Summary:  Wireworms cause damage in potatoes and other crops.  The most common form of crop protection (Neonicotinoid seed treatments) can deter wireworm feeding but do not kill the pest.  Fipronil (a phenylpyrazole insecticide) can kill wireworms and may reduce pest abundance in subsequent years and crops.  Fipronil (Regent ®) can be used on potatoes as an in-furrow spray.   The objectives of the project were to:

In Year 1: 1) test the efficacy of Fipronil to reduce wireworm damage in potato and in wheat planted in the next year and 2) compare Fipronil applied as a seed treatment to the in-furrow application. 

In Year 2: 1) test “carry-over” effects of year 1 treatments on wireworm abundance and damage in spring wheat.  

The experiment was conducted from 2014 to 2015 at the MSU-Western Ag. Research Center, Corvallis, MT with funding from the Montana Potato Advisory Committee.  The plots were located in a field with high wire worm densities.  Fipronil applied with Cruiser Maxx ® Potatoes, a common seed treatment, provided nearly complete protection from wireworm damage in potatoes and did not affect potato quality.  Applying Fipronil as a seed treatment was as effective as an in furrow spray.  Seed treatment may be more convenient and economical for producers.  Cruiser Maxx ® Potatoes alone did not reduce wireworm damage relative to two controls, Fungicide alone and an Untreated Control.  The effects of the treatments on wireworm abundance and damage in spring wheat were measured in 2015.  Fipronil applied in potatoes did have carry-over effects into the subsequent wheat crop. Wireworms were rare in plots were Fipronil was applied the previous year, averaging less than 1 worm per trap.  Similar to the results of the first year, the two Fipronil +Cruiser Maxx treatments were the only treatments that reduced wire worm abundance relative to the controls and Cruiser Maxx ® Potatoes alone did not reduce wireworm numbers. 

Background: Potato production provides over $40 million dollars in annual returns to Montana’s economy. Wireworm damage reduces potato producers’ profits by damaging potato and other crops grown in rotation with potatoes.  Wireworms are the larval stage of click beetle species that live in the soil and feed on a variety of crops. 

For more background on this pest complex visit: Bugwood or the Wireworm Research Group that collaborated with the station on this project.   

Montana potato growers would benefit from improved wireworm control.  The insecticides currently used for wireworm management (Neonicotinoids e.g. Cruiser Maxx Potato ™) deter feeding but do not reduce wireworm numbers and are utilize the same mode of action in potato and cereal grains, increasing the potential for wireworms developing resistance.   An alternate insecticide (Fipronil-phenyl pyrazole) is registered for use in potatoes. This insecticide has a different mode of action that causes wireworm mortality.  Previous research has shown that a single application of Fipronil in a wheat seed treatment causes nearly 100% mortality of Dusky wireworms (Agriotes spp.) and provides three or more years of crop protection (Vernon et al 2009).  Fipronil (Regent ®) use is not allowed currently in wheat but is registered to be used potatoes as an in-furrow spray.

By reducing wireworm densities in the soil, a single Fipronil application may only improve potato yields, but also provide protection and improve yields in subsequent crops.  This research is aimed at evaluating the efficacy of Fipronil against the wireworm biotypes commonly found in irrigated potato production systems in Montana (Limonius spp.).  

Methods:  The study occurred at the Western Agricultural Research Center, Corvallis, MT.  The area in which the experiment was conducted exhibited wireworm damage in previous cereal crops and wireworm densities in the spring of 2014 were high.  In 12 traps, 10 had at least one wireworm (Limonius californicus).  An average of 2.5 wireworms were collected per infested trap, well above the economic threshold of 1 worm per trap. 

Potatoes were planted in the first week of June into 12 ft x 12 ft plots in 4 rows spaced 3 ft apart.  Within a row, seed potatoes were planted one foot apart.  Seed potatoes were treated with one of five treatments (Table 1).  Treatments consisted of two no-insecticide control (fungicide alone and an untreated control), a common seed treatment (Cruiser Maxx ® Potatoes=CM) and two treatments where Fipronil was added to the CM treatment, either as an in furrow spray or as a seed treatment.  Note, whole potatoes were used in the controls and cut potatoes in the CM and CM+Fipronil treatments.  The experiment utilized a randomized complete block design with five blocks (see plot map below).  

The field was managed using standard fertilization and weed control practices.  All plots were irrigated (6 inches applied total in three, two inch irrigations) throughout the growing season.  


Table 1:  Treatments used in the experiment and the associated levels of wire worm damage on all potatoes in the sample. 





