Crops: Cole crops, also mustard and radish
Why is it a problem? The wax, shed skins, mold, 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, Thimphu, only experience problems with late-season broccoli crops.
Adults are tiny (2–3 mm long) and grayish-green. Their body is covered in grayish-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-colored head and body.
Adult cabbage aphid
All stages and all aerial parts of the crop are attacked. Direct feeding leads to visible wilting in plants. An early attack can cause leaf curl and stunted growth. Molds can grow on aphid honeydew, destroying the appearance of plants for sale. 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 directly instead of eggs). Reproduction occurs throughout the 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 grow quickly under favorable 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 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 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, as well as mustard and radish.
This pest is mainly a problem with late-season broccoli.
Regularly inspect crops for early symptoms of colony buildup, especially in areas and times where problems have been experienced before.
Effect of variety
Cultivars can vary in tolerance to aphids, but this has not been studied in Bhutan.
- Parasitoids and predators are important in keeping the pest in check, especially lady bird beetle larvae (coccinellids) and syrphid flies. The presence of mummified aphids is the indicative of parasitism in the field. These should be encouraged as much as possible by minimizing 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.
- Chemicals have to be used early owing to rapid population build-up.
- Recommended chemicals are Cypermethrin (1 ml per 1 liter 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.