Sweet potato weevil management in drought

 During “mid-drought”, when there is complete crop failure, it is important to manage the limited and available food resources such as sweet potato (kaukau). 

Sweet potato is the major stable food (across PNG), yet highly vulnerable to drought and frost.  

Mid-drought begins when food production is severely restricted by the lack of rain, and ends when there is return of good rains and further frosts cease to occur. 

Managing sweet potato weevil during mid-drought is an on-farm coping strategy recommended by NARI.

What is sweet potato weevil?

During a drought or after a frost, a small weevil commonly attacks sweet potato when the tubers are still in the soil. 

This is sweet potato weevil (Cylas formicarius), the worst insect pest of the root crop. The weevil bores holes in the tubers, causing them to taste bad and rot quickly. 

This insect pest is very difficult to control, even with insecticides. Weevils can cause losses of up to half of a sweet potato production and sometimes completely, especially during the dry season.

The adult sweet potato weevil is about six millimetres long with a long nose and reddish body. It may look like an ant but is a true weevil. It falls and pretends to be dead when disturbed.

During the day, the weevils rest under the sweet potato plants or in cracks in the soil. Male weevils are active at night while females feed and lay eggs during the day. 

The females prefer tubers for feeding and laying eggs. This means that tuber initiation and formation stages of the sweet potato plant influence the egg laying behaviour of the female weevils.

What do sweet potato weevils do?

Adults and larvae cause serious damage to the crop because they spend their entire life cycles on the host. 

Both the feeding and egg laying punctures degrade eating and marketing quality of tubers. 

Tubers stored with egg punctures will serve as a source of infestation for clean tubers stored beside them. 

Hatched larvae bore into sweet potato vines or tubers and start feeding and growing within the tissues. These cause most of the damage in tubers.

Obvious signs of weevil damage are thickening or malformation in tubers, feeding marks and frass (insect excreta) accumulation in the tunnels that show as dark coloured patches on the surface. 

The sweet potato tubers react to the damage by producing a poisonous substance with a bad smell that spoils the flavour. 

This poison is harmful to the lungs and heart of human beings and livestock. For this reason, weevil damaged tubers should not be used as human or animal food. The weevil-damaged tubers are also susceptible to Black Rot, a fungal disease that causes tuber rotting.

Can we control this weevil?

Sweet potato weevil is difficult to control effectively. However, farmers can prevent or minimise the damage by doing the following:

1. Cultural methods of weevil management

• Rotate sweet potato with other unrelated crops to reduce weevil population build up. In highly infested areas, farmers can select deep rooting and early maturing varieties so that the tubers are formed deep under the ground, making weevil access difficult. Early maturing varieties reduce weevil contact time and ensure early harvesting of good tubers.
• Always select clean planting material from weevil-free plants or a weevil-free field often obtaining about 20-30cm vine cuttings from the growing tips.
• Always cover the exposed tubers and soil cracks to prevent weevil entry.
• Always remove and destroy all crop residues to prevent weevil infestation in a new field or new planting. Remove other host plants like kangkong from the sweet potato field. 
• Cover the soil with mulch (e.g. grass) as it helps to keep the soil moist and prevent soil cracks. Mulch also provides a more favourable place for natural enemies.
• Allow bush to grow in the weevil-infested field and return later. This helps to break the life cycle and reduce the weevil population.

2. Natural enemies (biological control)

Protecting and allowing natural enemies to control the pest is very important in pest management. 

These organisms include earwigs, ants, ground beetles and spiders that can kill and eat the weevils. Ants’ nests can be moved to the sweet potato field 

to allow the ants to kill the weevils. 

Other organisms can live on the weevils and kill them eventually 

A fungus known as Beauvaria bassiana has proven to be fairly effective in killing the weevils. This pathogen lives in the soil and can be easily cultivated on coffee residue, wheat and rice straw.  The planting material and the soil can be treated with Beauvaria bassiana mixture to reduce the weevil population.

3. Chemical Control

The behaviour of the adults makes chemical control very difficult because they rest under sweet potato plants and in soil cracks during the day and are active at night. Hence, it is not only inconvenient but not practical for farmers to spray insecticides at night. This is an expensive option.

4. Pheromone traps

Sex pheromone traps (attractive chemicals) attract adult male weevils in large numbers that can be collected and killed, thus helping to reduce the male population.  Sex pheromone traps are not available in PNG at present

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• Next week’s article will focus on Simple Irrigation Systems for PNG