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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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A serious pest in
some locations. If populations are high, they can make it impractical to
grow susceptible crops on infested fields. More commonly, damage is patchy
within the crop.
Originally from southern South America, the whitefringed beetle is now widely
distributed, particularly in the cooler or more seasonal sweetpotato - growing
areas (southern USA, Australia, New Zealand and South Africa). In warmer
parts of East Africa, the
Rough Sweetpotato
Weevil causes similar damage.
The larvae live in the soil and gauge shallow cavities or
channels in
the storage roots. Damage resembles that of white grubs, except
whitefringed beetle larvae tend to leave rough ridges within the channels as
they are legless and do not move evenly over the surface. Damaged roots are not suitable for market or storage,
but may be fed to stock.
There is unclear evidence of larval feeding causing yield
reduction, but in some cases damaged storage roots may decay before harvest.
Adults are foliar feeders. They feed on the outer edges of
the leaves, producing characteristic notched or saw-toothed edges.
Adults are a type of weevil with a down-pointing snout (the family Curculionidae also includes the Rough Sweetpotato Weevil and West Indian Sweetpotato Weevil). For a weevil, they are very large, around 12 mm
long. They are light to dark grey-brown with characteristic white stripes
down the sides of their body. There is also a band of white hairs along
each side. They cannot fly, and have fused wing covers and only
rudimentary wings.
The white eggs are laid in small batches of around 10 to 60, on plant stems
or any other object near the soil surface, and covered with a sticky secretion.
The larvae
are legless, fat and slightly curved, growing up to 13 mm long. They are
white to yellowish with a light-brown head.
The pupae form in an earthen cell under the soil surface. They are
around 12 mm long and initially whitish but darken as they mature.
Life cycles are slightly different in winter and summer rainfall
areas. There is one generation per year, with usually larvae but
sometimes eggs overwintering in the soil. Some individuals may spend 18
months in the larval stage, having a two year cycle. Larvae generally pupate in late spring, in
an earthen cell usually between 5 and 15 cm below the soil surface. Pupation
lasts two to three weeks, but emergence may be delayed if the soil is dry and is
often triggered by heavy rainfall.
Adults live for two to five months and are present throughout the summer and
sometimes autumn. During the day they seek shelter and are most often found beneath
the canopy on stems and leaves near the soil. All adults are females
(males have occasionally been found in South America, but not in areas where the
species have been introduced), and reproduce by parthenogenesis (without
mating). They lay eggs in small batches throughout their life: one female
can lay over 2000 eggs, at more than 150 per day.
There are 7 larval instars, although some authors report up to
11. Most damage to storage roots
is done by the larger instars, from 4th to 7th. Hence most damage occurs
late in the growing season, when the current year's larvae have matured
sufficiently. Occasionally there are carry-over larvae from the previous
year which can cause earlier damage to a crop.
As adults are flightless, they do not spread rapidly. But
their wide host range ensures they can readily move from one field to the next.
Their introduction to new areas is believed to be usually as larvae in the soil
of potted plants, in tubers or as transported adults.
Host Range
Whitefringed beetles feed on at least 385 species of plants,
adults particularly preferring those with large broad leaves, including
cotton, peanut, soybean and many other leguminous crops. They are
considered a serious pest of a number of crops. Larvae also feed on roots
of orchard and forestry trees and can be a problem for tree establishment on
previously-cropped land.
Larvae can be collected before planting by burying baits of
corn, wheat or pieces of storage root, at about 15 cm depth. After about 3
weeks, remove a core of soil containing the bait, to about 30 cm depth, and examine it for larvae.
Adults can be found within the foliage in summer and autumn.
The leaf notching characteristic of their feeding is a reliable indicator of
their presence.
Cultivars vary in their susceptibility to attack by soil insects.
Varieties bred for resistance are available in some countries.
Repeated cultivation of the fields before planting will help
remove alternate food sources and expose the larvae to birds, other
predators and mechanical damage.
Soil-applied insecticides are commonly used to control the
larvae, but control is often poor. Insecticides are generally applied at planting,
and are required to maintain activity through the life of the crop.
However, the effectiveness declines over time and survival of only a small
percentage of the previous year's larvae can generate a new population
sufficient to cause damage as the crop matures. Foliar sprays to control the
adults can greatly reduce egg-laying and consequent damage by larvae, but must be done regularly (every one to two
weeks) after the first adults are detected.
In some locations it may be feasible to plant earlier (eg. by
raising sprouts under plastic shelters) so that the crop is harvested before the
new season's larvae mature.
Rotation with poor hosts such as cereals is critical and can
significantly reduces the weevil population.
Dixon
W.N., 2003. Whitefringed Beetles. Factsheet EENY-294 (originally published
as DPI Entomology Circular 309), one of a series of Featured Creatures from the
Entomology and Nematology Department, Florida Cooperative Extension Service,
Institute of Food and Agricultural Sciences, University of Florida. Published:
June 2003.
http://edis.ifas.ufl.edu/IN572
Matthiessen, JN. 1991. Population phenology of whitefringed weevil,
Graphognathus leucoloma (Coleoptera:Curculionidae), in pasture in a
Mediterranean-climate region of Australia. Bulletin of Entomological Research
81: 283-289.
Morgan E.R., Gianessi, L. and Marcelli M., Crop Profile for Sweetpotatoes in
Mississippi. Mississippi State University Pesticide Impact Assessment Program
with National Center for Food and Agricultural Policy,
http://pestdata.ncsu.edu/cropprofiles/docs/mssweetpotatoes.html
Sorensen, K.A. 2005. Whitefringed beetle on sweetpotato. Insect Note
#44, Vegetable Insect Pest Management, Department of Entomology, North Carolina
State University.
http://www.ces.ncsu.edu/depts/ent/notes/Vegetables/veg44.html
Zehnder, GW 1997. Population dynamics of whitefringed beetle (Coleoptera:
Curculionidae) on sweet potato in Alabama. Environmental Entomology 26: 727-735.
Zehnder GW, Briggs TH and Pitts JA 1998. Management of whitefringed beetle (Coleoptera:
Curculionidae) grub damage to sweet potato with adulticide treatments. Journal
of Economic Entomology 91: 708-714.
Contributed
by:
Jane O'Sullivan
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