Weevils & other sweet potato pests

The sweet potato weevil (Cylas spp) is arguably the most destructive pest of this crop.

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The sweet potato weevil can lay up to 250 eggs in the plant’s roots or stems. The larvae burrow into the tubers, causing rot.hat you can become aware of it only at harvesting – by which time a high percentage of the roots might be unmarketable. These weevils are a dark metallic blue or black. They are about 6mm long and have a slender, prominent snout which makes them easy to identify. Although they eat the leaves, making small holes in them, the damage is not readily noticeable. For this reason, you should frequently check for the pest by looking for tunnels on the thick stems that enter the ground and on any exposed roots.

The adults can live for up to three months, although in warm weather, the life cycle can be as short as one month. During this period, they lay up to 250 eggs. The female chews into the stem or root and lays a single egg at a time. The larvae tunnel into the stem or root, causing it to become bitter and rot. During a severe infestation, the root may have so many holes that it looks like a sponge.

Beware of cracks in the soil
Soils with a fairly high clay content often crack above the developing roots, giving the weevils easier access to the roots. So make sure that the ridges are ‘earthed up’ sufficiently to minimise this risk. If weevils are present and the roots are well covered, the damage is likely to be confined to the roots near the stems.

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Like hawk moths, this weevil can also attack other members of the family, such as Convolvulus and Ipomoea (morning glory).
Look out for these weeds and remove them. Don’t discard damaged roots – they will be a source of re-infestation. Destroy them completely. 

Rough sweet potato weevil (Blosyrus spp)
This is another species of sweet potato weevil. It is brown with indistinct lighter and darker patches and has a much less pronounced snout. It is slightly larger – 8mm to 9mm long – than the typical sweet potato weevil. If disturbed, it falls to
the ground and plays dead. In contrast to the Cylas species, they eat the leaves from the edges, not the middle. Also, unlike their cousins, the larvae bore into the soil to find developing roots. Should you come upon signs of these two weevil species, contact your pest control representative immediately for advice on spraying.

The sweet potato weevil can lay up to 250 eggs in the plant’s roots or stems.
The larvae burrow into the tubers, causing rot.

Sweet potato leafminer

The adult sweet potato leafminer is a small, slender, light-coloured moth, which lays up to 300 eggs, deposited singly along the midribs on the underside of leaves. The larvae develop rapidly, and in warm conditions the life-cycle can be less than a month. The larvae are light-coloured with red spots along their sides. They tunnel into the leaves making ‘windows’. When the larva is nearly fully grown – about 7mm long – it sometimes leaves its tunnels and starts another elsewhere. Because the larvae tunnel within the leaves, they are best controlled by a systemic product. Only a percentage will be killed with a contact product. They can do considerable damage, and as with the other sweet potato pests, can also occur in other members of the sweet potato family.

Other pests

There are a few more sweet potato pests that can appear from time to time but I have never had to apply control measures for them. The brightly coloured tortoise beetle, for example, can eat the leaves. In the unlikely event that you will need to control this pest, you can choose from several effective insecticides.

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