Crop: Generalist, but is only a pest of potato
Why is it a problem? Aphids are sap-sucking insects that at high densities can cause leaf curl and stunting, resulting in poor potato production. However, globally green peach aphid is known as one of the most important virus vectors, being capable of transmitting over 100 viruses on a wide range of crop hosts. In Bhutan their greatest impact is expected to be their role in transmitting potato viruses (especially Potato Leaf Roll Virus), although this has not yet been confirmed.
Where and when is it a problem? They are common in all potato growing areas but rarely, if ever, cause production losses in the main potato-growing areas. However, they are an important consideration in the production of seed potato owing to their likely role as virus vectors. At 2,400-2,700 they are expected to be an important virus vector, especially prior to the monsoon. Their pest potential is expected to be greatest at low altitude, but this hasn’t been quantified.
Immature (nymphs) and adults are soft bodied and pear-shaped, and pale green to yellow or pink. They are visible to the naked eye (up to about 2 mm long). Adults have a winged stage. Aphid colonies can be easily found in plant terminals and on the underside of leaves.
Damage is caused by nymphs (immatures) and adults sucking sap from young leaves. Sucking of sap causes leaf curl and stunted growth. They also excrete sweet sticky substances (honeydew). The black sooty mould that often grows in the honeydew, although not directly harming the plant, may block photosynthesis and reduce yield.
Confusion with other pests: Many aphid species have been reported in Bhutan. Peach-potato aphid (also known as green peach aphid) is thought to be the main species on potato. Confirmation of identification needs to be done by a specialist.
Lifecycle: Nymphs and adults suck the plant sap, weakening their host. They can pass through many generations in a year. They are mostly found under leaves.
Dispersal: Winged females can travel long distance in the wind, including to colonise summer hosts.
When can damage be expected? Populations can increase very fast with increasing temperatures at 2,400-2,700m. This also coincides with the planting times for potato at those altitudes. Greater than 100 aphids per 100 compound leaves is not uncommon. Aphid populations normally decline with the onset of the monsoon. High temperatures, hard rains and damp weather reduce populations. Population studies have not been done at lower altitudes.
Hosts: This aphid species utilises many different hosts. In Bhutan it has only been reported as a pest of potato, although it has also been recorded off cole crops, mustard and lettuce. Elsewhere it is thought to use Prunus species (including peach – Prunus persica) exclusively as a winter host but to feed on over 40 different families in summer, including many important crops.
Aphid monitoring and management is most important in ensuring that the production of commercial seed potato is virus-free. It is rarely, if ever, a problem for the production of table potatoes in the main potato growing areas (> 2,000 m asl). Specific management may be needed at low altitudes, especially if potato production increases there, but no studies have yet been conducted there.
Monitoring is critical when growing seed potatoes commercially. The standard methodology is to count the number of aphids per 100 compound leaves with the threshold density for growing quality seed potato set at 20 aphids per 100 leaves. At mid to high altitudes monitoring on commercial seed potato farms should be done from planting through to at least the start of the monsoon season. Routine aphid monitoring for general potato production at those altitudes is not necessary. Not enough is known about aphids at low altitudes to be able to make monitoring recommendations.
Effect of variety
- Parasites and predators are important in keeping aphid pests in check, especially the larvae of syrphid flies and ladybird beetles. These should be encouraged as much as possible through limiting the use of insecticides.
- The easiest way to avoid virus transmission by aphids is to grow seed potato above about 2,800 m asl where aphids do not naturally exceed threshold densities.
- Remove all weeds which act as secondary hosts, and also volunteer plants (self-grown potatoes).
- Pick heavily infested leaves and destroy.
- Properly store seed potatoes as tuber sprouts are readily colonised by aphids.
Use of insecticides is rarely, if ever, needed in the main potato growing areas for table potatoes. Contact NPPC for advice if outbreaks occurs. There are organic solutions available for managing aphids. NPPC can provide specific advice for growing certified seed potatoes.