the importance of efficient biological control.

The potato aphid (Macrosiphum euphorbiae) is a 2-4 mm tall, elongated aphid with relatively long legs. The antennae are longer than the body. The eyes are conspicuously red. The cauda (tail) is relatively long, and the siphunculi are long with a dark tip. The potato aphid is usually green, but sometimes yellowish or pink. The larvae have a dark longitudinal line on the back. The mobility is also typical. Moreover, the aphid easily lets itself fall.

Although this aphid hibernates in North-America usually on roses, in Europe it passes the winter usually in the greenhouse. The potato aphid has more than 200 host plants such as tomato, eggplant, sweet pepper, chrysanthemum, rose, pelargonium, tobacoo and potato. It is often found on stalks or younger parts of a plant, which causes crimped tips that look like virus infestations.

The glasshouse potato aphid (Aulacorthum solani) is a middle-sized, round-oval aphid 1.8-3 mm long. Characteristic are the dark bands on the antennae, which are longer than the body. The cauda and spiracles are medium-long. Around the base of the spiracles dark green patches can be seen. This aphid usually has a glossy yellowish green colour, but this can vary from white-yellowish green to browny green.

The glasshouse potato aphid has no sexual phase, thus always reproduces viviparously on several host plant species. In the open field, potato and several bulb crops are attacked. In the greenhosue the main hosts are sweet pepper, chrysanthemum, tomato, eggplant, lettuce and bean.

Aphids can cause damage to crops in several ways:

1. They extract nutrients from the plant, which affects plant growth. Infestations on young leaves may give deformations later on.
2. The excess of sugar they absorb is secreted as honeydew, on which sooty moulds can grow that foul the plant.
3. They can transmit viruses.
4. They can bring toxic substances in the plant.

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