Red ant

Dorylus orientalis

Crop: Generalist, but can seriously impact vegetable crops including potato and Cole crops

Why is it a problem? Red ants live in the soil and can feed on roots and tubers of a wide variety of plants. They make holes on the surface of potato tubers, reducing tuber yield as well as market prices. They can also kill seedlings of a diverse range of plants including cole crops. Ants can prevent potato cultivation in badly-affected fields. Currently there are no effective management options.

Where and when is it a problem? Red ants are mainly a problem at lower elevations to about 600 m asl where it can be a chronic problem across significant areas. There have also been isolated reports of serious impacts from farms at higher elevations, such as near Talo (2,300 m asl).

IDENTIFICATION

 

Red ant

Adult: Worker ants are 3-6 mm long, sterile females with no functional eyes. Female queens and males are comparatively larger and look like wasps. The queen is brown and wingless. Males are reddish-brown to dark brown and have two membranous wings.

SYMPTOMS

The pest makes holes on the surface of potato tubers. Damaged plants wilt (quite similar to that caused by bacterial wilt) and can eventually die. Initially wilting will be temporary, occurring during the day and then recovering. Ants should normally be found in association with affected plants.

Red ant damage on potato
Red ant damage on potato

Confusion with other pests: Other, harmless, ant species can be present in fields, but red ants are the only ones that will be closely associated with roots and tubers. Red ants can also be superficially confused with termites which are also harmless. White grub also cause surface feeding damage to potato tubers, but holes caused by white grubs are much larger.

BIOLOGY

Lifecycle: Reds ants are social insects with a queen, female workers and males. They live in a nest, usually built 1.5-2 m under the ground. Subterranean worker ants forage for food along underground trails.

Dispersal: Adults have winged forms.

When can damage be expected? Once established red ants can become a chronic problem within a field.

Hosts: Red ants can feed on underground stems, tubers, and roots of a wide range of plants including potatoes and cole crops.

MANAGEMENT PRACTICES

There are currently no recommended management practices for this species in Bhutan. Management studies by the NPPC are underway. Red ant is also a serious problem in Nepal and is relatively well studied there. Management practices developed there are provided below, but have not yet been tested in Bhutan.

On-farm monitoring

The presence of red ant can be indicated by wilted plants and damaged tubers, and confirmed by the presence of red ants around affected roots. Locating the actual nests is very difficult.

Effect of variety

Unknown.

Non-chemical management

Flooding of fields may work, but is often impractical when crops are not being grown in rotation with paddy rice.  In Nepal application of Azadirachtin (0.00045%) at 100 ml per plant in soil near plant stems has been found to be effective, as has the use of cow urine (diluted one part urine to five parts water) to saturate the soil.

Chemical management

  • There is currently no  pesticide recommendation for Bhutan. Standard pesticide applications will not work as the ants live deep underground.
  • NPPC is currently testing new management methods, including spinosad and bait boric acid.

 

Version: NPPC 2017. Red ant V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact: nppcsemtokha@gmail.com
Image acknowledgements: NPPC.

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