Sweet potato weevil (Cylas formicarius)

Sweet potato weevil can be devastating in Africa. losses between 73%¹-100%²[/efn_note]³ have been reported. The weevil damages crops in every life cycle. The most damage is through the female that lays eggs in the cracks of soil and exposed roots. The roots are covered with unsightly punctures that makes the tubers unmarketable. The larvae hatch in a week and feed, leaving a tunnel of destruction with excrement. The excrement, with terpene stench, makes it unmarketable and not fit for human and animal use⁴. Adults are good flyers so they spread easily to neighboring fields, starting new populations. Storage roots and runners are the most important spreading medium. With heavy infestations the vines turn yellow ⁵, initially, then darkens, cracks, and collapses.

Sweet potato varieties do not have a significant resistance to the weevil, but it is found that kairomone concentrations on the root surface can act as an ovipositional stimulant for female weevils⁶. So selecting varieties with higher kairomone concentration can lower damage⁷. Varieties that have deep roots/tubers are also less susceptible to weevil damage⁸. Treating the first 35cm of vines with Beauveria bassiana also helps to reduce weevil infestation. Removing crop residues from the field also helps against weevil infestation and propagation. Micro-climate does play a role in infestation levels. Through regular scouting every year and good record keeping, patterns can emerge when the weevil is most active and causes most damage. Minimum economic crop loss was found when sweet potatoes were planted during early November in Jamalpur⁹.

The most basic control measures are to keep infested material to the minimum. New transplants must not come in contact with old fields or planted as far as possible to old ones. Destroy all left over runners and tubers after harvest. Remove new sprouting tubers that are left over from the previous year. Ridging the tubers in the field will reduce the sweet potato weevil from reaching the storage roots to lay their eggs. A very easy way to reduce sweet potato weevil is to interplant with pepper, which is not only cost effective, but beneficial to the soil as well.¹⁰

There are three active ingredients registered in South Africa:

• deltamethrin
• gamma-BHC
• triazophus

Credit of featured image by Author Graham Wise from Brisbane, Australia https://commons.wikimedia.org/wiki/File:Cylas_formicarius_(14576849194).jpg

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