| CLICK
BEETLE (adult) and WIREWORM (larva)
Wireworms
are the larvae of many species in several genera called click beetles.
Wireworms cause the damage not the adult click beetles. Since every
geographic location has its own set of species, to know which wireworms
are causing damage to potato, it is necessary to trap them and identify
those in the potato field. An infested field will remain infested
for three to six years and planting there should be avoided.
Description
Adults
are slender beetles whose appearance varies greatly since many genera
and species comprise the group called click beetles which produce
wireworms.
Larvae
from click beetles are called wireworms and these are the damaging
form of the insect. During the early stages lasting one to two years,
wireworms are very small and white. Mature larvae lasting two to
six years are hard-shelled with dark transverse bands along the
length of their body. The body color is a shiny yellow to rust.
There are six (three pairs) slender legs toward the head region.
At the head, there is a pair of pincer-like protrusions. Wireworms
are 1/2 to an inch long at maturity and easily visible in traps
and occasionally may be found in or hanging from a potato tuber.
Eggs
are laid and pupa are formed deep in the ground; eggs are laid in
grassy areas.
Life
Cycle
 Adults
(click beetles) are relatively common in the summer and are often
attracted to lights at night. Their presence does not necessarily
indicate future problems. They overwinter in survive in the soil
for several months, overwintering, and emerge in the spring. Once
mated, click beetles seek egg-laying sites in grassy areas which
may be pastures or other sod areas. For this reason, wireworms,
the larvae, are most likely to be a problem in fields that have
recently been broken out of sod or grass crops. Click beetles also
may seek to deposit eggs in areas of cultivated fields where grassy
weeds are a problem or in cereal crops. Eggs hatch in a few days
to weeks, and the larva or wireworm emerges.
Wireworms
emerge in the soil and may survive for two to six years. In the
winter, they survive about two feet deep and as the temperature
worms move up through the ground to the top two to three inches
of soil. Wireworms move up and down in the soil during the season
depending on temperature. They prefer soil temperature to be 50
to 60oF. After wireworm achieve full maturity during the summer,
they will pupate in the soil, and the pupae will transform into
click beetles after a few days.
Damage
Wireworms,
the larvae of click beetles, damage potato by feeding primarily
on tubers. The damage appears as straight, round holes with smooth
walls. It is not clear whether the wireworm feeding is due to a
search for shelter or due to thirst and not actually eating. Early
in the season around planting, wireworms may drill into seed tubers
or seed-pieces thereby weakening them possibly resulting in secondary
infection especially by soft rot, in reduced stands and in weakened
young plants. Wireworms also can feed on young sprouts with the
same results. During the season, although not common, wireworms
could damage roots and underground stem. For potato, the major damage
is later in the season when tubers are maturing after bulking. Late-season
damage will result when larvae feed on tubers causing feeding scars
or feeding tunnels. The result of this feeding is reduced quality
of the harvested tubers. Wireworm damage is most likely the result
of feeding by larvae that are 2 or more years old. In most situations
larvae of several ages can be found in an infested field. This does
not affect seed production as seed vigor is not affected but the
damage causes culling for the fresh market. In processing, potato
chips will show gaps along the margin where the hole was located
and likewise french fries will show the gap at their ends.
Soil
Sampling
Wireworms
would likely cause significant problems only in a proportion of
a field. Even in those fields where they are present, wireworm damage
may be spotty. Soil sampling and baiting are somewhat effective
at determining if wireworms are present in fields to be planted
to potato. Wet wheat flour or germinating wheat, barley or oat seeds
can be used to attract wireworm larvae into buried bait stations.
These pests seem to be attracted to fermenting grain seeds. Wireworms
can be collected by sifting soil samples through window screen.
Sampling and baiting must be carried out when soil temperatures
are 50 to 60oF to ensure that wireworms are active near the surface
of the soil. Threshold levels have been released based on the number
of bait traps with wireworms. Economic risk assessment are based
on 3% of harvested tubers having wireworm holes. When baited traps
have two to four wireworms on the average, the level is considered
very high and the application of a chemical at planting is often
recommended. If the traps have more than four wireworms, the level
is considered extreme and the recommendation is not to plant potato
in that field. A rough estimate is that an average of one wireworm
per trap is equivalent to 20,000 wireworms per acre.
Table.
Risk of economic damage from wireworms: Economic damage is taken
as 3% of tubers damaged at harvest.
| Average
number of wireworms per bait |
Risk
of economic damage |
| 0 |
less
than 10% chance |
| less
than 0.5 |
10
to 33% chance |
| 0.5
to 1.0 |
34
to 50% chance |
| 1
to 2 |
50
to 74% chance |
| 2
to 4 |
74
to 90% chance |
| more
than 4 |
greater
than 90% chance |
|
Bechinski,
E.J., Sandvol, L.E., Carpenter, G.P. and Homan, H.W. 1994. Integrated
Pest Management Guide to Wireworms in Potatoes. Univ Idaho Coop
Ext Circ #760.
Management
Complications
dealing with wireworm life cycles and biology make wireworms a difficult
insect pest with which to deal.
Biological
Because
of wireworms long life cycle, growers need to be aware of the cropping
history of a field and also the severity of grassy weed problems
in all of the fields areas. Fields with cereal cropping history,
a history of grassy weed problems and newly cultivated soils need
to be avoided to minimize wireworm problems. Crop rotations would
have to exceed six years unless bait-traps indicate that wireworms
are no longer present. Planting after an alfalfa field that has
been in production for several years is excellent. Although not
reported, personal observations suggest wireworm preferences for
different varieties. For instance, wireworms have a distinct liking
for Krantz and seem to have a moderate preference for Russet Norkotah.
They do not have a particular taste for chipping potatoes nor for
Shepody.
Chemical
If
soil sampling or baiting indicate a significant presence of wireworms
and planting in the field cannot be avoided, the options for wireworm
control are to apply a pre-plant broadcast or planting-time application
of an insecticide. Control of wireworms is difficult resulting in
high use rates and incomplete control. This is the result of the
difficulty in trying to move the insecticide down into the soil
in a high enough concentration to obtain wireworm control and for
an insecticide to last long enough to protect mature tubers near
harvest. Where wireworm populations are very high and land cannot
be rotated to a less sensitive crop than potatoes, soil fumigation
is an option. Foliar treatments are not effective nor available.
Quick
Review
Appearance:
- Adult
- click beetles comprises many genera of insects
- Larvae
- wireworms, shiny yellow to rust with dark bands, six frontal
legs and pincer mouth parts, up to 1 inch long
Life
Cycle:
- Wireworms
survive in the ground for up to six years
- Adults
live for up to a year
- Full
life cycle is three to seven years in the same field
Damage:
- Wireworms
eat holes in tubers and other underground organs
- Mostly
tuber quality loss, some yield loss possible due to stand
Management:
- Biological
- avoid field, plant after several years of alfalfa
- Chemical
- soil-applied, long-residual product, soil fumigation
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