Aphids
Beginner
Scientific Names
Myzus persicae, Brevicoryne brassicae, Lypaphis erysimi
Identification
- Small, pear-shaped insects, with cornicles or “tail-pipes” near the tip of their abdomen
- Typically found in colonies located on the underside of the leaf, or along the stem or petioles
- Aphids pierce the leaves and suck the sap from many vegetable crops, causing leaf distortion
- Aphids act as a primary vector of several economically significant virus disease, spreading the virus as they feed
Often Confused With
Tarnished plant bugs
Leafhoppers
Period of Activity
Aphids are present throughout most of the growing season. Populations build quickly during hot, dry weather. Aphid populations are often kept below threshold levels where predators (ladybird beetles, lacewings, minute pirate bugs, etc. ) are present in abundance.
Scouting Notes
Carefully inspect 10 groups of 10 plants across the field. Check the underside of leaves from the top, middle and bottom of plants. Watch for the presence of aphids on the developing plant.
Threshold
None established. The degree of damage depends on the crop grown, its stage of development and the number and species of aphid present. Examine seedlings and young transplants frequently and treat as required to prevent damage. Protect foliage or flower stalks intended for harvest, as well.
Advanced
Scientific Names
Myzus persicae, Brevicoryne brassicae, Lypaphis erysimi
There are three aphids commonly associated with cruciferous vegetables in Ontario: the green peach aphid (Myzus persicae), the cabbage aphid (Brevicoryne brassicae), and the turnip aphid (Lypaphis erysimi).
Identification
Wingless green peach aphids are uniformly pale yellow, green or pink. They are opaque in appearance rather than shiny. On crucifers, green peach aphids are most often found on the lower surfaces of older leaves or on flower shoots.
Wingless, summer forms of the cabbage aphid are greyish-green and covered with a powdery greyish-white wax, while winged forms lack the heavy wax coating. The turnip aphid is similar in appearance and habits to the cabbage aphid. Adults are a dusky greenish-brown, but the body of the turnip aphid is more broadly rounded and all stages appear darker.
Aphids have long, piercing mouth parts which allow them to suck juices from the plant. The cabbage aphid injects a toxin into the plant when it feeds which causes leaf cupping and a yellowish mottling. The turnip aphid does not inject a toxin. Aphids produce a sticky substance called “honeydew” which can promote the development of secondary moulds (sooty mould). The chief damage caused by these aphids to the crop consists of contamination by themselves and their cast-off skins, the development of sooty mould and transmission of viral diseases.
Often Confused With
Tarnished plant bugs
Leafhoppers
Biology
In Ontario, aphids can be found on crucifers beginning in late spring. Aphids can take two forms, a winged form, capable of migrating across considerable distances, and a wingless form, that remains in place on one plant. As aphids become crowded, or the host plant begins to deteriorate, winged forms are produced. When these winged forms find suitable hosts, they produce wingless offspring. These wingless offspring are capable of reproducing without mating resulting in 'colonies' of wingless aphids that remain in place on the host until conditions deteriorate and more winged forms develop.
The green peach aphid overwinters outdoors as a small black egg on the twigs of peach and other Prunus species. Several generations develop on peach in the spring before the winged forms are produced which migrate to a large number of summer hosts including all of the cruciferous crops. The wingless forms then develop on the summer host. Significant numbers reproduce indoors on numerous plant species and will escape outdoors when the weather becomes favourable. As well, bedding plants and transplants can become infested in the greenhouse.
In addition to direct feeding damage, aphids also vector viruses such as turnip mosaic virus. Aphids transmit the virus in a non-persistent manner, meaning that an aphid can acquire the virus from an infected plant and transmit it to a healthy plant after feeding for a very short period of time. Insecticides are not an effective means of controlling the disease, as infection occurs very rapidly and plants will become infected before aphids are killed. Additionally, aphids colonizing crucifers and winged migrant aphids are both important vectors of the virus.
Period of Activity
Aphids are present throughout most of the growing season. Populations build quickly during hot, dry weather. Aphid populations are often kept below threshold levels where predators (ladybird beetles, lacewings, minute pirate bugs, etc. ) are present in abundance.
Scouting Notes
Carefully inspect 10 groups of 10 plants across the field. Check the underside of leaves from the top, middle and bottom of plants. Watch for the presence of aphids on the developing plant.
The green peach aphid tends to be distributed fairly uniformly in a field, but numbers can vary between crops and between fields. Populations are often higher initially along field margins and hedgerows.
Cabbage aphids form large dense colonies in the hearts of plants or on leaves and flower stalks. Because this aphid can severely retard the growth of plants, susceptible crops need to be thoroughly inspected and treatment is usually required if any infestations are found. In the case of cabbage aphid, spot-treatment may be practical.
Colonies of the turnip aphid are not as dense as those of the cabbage aphid and their feeding does not cause the same degree of leaf curl.
Thresholds
None established. The degree of damage depends on the crop grown, its stage of development and the number and species of aphid present. Examine seedlings and young transplants frequently and treat as required to prevent damage. Protect foliage or flower stalks intended for harvest, as well.
Root crops (e.g. rutabaga) can sustain large populations of green peach aphid and turnip aphid later in the season, whereas leafy crops (e.g. broccoli) can only sustain small numbers due to possible contamination of the crop at harvest.
Management Notes
- After harvest, crop residues that are heavily infested with aphids should be worked into the soil. This will prevent aphids from moving into later plantings and will also reduce the numbers of cabbage and turnip aphids surviving through the winter.
- Transplants should be examined for aphids and treated before placing in the field. Even relatively small numbers of aphids can increase rapidly and damage transplants.
- There are numerous species of predatory and parasitic insects which serve to regulate aphid populations. Important predators include the ladybird beetles, minute pirate bugs, lacewing larvae, syrphids, predatory midges, ground beetles and spiders.
- Aphids on crucifers are also commonly attacked by small parasitic wasps such as Diaretiella rapae. Aphids that contain a developing parasite swell and turn a papery grey or brown.
- Aphid numbers often increase dramatically after the application of insecticides which are detrimental to beneficial insects. Broad-spectrum materials such as pyrethroids should be used only as required in an IPM program where aphid control is a consideration.
- Later in the season, large numbers of aphids frequently become infected with fungal diseases. Outbreaks of disease are favoured by dense plant canopies, large aphid populations, and moist conditions. Periods of heavy rain can directly dislodge aphids from plants and promote the development of these diseases.
