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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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Elateridae, Chrysomelidae
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Soil-dwelling
beetle larvae can cause considerable damage to storage root marketability in
some areas. As wireworms, rootworms and flea beetle larvae cause similar
damage, the problem is sometimes referred to as the "WDS complex", where the contribution of particular
species to the damage is not specified. Secondary infection of the wounds
may cause rot of some storage roots, but more often the scars heal. The
roots are still edible, but may not be marketable.
Damage by these soil insects was only reported from sub-tropical to temperate
production areas. No reports were found from tropical countries: they may
be present but of less concern.
Wireworms are widespread, with different species and genera present in various countries.
In USA, various species of the genus Conoderus have been reported, including C. scissus Schaeffer,
C. rudis (Brown), C. vespertinus (F.), C. amplicollis (Gyllenhal)
and C. falli Lane, as well as the corn wireworm, Melanotus communis
(Gyll.), which is more common in cooler areas. Species of the genus
Agrypnus are widely distributed, and in Australia, Hapatesus hirtus
Candeze was recorded.
Cucumber beetles (rootworms) are native of North America. All
references found to cucumber beetle and flea beetle damage to sweetpotato were
from USA, and it is not known if they are a problem elsewhere.
Storage root surface is damaged with round to irregular crater-like
holes or
shallow channels, usually less than 5 mm deep but sometimes over 1 cm. The holes are larger than those produced by
sweetpotato weevils, and
are often referred to as "shot-holes" by farmers. Older scars
may appear healed, and may be deepened by the expansion of the root.
Recent scars have ragged edges and may contain
frass.
Initially the larvae make a small hole in the skin of the
sweetpotato, then eat an enlarged cavity just under the skin. In the case
of flea beetle larvae, they may make short winding tunnels below the skin. The skin over the
cavity dies, leaving ragged edges on recent scars and an open cavity on
healed scars.
Wireworm scars are usually randomly scattered over the root,
while those of cucumber beetle are usually in groups. Wireworm damage is
reported to be greatest late in the season producing many fresh scars at harvest
(but this may only apply to Conoderus spp with a single-year lifecycle), while cucumber and flea beetles often attack
at early stages of storage root development, resulting in mainly healed-hole injury.
Cucumber beetle scars are most commonly round, while flea beetles produce some
elongated or irregular channels as a result of tunnelling.
Flea beetle larvae feed primarily on the fibrous roots, but when
populations are high, they can cause significant damage to storage roots.
The larvae of all species live in the soil and don't remain inside storage
roots for prolonged periods. Thus they may not be easily detected during
harvest.
Adult click beetles do not feed on the crop. Cucumber
beetle adults feed on leaves producing irregular-shaped holes. Sweetpotato
flea beeltes produce narrow brown tracks or grooves, while the Systena
flea beetles characteristically make small circular shot-holes on the leaves.
Wireworms
The larvae
are slender, 1 to 4 cm long, with a hard, often shiny and distinctly segmented
cylindrical to flattened body. They can vary in colour from cream to
yellowish to
red-brown, usually with darker brown heads and tails. Their three pairs of short legs
on segments behind the head are barely
visible from the
dorsal view. They have bristly hairs at the sides of body segments,
and a flattened tail segment that is typically forked or notched in the centre and serrated
around the margin.
Adults are typically small (less than 1 cm) elongated oval-shaped beetles
with distinctly segmented antennae, pitted bodies and may have various markings on the
elytra (wing
cover). They are referred to as "click-beetles" because of their ability to
"click" themselves upright when placed on their backs.
Cucumber beetles
Eggs are oval, orange-yellow colour and found in clusters of 25-50 on the
underside of host leaves.
The larva is a soft-bodied cylindrical grub, about 12-20 mm long when mature.
It is cream to yellowish with a dark head and six brown legs.
The adult beetles are around 5-6 mm long with domed, smooth, shiny
elytra carrying
distinctive markings. The 12 spotted cucumber beetle has a yellow to
greenish body with black spots on the elytra, and black head and legs. The
banded cucumber beetle is greenish with a red head and broad yellow bands across
its elytra.
Flea beetles
The flea beetle species that feed on sweetpotatoes (both foliage and roots)
are the pale-striped flea beetle (Systena blanda), the elongated flea
beetle (Systena elongata), and the sweetpotato flea beetle (Chaetocnema
confinis). They are small beetles around 2 mm long, and have strong
rear-pointing back legs with enlarged femurs, with which they jump like a flea when disturbed.
The creamy coloured eggs are laid in the soil. The larvae are white,
soft-bodied and grow to around 9 mm long. They have brown heads and a
fleshy pointed tail.
Wireworms
Most species of wireworm feed on decaying plant material.
Relatively few species feed on living plants (feeder roots as well as storage
roots and tubers). These may be pests in a range of crops, particularly
affecting seedling establishment of cereal and vegetable crops.
Larvae are most abundant in
the upper layers of the soil, with younger instars concentrated in the upper 10
cm and older instars often found somewhat deeper, to around 30 cm. They
are highly mobile in the soil, and don't stay at one feeding site for prolonged
periods.
