Sweet potato weevil management

 By JAMES LARAKI

SWEET potato weevil (Cylas formicarius) is the worst known pest of sweet potato (kaukau). 

The weevil bores holes in the tubers, causing them to taste bad and to rot quickly. 

The pest is very difficult to control, even using insecticides. 

Weevils can cause the loss of up to half the sweet potato produced and sometimes can lead to the loss of the entire crop, especially during dry seasons.

During a drought or after a frost, sweet potato tubers still in the soil are commonly attacked by the weevil, which if not controlled, can contribute significantly to food shortage.

The adult weevil is about six millimetres long with a long nose and has a reddish body. 

It could be mistaken for an ant. 

It falls and pretends to be dead when disturbed.  

Like most insects, it undergoes four different stages in its life cycle (i.e. egg, larva, pupa, adult) and takes about 33 days to complete a life cycle. 

The time it takes to complete each stage depends mainly on temperature; the higher the temperature, the faster the development of the life cycle.

Hot and dry weather is favourable to weevil attack of sweet potato because the tubers are more easily reached through cracks in the soil. 

The adults can live up to 94 days and produce about 50–250 eggs, with most of the eggs produced during the first 50 days.

Management of sweet potato weevil

Sweet potato weevil is difficult to control effectively, even with the use of insecticides. 

However, farmers could prevent or minimise the damage through a number of cultural, biological and chemical control methods.

Cultural methods

• Crop rotation – rotating sweet potato with other unrelated crops will be helpful to reduce weevil population build up;
• variety selection – in highly infested areas, it is useful to select deep rooting and early maturing varieties so that the tubers are formed deep under the soil, making weevil access difficult. Early maturing varieties would reduce weevil contact time and ensure early harvesting of good tubers;
• clean planting material – always select clean planting material from weevil-free plants or a weevil-free field;
• earthing up – always cover the exposed tubers and soil cracks to prevent weevil entry;
• sanitation – always remove and destroy all crop residues to prevent weevil infestation in a new field or new planting. Remove other host plants like taro Kongkong from sweet potato field;
• mulching – cover the soil with mulch as it helps to keep the soil moist and prevent soil cracks. Mulch could provide a more favourable environment for natural enemies of the weevil, and, 
• Fallow – allow bush to grow in weevil-infested field and return later. This helps to break the life cycle and reduce weevil population.

Natural enemies 

(biological control)

Protecting and allowing natural enemies to control is very important and is the most effective way to manage pests, not only for sweet potato weevil but for other pests as well. 

Biological control includes predators, parasites and pathogens. 

Predators include other organisms that may kill and eat the weevils. 

Parasites include organisms living on the weevils and eventually killing them. 

Pathogens are microorganisms like a fungus or bacteria that may kill weevils. 

For example, the fungus, beauvaria bassiana, has proven to be fairly effective in controlling weevils. 

This pathogen lives in the soil and it can easily be cultivated on coffee residue and rice straw. 

For effective control, planting material and the soil can be treated with Beauvaria bassiana mixture to reduce weevil population.

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. Chemical control is not recommended as it is an expensive option and for concerns.

Pheromone traps

Using sex pheromone traps (attractive chemicals release by females) attract adult male weevils in large numbers that can be collected and killed, thus help to reduce the male population. 

Sex pheromone traps are not available in Papua New Guinea at present but is an option that could be explored.

The incident of weevil attack and its control is very important as sweet potato is an important staple food in PNG. 

The issue as reported in 1997 drought was that weevils were found to destroying sweet potato tubers, contributing significantly to the food shortage, particularly in the highlands. 

Thus the measures discussed above and other options to control weevil become useful in our drought preparedness activities and awareness.