Potato XXII – 2
Wireworms
Whitney S. Cranshaw
Wireworm damage.
Field Biology and Identification
Wireworms are the larval stage of a family of beetles commonly called
"click beetles." There are several species of wireworm whose life
cycle may require one or more years per generation. The insects usually overwinter in the adult stage. Adults are brownish or even
blackish in color, elongate and tapering toward each end but more so toward the
rear. The earliest stages of larvae are very small and white, later stages have
a characteristic hard-shell appearance and a shiny yellow to reddish-brown
color with six slender legs. Mature larvae range from 1/2
to one inch (12 to 25 millimeters) in length, depending on the species.
Females deposit eggs in the soil. When the eggs hatch the larvae feed on tubers. In potatoes, wireworms usually stay in the top two or three inches (50 to 80 millimeters) of soil. During winter, they move as far as two feet (0.6 meter) deep. The larvae mature in from two to five years. Fully developed larvae form pupation chambers in the soil and pupate. Adults emerge from the pupae and remain in the soil until the following spring.
Plant Response and Damage
Wireworms feed on seed pieces in the spring, occasionally damaging young
shoots. The damaged seed pieces secondarily become infected with bacteria or
fungi, and plants grow weakly or fail to emerge. Later in the summer, wireworms
bore into tubers, leaving straight round holes which usually heal over and do
not become infected with rotting microorganisms.
Management Approaches
Biological Control
No applied biological control practices exist for control of wireworms.
Cultural Control
Since wireworms have a developmental period of several years, the cropping history of each field is important when planning to grow potatoes. Wireworms are usually present following small grains, corn, and beans, in weedy alfalfa fields, and pastures. Pure stands of alfalfa are less likely to harbor wireworms. Wireworm infestations tend to be spotty, and often reoccur in the same locations for several years.
Chemical Control
Elimination of wireworms requires control measures to be applied for several
years. Seed treatment of small grains in the rotation will help reduce buildup.
Sampling/Surveying/Timing of Sampling
Wireworms can be sampled in the spring if soil temperatures are above 45°F (7°C) and the soil is neither saturated nor completely dry. Two methods are effective: by pouring soil samples through 1/4 inch hardware cloth over window screen, or by baiting. When using the screen method, use a two-handled post-hole digger to collect soil samples randomly through the field, to a depth of at least six inches (25 centimeters), sift the soil through the screens and count the wireworms collected. The post-hole digger will pick up about 1/4 square foot area of soil, so the number of wireworms collected can be multiplied by four to give number per square foot. Total wireworms found multiplied by four and divided by the number of soil sample collected, equals the average number of wireworms per square foot. Bait station traps may be useful for wireworm sampling in potatoes. Each bait station should be dug four inches deep (10 centimeters) and nine inches (23 centimeters) wide. Place one-half cup of a corn-wheat seed (untreated) mixture in the bottom of the hole, add water if the soil is dry. Fill the hole and mound it slightly, then cover it with black plastic. The plastic can be held in place by spreading dirt around the edge. The plastic increases the soil temperature. The bait stations should be in place two to three weeks before planting. Because of variability, two bait stations per acre is recommended. Check the bait stations prior to planting and record the number of wireworms in each.
Economic Thresholds
0.4 wireworms per square foot is the threshold for control in potatoes using the screen method. This average is equal to about 17,000 wireworms/acre
By counting the larvae in and around the bait station, the potential for a
wireworm problem can be determined. An average of one wireworm larvae per bait
station (assuming two stations/acre) is the equivalent
to 20,000 wireworms/acre, and is the threshold for control. Insecticides listed below may reduce wireworm
damage but are unlikely to give control exceeding 80 to 90 percent.
Product List for Wireworms:
Insecticide |
Product per
|
Preharvest Interval, Remarks |
|
Thimet 20GR,
Phorate 2OGR Rampart 10GR Phorate 15GR |
11.3-17.3/1000 row-ft 23-35 oz/1000 row-ft 13.5-23.3 oz/1000 row-ft |
Thimet, Rampart and Phorate are
phorate-based products. Use as a planting time
band. 24 hour reentry after application. |
|
Mocap 15GR |
27 oz/1000 row-ft |
24 hour reentry. Apply as
a 12 inch band over row at planting. |
|
Mocap 10 G |
2.1 lbs per 1000 row-ft 40-60 lbs/A (broadcast) |
Band Applications in 12
inch band mixed with top 2 to 4 inches of soil. Broadcast treatment should be made within
two weeks of planting. |
|
Mocap ECR |
4.4 fl oz per 1000 row-ft 2.7-40 qts/A
(broadcast) |
Band Applications in 12
inch band mixed with top 2 to 4 inches of soil. Broadcast treatment should be made within
two weeks of planting. |
|
TenaxR |
Variable, depending on row spacing |
48 hour reentry. Avoid
direct contact of granules with seed piece. Tenax
is a fonfos/phorate combination. |
|
RRestricted
Use Pesticide. 1Labelled for chemigation.
|
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The information herein is supplied with the understanding that no
discrimination is intended and that listing of commercial products, necessary
to this guide, implies no endorsement by the authors or the Extension
Services of
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Categories: Potato, Insects, Wireworms
Date: