Sweet Potato weevil
Namibian crop pests  #  50
(Cylas spp.) Order: Coleoptera Family: Brentidae

There are approximately 25 Cylas species. This weevil has not been identified to species level yet. The description is based on Namibian field material and the CABI Crop Protection Compendium notes on  Cylas puncticollis and Cylas formicarius.

Sweet potato tuber showing deep
weevil attacks

Same sweet potato tuber cut open to reveal
larva (centre) and tunnels. Discolouration of the white flesh happened before the photographer could finish the photo.

Adult sweet potato weevil,
5 mm long
Note: 
This troublesome pest is not reported from Omusati, Ohangwena and Oshana regions yet. Please be very careful if you bring sweet potato cuttings or tubers from other areas into these regions. 

 

Common names: 

 No vernacular names were recorded. 

Pest status:

 Partly unknown. The specimens all come from the same farm, 50 km northwest of Tsumeb, Oshikoto region. It is mainly mentioned from Tsumeb area and Kavango and Caprivi regions but may also have been confused with another and larger leaf weevil. 
To my knowledge, this species has not yet been noticed in the Ohangwena, Oshana and Omusati regions. This may change due to promotion of sweet potato growing in all regions. It is possible that the sandy soils in north west do not support the pest.

From literature, this species is known to host on other Ipomoea species as well and Maize and Sesamum may also be host crops. In other African countries it often causes considerable losses in sweet potato fields.

Description:

 Symptoms are mainly noticed at time of harvesting. The tubers show deep tunnels and frass and are completely damaged. Damage and spread of adult weevils may continue to develop after harvest. The tuber flesh is often discoloured and tubers are not marketable or edible any longer. As the tubers often develop very bitter substances they cannot even be used for animal fodder.

The larva is white and a few mm long. It tunnels deep into the tubers or stems of sweet potato.

The adult beetle is a shining black weevil (snout beetle), 5-6 mm long. It looks like an ant. Several older books report that Cylas have wings and can fly quite well, but according to CABI they are wingless or can hardly fly. 
 

Control:

 I am not aware of control measures from Namibia yet so here is summary from literature. 

Crop rotation
Crop rotation has been the most successful control in other countries. As the weevils do not move very fast infestation is reduced by moving the new plantings as far from old sweet potato crops as possible.

The infestation spreads via old tubers and debris from sweet potato fields. It is therefore very important to remove all debris from harvested sweet potato fields.

Use young clean cuttings.
Weevils are often spread with infested cuttings. Especially the older cuttings are more infested than young shoots. Select younger cuttings from clean motherplants or dip the cuttings in  Malathion or another insecticide before planting.

Variety differences ("resistance")
There are some claims about "resistant" varieties in other countries but they may not be useful for Namibia where we only have few released varities. It may, however, be useful to select varieties that set tubers deep in the ground as the weevil can only burrow a centimeter or so through the soil.

Hilling up the furrows to fill cracks
It is believed that weevils mainly attack roots and tubers through surface cracks in the hills. If your soil has a tendency to crack open, extra hilling up may reduce the weevil attack.

Chemical control
Weevils are difficult to control with chemicals as they live so hidden in tubers and stems. The "A Guide to the use of Pesticides and fungicides in the Republic of South Africa" mentions control with:

  • deltamethrin  (50 mls /100 litres)
  • triazophos     (100 mls / 100 litres)
  • trialomethrin  (40 mls / 100 litres)
All three chemicals are applied when the pest is noticed and repeated at 3 weeks interval.

Note:  
Do not use Gamma-BHC or DDT although they are mentioned in older books.. 
 

Other notes:

 		 Thanks to Wilhelmina, Marion and Jurgen for helping out by making the first specimens and tubers available for identification and photography.

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