A great many gardeners and allotment holders grow cabbages or other members of the brassica family each year, so will no doubt be familiar with a common garden pest – the mealy cabbage aphid.
This strain of aphid – grey/green in colour and covered with a powdering of white, mealy wax - just loves brassicas but has a particular penchant for cabbages, brussels sprouts, swedes and cauliflowers, and the wide open spaces (to an aphid) of the leaves allow very large populations to rapidly build up, starting around July and peaking in September and October.
Understandably, the leaves soon show signs of distress with such an aphid feeding frenzy taking place upon them – the typical symptoms are distortion of shape and the appearance of yellow patches. This is not a direct threat to the survival of established plants, although a check in growth is often recorded - the real damage occurs to young seedlings and transplants, normally a stage in a succession of sowings later in the year, when the constant sucking of sap can cause the plants to die. In a two pronged attack, the mealy cabbage aphid is also a vector of two viruses; both cauliflower and turnip mosaic.
What time of year are the aphids likely to appear then? Eggs which have over wintered hatch around April and the young aphids start to feed on brassica leaves. The late spring and early summer sees the pioneering winged aphids fly off to establish colonies on younger plants, which reach mass proportions by October. The cycle starts again as eggs can survive the winter on leaves and stems of brassicas – some adults will even pull through the winter if the weather’s not too severe.
Now we know the life cycle and movements of this pest, what about the all important subject of prevention and control? As the earliest generation of young aphids are extremely unlikely to emerge before April, getting the first wave of brassica plants off early is a great place to start. By using various protection measures such as fleece and polythene, an early sown crop should be established enough by April to withstand attack.
The introduction of flowering plants will attract natural aphid predators, which will gleefully devour the pests throughout the year, and of course you can simply destroy the earlier outbreaks by hand. Physically blasting the aphids from the leaves with a jet of water is always satisfying, and there are numerous organic pest control sprays available, the majority of which contain natural fatty acids which block the aphids breathing holes, causing death by suffocation. Unfortunately, these liquids can also knock out the beneficial insects you have worked hard to attract, so consider your options before reaching for the spray gun.
As the aphid eggs pass the winter on brassica stems and leaves, removing and destroying any lingering plants by April at the latest will severely decrease the early population. Allotmenteers may find this a problem if neighbours do not follow suit, but on the whole the practice of removing spent plants is the most disruptive to this pest. Finally, the option of chemical control is available – any insecticide with the active ingredient of bifenthrin is unlikely to be welcomed by the aphids, but again, the need to consider any natural predators exists.
The mealy cabbage aphid - not the most destructive of garden pests, and with a dash of good horticultural practice and general garden husbandry, one that shouldn’t cause too much distress to your brassica crops.