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Class |
Insecta |
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Order |
Coleoptera |
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Family |
Curculionidae |
Euscepes postfasciatus rivals the common
sweetpotato weevil,
Cylas formicarius as the most important pest of sweetpotato
in the Caribbean and Central America.
Damage is similar to that caused by
Cylas formicarius.
The West Indian sweetpotato weevil is present
throughout sweetpotato-producing regions of the Americas, and in Japan, where an
eradication program is currently underway. No published reports have been
found of its presence elsewhere.
The adult weevils feed within
vines and storage roots, and the larvae cause most damage by tunnelling within
the storage roots. The damage is characterized by
small feeding and ovipositional punctures on the surface and larval tunnels
filled with frass throughout the flesh of the storage roots. The weevil induces
the roots to produce terpenes, giving them a bitter taste and unpleasant odour,
so that even partly damaged roots are unfit for human and animal consumption.
Egg. The ovoid eggs are yellow to greyish yellow, and laid singly
in shallow cavities in the storage roots, and covered with a faecal plug.
They are similar in size to those of other sweetpotato weevils.
Larva. The white larvae are legless
and less than 1 cm in length.
Pupa.
The pupae are creamy white and found within the tunnels in roots and stems.
Adult. The adult weevil is
reddish brown to near black, with a compact body covered with short bristles,
which are arranged in parallel rows on the abdomen. The head is small and
protrudes little from the thorax except for a prominent, downward-curving snout.
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 fecal 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, she finds storage roots
in which to lay her eggs by entering through soil cracks.
Hot, dry weather favours weevil development. At optimal temperatures of 27-30oC,
C. formicarius completes development (from egg to adult) in about 33
days. Adult longevity is about 75-105 days and females lay between 100 and 250
eggs in this period. At suboptimal temperatures, development takes longer.
Detection of early infestation is difficult as adults are most active by night.
Early detection can be done by checking the base of the plant for feeding
punctures and exit holes. 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.
As no sex pheromone is available for this species, pheromone traps can not be
used to evaluate the weevil populations. Recently, green
light-emitting-diode (LED) traps and pitfall traps have been used successfully
to monitor this species.
Wild species of Ipomoea are important host plants.
When sweetpotato weevil populations are high, no single control method
provides adequate protection. The integration of different techniques (IPM)
, with emphasis on the prevention of infestation, provides sustainable
protection.
Cultural control
Cultural control practices have proven 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.
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Sanitation and
use of uninfested vine tips as planting material.
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Crop rotation.
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Timely
planting and prompt harvesting to avoid a dry period.
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Hilling-up
of soil around the base of plants and filling of soil cracks.
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Applying
sufficient irrigation to prevent or reduce soil cracking.
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(Sex
pheromone
traps are species-specific, and have not been developed for E.
postfasciatus).
Chemical control
Dipping planting material in a solution of an insecticide (such as carbofuran
or diazinon) for 10-15 minutes prior to planting can control sweetpotato weevils
for the first few months of the growing
season. Dipping longer than necessary can burn and kill the planting
materials. Spraying insectides after planting is not (cost)-effective.
Host resistance
Cultivars with immunity or a high level of resistance are not available. Some
cultivars have been shown to be less attractive to West Indian weevils in
free-choice tests for feeding and oviposition. However, whether this can
be equated to less damage in the field has not been demonstrated. Early maturing
and deeply rooted varieties expectedly get less weevil damage and can even
escape weevil infestation.
Biological
control
Predators and
parasitoids which attack Cylas species generally also attack E.
postfasciatus. There is little evidence that naturally occurring
predators and parasitoids effectively control weevil populations.
More promising
biocontrol agents are the entomopathogenic fungi (especially Beauveria
bassiana) and nematodes (especially Heterorhabditis spp.)
Applications of inoculum have been shown to reduce weevil populations and damage
in the field. However, these pathogens are poorly persistent in the field,
and the cost of inoculum is likely to be prohibitive for most semi-subsistence
growers.
Sex pheromone trap
systems, while proven moderately effective for controlling the common
sweetpotato weevil, are not currently available for E. postfasciatus as
the pheromone has not been synthesized.
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.
Jansson, R.K. 1992. Biological approaches for management of weevils of
root and tuber crops: a review. Florida Entomologist 75, 568-584.
Moriya, S. and Miyatake, T. 2001. Eradication programs of two sweetpotato
pests, Cylas formicarius and Euscepes postfasciatus, in Japan with special
reference to their dispersal ability. JARQ - Japan Agricultural Research
Quarterly 35, 227-234.
Nakamoto, y. and Kuba, H. 2004. The effectiveness of a green light
emitting diode (LED) trap at capturing the West Indian sweet potato weevil,
Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculoinidae) in a sweet potato
field. Applied Entomology and Zoology 39, 491-495.
Wanderley, P.A., Boica, A.L. and Waverley, M.J.A. 2004. Resistance of sweet potato
cultivars to Euscepes postfasciatus Fairmaire (Coleoptera: Curculionidae).
Neotropical Entomology 33, 371-377.
Contributed by:
Jane O'Sullivan
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