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Class
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Insecta
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Order
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Coleoptera
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Family
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Curculionidae
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These weevils are serious pests in the sweetpotato growing areas
of Africa.
C. puncticollis and C. brunneus are confined to Africa. They
often occur simultaneously and might both emerge from one infested storage root.
Damage symptoms are similar to those of the common sweetpotato weevil
Cylas formicarius. Adult sweetpotato
weevils feed on the epidermis of vines and leaves, but damage is hardly
noticeable. Adults also feed on the external surfaces of storage roots, causing
round feeding punctures. The developing larvae of the weevil tunnel in the vines
and storage roots, causing significant damage. Frass is deposited in the
tunnels. In response to damage, storage roots produce toxic terpenes, which
cause a greenish discolouration, an off-taste and distinct odour. Depending on
the marketing and food security situation, the storage root is considered a
total loss or the affected part is cut off and the remaining part used as food.
Larval feeding inside the vines causes malformation, thickening, drying and
cracking of the affected vine.
Egg. The eggs of both species are shiny white and
roundish.
Larva. The legless larvae are
white and curved and have an orange head capsule. The last instar larvae of C.
puncticollis are distinguishably larger than those of C. brunneus.
Pupa. The pupae are white, with
that of C. puncticollis being the larger in size.
Adult. Adult Cylas spp. are
elongated and ant-like. C. puncticollis is completely black while C.
brunneus is not uniform in its colour. Most specimen are bicoloured like C.
formicarius, while others are black like C. puncticollis.
Average-sized adults of C. brunneus are smaller than adults of C.
puncticollis, which are 7 mm long.
Cylas spp. are found in almost all zones of sweetpotato production in
Africa, and are considered a severe constraint in relatively dry agro-ecological
zones and/or during dry growing seasons.
All sweetpotato weevil species have a similar life history. The adult female
lays eggs singly in cavities excavated in vines or in storage roots, preferring
the latter. The egg cavity is sealed with a protective, grey faecal plug. The
developing larvae tunnel in the vine base or storage root. Pupation takes place
within the larval tunnels. A few days after exclusion, the adult emerges from
the vine or storage root. Because the female cannot dig, it finds storage roots
in which to lay the eggs by entering through soil cracks.
Adults of all species may be conveniently sexed by the shape of the distal
antennal segment, which is filiform (thread-like, cylindrical) in males and
club-like in females.
At 27oC C. puncticollis has a total developmental period of
20-28 days, whereas C. brunneus takes 32-41 days. Adults of the first
species live an average of 140 days, whereas the latter dies after about 92
days. Both C. puncticollis and C. brunneus females lay
around 100 eggs in their lifetime.
The insects prefer sweetpotato. Cylas spp. can complete all their life
stages on several wild Ipomoea spp.
Detection of infestation is difficult, as adults are most active by night.
Early detection of the pest can be done by checking for swollen vine bases and
infested exposed roots. When damage has been detected, several plants could be
dug up to establish an idea of the damage level to the roots. This could lead to
the decision for an early harvest. The species-specific pheromones of the Cylas
species that are released by female weevils and attract males have been
identified.
When sweetpotato weevil populations are high, no single control method
provides adequate protection. The integration of different techniques, with
emphasis on the prevention of infestation, provides sustainable
protection.
Cultural control
Cultural control practices have proved to be effective against the
sweetpotato weevil and should be the main basis of control. The local growing
conditions and the use of the crop (commercial or for own consumption) will
indicate which practices should be applied. Cultural practices include:
- Use of uninfested planting material, especially vine tips.
- Crop rotation.
- Removal of volunteer plants and crop debris (sanitation).
- Timely planting and prompt harvesting to avoid a dry period.
- Planting away from weevil-infested fields, and/or using a barrier crop.
- Hilling-up of soil around the base of plants and filling of soil cracks.
- Removal of alternate, wild hosts.
- Flooding the field for 24 hours after completing a harvest.
- Applying sufficient irrigation to prevent or reduce soil cracking.
Chemical control
Dipping planting material in a solution of an insecticide (such as carbofuran
or diazinon) for 30 minutes prior to planting can control sweetpotato weevils
for the first few months of the growing season. Spraying insecticides after
planting is not (cost)-effective.
Host resistance
Varieties with immunity or a high level of resistance are not available. Some
varieties have low levels of resistance. Others escape weevil damage because
their storage roots are produced deep in the soil or because they mature quickly
and can be harvested early.
Sex pheromones
Several good and simple trap designs have been produced making use of locally
available materials. Mass-trapping of the Cylas species with their
specific sex pheromones will reduce the numbers of males, but the beneficial
effect on infestation rate and yield is limited. The availability and price of
pheromones will restrict the number of users.
Biological control
Promising biological control agents for sweetpotato appear to be the fungi Beauveria
bassiana and Metarrhizium anisopliae and the nematodes Heterorhabditis
spp. and Steinernema spp. The fungi attack and kill adult weevils,
whereas the nematodes kill the larvae. A solution of B. bassiana could be
used as a pre-planting dip.
Ants, spiders, carabids, and earwigs are important generalist predators that
attack weevils. The ants Pheidole megacephala and Tetramorius guineese
are used as biological control agents in Cuba.
Ames, T., Smit, N.E.J.M., Braun, A.R., O’Sullivan, J.N., and Skoglund, L.G.
1996. Sweetpotato: Major pests diseases, and nutritional disorders.
International Potato Center (CIP). Lima, Perú. 152 p.
Gonzales, S.S. 1925. The sweetpotato weevil (Cylas
formicarius Fabr.), Philippine
Agriculturist 14:257-281.
PANS. 1978. Pest Control in Tropical Root Crop. Manual No.
4. Centre for Overseas Pest Research.London.
235pp.
Smit, N.E.J.M. 1997. Integrated Pest Management for sweetpotato in Eastern
Africa. PhD thesis Agricultural University Wageningen, 151 p.
Smit, N.E.J.M. and van Huis, A. 1998. Biology of the African Sweetpotato
Weevil Species Cylas puncticollis (Boheman) and C. brunneus
(Fabricius) (Coleoptera: Apionidae). Insect Science and its Application, 18:
93-100.
Smit, N.E.J.M., Downham, M.C.A., Laboke, P.O., Hall, D.R. and Odongo,
B. 2001. Mass-trapping male Cylas spp. with sex pheromones: a potential
IPM component in sweetpotato production in Uganda? Crop Protection 20:643-651.
Wolfe, G.W. 1991.
The origin and dispersal of the pest species of Cylas with a key to the
pest species groups of the world, pp. 13-44. In: Sweetpotato Pest Management, A
Global Perspective (Edited by R.K. Jansson and K.V. Raman). Westview Press,
Boulder, USA.
Contributed
by: Nicole Smit
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