Species differ widely in the length of their lifecycle (1 to
7 years) and the number of larval instars (around 4 to 15). For
multiple-year species, larval instars
of varying age can be found all year. The final larval instar is
longer-lived than the earlier ones, and is the most damaging. Pupae form
in earthen cells in the soil near the food source. Pupation may take one
to three weeks, but emergence may be delayed if soil is dry. Adults generally hide under leaf
litter or soil clods during the day. They do not feed on the crop.
Cucumber beetles
Larvae of both the banded cucumber beetle (Diabrotica balteata)
and the spotted cucumber beetle (Diabrotica undecimpunctata howardii
Barber) feed on the storage roots of sweetpotatoes.
Like click-beetles, adults lay eggs near the base of plants,
often under fallen leaves. The larval stage lasts from eight to 30
days, and pupae are found in earth cells just below the soil surface.
There are two to three
generations per year.
Flea beetles
Eggs are laid in the soil near the base of host plants.
Larvae feed for about three weeks. The whole life cycle takes around 30
days in warm weather, and there are typically several generations per year.
Host Range
Wireworms have a very broad host range, including many weeds
grasses, cereal and vegetable crops.
Cucumber beetles prefer cucumber, melons, pumpkin and squash but
may feed on a wide range of crops, including corn, peanuts, potato and wild
grasses.
Particular species of flea beetle tend to specialize on one or a
few host plants, but preferences differ with each species.
Wireworms may be quite difficult to find in soil samples and in
damaged roots. The most reliable method for detecting them and determining
population density is through the use of baits, such as buried pieces of storage
root or germinated grains left in the ground for 1 to 3 weeks, the optimum time
depending on species.
Adult click-beetles are only active at night and shelter under
plant debris or soil clods during the day. They have been collected by inspecting under clods
in bare fields before planting, or using black light traps placed above the crop.
Adult flea beetles and cucumber beetles are active during the day, and can be
collected using sweep nets.
Cultivars vary in their susceptibility to attack by soil
insects, and use of resistant cultivars is the most reliable method of damage
control. Some cultivars show parallel resistance to weevils, but this is
not always linked.
Controlling weeds along borders of the field and plowing under
crop debris destroys overwintering and egg laying sites. Repeated
cultivation of the fields before planting will help remove alternate food
sources and expose the larvae to birds and other predators.
Waterlogging of soil will reduce the insect population.
Soil-applied insecticides are usually effective in reducing
damage, but do not eliminate the pest.
Experimental control methods, using parasitic nematodes, have
been successful in controlling damage.
Chalfant, R. B., Bondari, K., Sumner, H. R., Hall, M. R. (1993). "Reduction
of Wireworm (Coleoptera, Elateridae) Damage in Sweet-Potato with Insecticides
Applied by Chemigation." Journal of Economic Entomology 86(1): 123-130.
Chalfant, R. B., Hall, M. R., Johnson, A. W., Seal, D. R., Bondari, K.
(1992). "Effects of Application Methods, Timing, and Rates of Insecticides and
Nematicides on Yield and Control of Wireworms (Coleoptera, Elateridae) and
Nematodes (Tylenchida, Heteroderidae) That Affect Sweet-Potato." Journal of
Economic Entomology 85(3): 878-887.
Fronk, W. D. and L. E. Peterson (1956). "Wireworm Control in Iowa Sweet
Potato Fields." Journal of Economic Entomology 49(4): 479-481.
Horne, P.A and Horne, J.A (1991). The Life-History and Control of Hapatesus
hirtus Candeze (Coleoptera: Elateridae) in Victoria. Australian Journal of
Agricultural Research 42, 827-34.
R.P Griffin and J. Williamson, Clemson Extension – Home & Garden Information
Center. Clemson University.
http://hgic.clemson.edu/factsheets/HGIC2215.htm
E. R.
Morgan, L. Gianessi and M. Marcelli, Crop Profile for Sweetpotatoes in
Mississippi. Mississippi State University Pesticide Impact Assessment
Program and National Center for Food and Agricultural Policy.
http://pestdata.ncsu.edu/cropprofiles/docs/mssweetpotatoes.html
Schalk, J. M., Bohac, J. R., Dukes, P. D., Martin, W. R. (1993). "Potential
of Nonchemical Control Strategies for Reduction of Soil Insect Damage in
Sweet-Potato." Journal of the American Society for Horticultural Science 118(5):
605-608.
Seal, D. R. and R. B. Chalfant (1994). "Bionomics of Conoderus-Rudis (Coleoptera,
Elateridae) - Newly Reported Pest of Sweet-Potato." Journal of Economic
Entomology 87(3): 802-809.
Seal, D. R., Chalfant, R. B., Hall, M. R. (1992). "Effectiveness of Different
Seed Baits and Baiting Methods for Wireworms (Coleoptera, Elateridae) in
Sweet-Potato." Environmental Entomology 21(5): 957-963.
Seal, D. R., Chalfant, R. B., Hall, M. R. (1992). "Effects of
Cultural-Practices and Rotational Crops on Abundance of Wireworms (Coleoptera,
Elateridae) Affecting Sweet-Potato in Georgia." Environmental Entomology 21(5):
969-974.
Sorensen, K.A. 1993. Cucumber beetles. Vegetable Insect Note 25, North
Carolina Cooperative Extension Service, North Carolina Stat University.
http://www.ces.ncsu.edu/depts/ent/notes/Vegetables/
Contributed
by:
Jane O'Sullivan
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