Pest insects in brassica crops: aphids
Brassicas can be infested by several species of aphid. Find out about the most important ones, what damage they cause and what you can do to limit their activity.
Brassica crops can be infested by several species of aphid, including:
- Cabbage aphid (Brevicoryne brassicae)
- Peach-potato aphid (Myzus persicae)
- Potato aphid (Macrosiphum euphorbiae)
The cabbage aphid feeds only on brassica species, while the other species have a wider range of hosts. In the UK, all species overwinter as mobile aphids on a range of cultivated and wild host plants, including oilseed rape. Some cabbage aphids may also overwinter as eggs. In the spring, aphid development becomes more rapid and winged forms are produced. These fly to new hosts, from which infestations of wingless aphids may develop. Further winged forms are produced later in the summer and may infest new crops.
All species cause direct damage if they exist in high enough numbers, particularly the cabbage aphid, which can distort plant foliage and may kill smaller plants.
All species are contaminants and produce honeydew that can support sooty moulds, which also become a contaminant. The peach-potato aphid, in particular, can transmit plant viruses, especially Turnip yellows virus, which may reduce yield significantly.
Monitoring and forecasting
All three species are captured in the network of suction traps run by the Rothamsted Insect Survey. The information on captures is updated weekly, and bulletins released on the website on a Friday contain information on captures for the week ending on the previous Sunday.
Bespoke monitoring services using yellow water traps are available. Regular crop walking is important to identify developing problems. In early March each year, the Rothamsted Insect Survey releases long-term forecasts. These predict the timing of the first migration of all three species into crops in the spring/early summer and their relative abundance early in the year. Over the last 50 years, the timing and size of aphid migrations have shown the strongest correlation with the mean January/February air temperature. This is used to produce forecasts for different locations.
Non-chemical control methods
There are no established non-chemical methods for reducing the overall population of aphids. However, all species are attacked by a range of predators (e.g. ladybirds, hoverfly larvae, lacewing adults and larvae) and parasitoid wasps. In addition, aphids can be infected with a naturally occurring fungus that can lead to considerable mortality within an infestation. A combination of predators, parasitoids and fungi often leads to a rapid decline in aphid numbers in the middle of the season. This is known as the ‘aphid crash’.
Fine mesh netting used to exclude cabbage root fly may also exclude winged aphids if the mesh size is sufficiently small. However, if aphids manage to access the crop, for example, through a hole, their numbers can increase very rapidly.
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