Crops: Cole crops, also mustard and radish
Why is it a problem? The wax, skins, mould and honey dew associated with cabbage aphid colonies can spoil the appearance of Cole crops for market. This is primarily a problem for broccoli where the heads can become unsaleable.
Where and when is it a problem? Although it is expected to occur in all Cole crop growing areas, losses remain unquantified. Farmers in Bjemina Valley only experience problems in late-season broccoli crops.
Adults are tiny (2-3 mm long) and greyish-green. Their body is covered in greyish-white mealy wax, which is also secreted onto the surface of plants and extends throughout the colony. Winged aphids (alates) are slightly longer and have a dark-coloured head and body.
All stages and all aerial parts of the crop are attacked. Direct feeding leads to visible wilting of plants. Early attack can cause leaf curl and stunted growth. Moulds can grow on the honey dew produced by aphids, spoiling the appearance of plants for market. Often the first signs of attack are small bleached areas on the leaves. Leaves turn yellow and become crumpled, protecting the aphid colonies within.
Confusion with other pests: The waxy colonies that are formed are unlikely to be confused with any other aphid.
Lifecycle: Reproduction is mostly parthenogenetic (asexual – without mating) and vivaparous (producing young ones directly instead of eggs). Reproduction occurs throughout summer and where winter is mild. In colder regions, sexual reproduction may also take place. In Bhutan eggs are laid during November-December and probably hatch towards the end of March. There are four nymphal stages prior to the adult stage. Crowding of nymphs in spring and summer usually results in the production of alates (winged aphids). Populations can rapidly increase under favourable conditions, with the lifecycle taking 11 to 45 days depending on temperature.
Dispersal: Winged adult stages are produced under some conditions.
When can damage be expected? This pest is poorly studied in Bhutan. However, farmers in Bjemina Valley who were surveyed in 2016 reported that cabbage aphids are only a problem in late-season broccoli (November to December). Cabbage aphids were rare during field surveys conducted in the July at the same farms.
Hosts: Cabbage aphids are virtually restricted to the family Brassicaceae. In Bhutan it is a common pest of Cole crops, and also on mustard and radish.
This pest is mainly a problem in late-season broccoli. Monitoring is necessary to detect outbreaks in time. Insecticides need to be used for serious outbreaks.
Regularly inspect crops for early symptoms of colony build up, especially in areas and times when problems have been experienced before. Act quickly once detected.
Effect of variety
Cultivars can vary in tolerance to aphids, but this has not been studied in Bhutan.
- Parasites and predators are important in keeping the pest in check, especially lady bird beetle larvae (coccinellids) and syrphid flies. These should be encouraged as much as possible by minimising the use of chemicals.
- Stick to recommended application rates of nitrogen fertilizers. Over-fertilization makes plants more succulent and therefore more vulnerable to aphids.
- Maintain healthy crops as plants in poor condition are more prone to attack.
- On a small scale, aphids can be washed or rubbed off infested plants, or infested plant parts removed.
- Soap solutions have been recommended elsewhere, as have botanicals such as neem.
- Chemicals have to be used early owing to rapid population build-up.
- Recommended chemicals are Cypermethrin (1 ml per 1 litre of water) and Chlorpyrifos (4 ml per 1 litre of water). A waiting period of at least two weeks prior to harvest needs to be observed.