Cruiser Maxx ® Potatoes (CM)


Actara ® insecticide (Thiamethoxam)

Maxim ® L PSP fungicide (fludioxonil)

Systemic Insecticide Fipronil (F)

Average percentage of potatoes damaged by wire worms

UTC= untreated control





CM=Cruiser Maxx






Yes (in furrow)




CM+ F Seed treat



Seed treatment


CM+ F In furrow



In Furrow



Data Collection and Analysis: In each plot, two rows near the center of the plot were excavated and all potatoes were collected on September 22, 2014. Small potatoes (<1 inch in diameter) were removed as they exhibited no wireworm damage and would not be harvested by a mechanical harvester. The remaining 20 to 55 potatoes (~5-10lbs) per plot were sorted into large (>2 inches in diameter) and medium sized potatoes (<2 inches in diameter). The number, average weight, number of tubers with wireworm holes, and average number of holes in the wireworm damaged tubers were recorded for each size class in each plot. Yield was not measured. However, we were able to estimate yield loss due to wireworms assuming all damaged tubers would be culled. Differences among treatments in potato quality (proportion of large potatoes in sample and mean weights of large and medium sized potatoes) and extent of wireworm damage (probability of wire worm damage and severity of damage (mean number of holes in damaged tubers)) were analyzed using ANOVA with a random effect of block. When block did not explain variance in a response variable, the random effect was removed to increase the power to detect differences. Probabilities were converted to log (odds) for analysis.

In 2015, wireworm abundance and damage to spring wheat were sampled in the same plots. No additional treatments were applied. Wireworm abundance was sampled three times during the month of May. The traps which consisted of two cups of untreated wheat seed in nylon stockings. The traps were soaked in water overnight in speed germination. The trap was buried in the soil at a depth of 4 to 6 inches and left for 10 days. After the traps were removed from the plots, wireworms were collected from each trap and surrounding soil using a Berlese Funnel. The total wireworm collected per plot were compared using ANOVA.
Spring wheat was planted in the plots in Mid-May. Wireworm damage was assessed by measuring the density of wheat seedlings in Mid-June. Wheat stand density was highly variable due to poor soil-seed contact at planting. The wheat stands were similar among treatments but this is likely due high variance in stand density obscuring treatment effects.

Potato Quality: Quality measures were not affected by treatments. The proportion of large potatoes was similar among treatments (F4,16=1.0, p>0.1) with an average of 76% of the potatoes. Mean potato weights for total sample (F4,16=1.75, p>0.1) and large potatoes (F4,16=0.8, p>0.5) were not affected by the treatments. Across treatments large potatoes weighed an average of 119 g or about ¾ lbs. Average weight of medium potatoes tended to be lower in the two Fipronil treatments (mean=37.2g) than the other treatments (mean=41.2g) (F4,16=2.4, 0.1>p>0.05).

Wireworm Damage
Risk of Wireworm Damage: The treatments affected the likelihood of wireworm damage with Fipronil providing superior protection. Overall, the probability of damage was highest in the treatments without insecticide (UTC and Fungicide), intermediate in Cruiser Maxx alone, and lowest in the two treatments that contained Fipronil (Table 1). To compare treatment effects, separate analyzes were conducted on large and medium sized tubers. Medium sized tubers, due to both age and size, are smaller targets for wireworms and were less likely to experience than large tubers (paired t-test of the 17 plots that experienced some wireworm damage, p<0.005). Averaged among treatments without Fipronil application, 40.6% of large tubers were damaged by wire worms compared to 20.6% of medium sized tubers (Figure 1).

In large potatoes were risk of wireworm damage was highest, the differences among treatments were large (F4,20=19.6, p<0.00001, Figure 1). Forty percent (95% C.I.=33-50%) of untreated potatoes were damaged by wireworms. The likelihood of wireworm damage was not reduced by treating the seed potatoes with fungicide or Cruiser Maxx. The absence of effect of the Cruiser Maxx treatment is in part due to high plot to plot variation in wireworm damage. Percent of potatoes with wireworm damage ranged from 12 to 45 % in Cruiser Maxx, 23 to 85 % in Fungicide alone, and 11 to 83% in the Untreated Control. Adding Fipronil, either in furrow or as a seed treatment, reduced the rates of wireworm damage to an average of less than 3% (95% C.I.= 1 to 5%). The method of Fipronil application did not alter the protection against wireworms.

In the medium size potatoes, damage rates differed among the treatments (F4,20=3.4, p<0.05, Figure 1) but treatment effects were not as large or as consistent as in the large potatoes. Similar to the large potatoes, Fungicide alone and Cruiser Max treatments did not reduce rates relative to the Untreated controls. There was no damage recorded in the two Fipronil treatments. In the non-Fipronil treatments damage levels were highly variable, limiting the ability to detect treatment differences. Rates of wireworm damage ranged from 0 to 50% in CM, 0 to 30% in fungicide alone, and 0 to 56 % in UTC. Due to the highly variable damage in this size class and an artifact of the transformations required for the analysis, all treatments were similar in Post-hoc tests (Tukey’s HSD p>0.1).

Effect on risk of wireworm damage
Potato Yield:  Yield was not directly measured but assuming damaged potatoes would be culled, the Fipronil application would reduce yield losses due to wireworms dramatically. In the UTC, Fungicide, and CM treatments, 30 to 40 % of large and 10 to 23% of medium potatoes would be culled. In contrast, only 1 to 5% of the large and no medium potatoes would be damaged and require culling in the two Fipronil treatments.

Table 2: Severity of damage across treatments and potato sizes.  Average number of wireworm holes in damaged tubers are presented.  Not all plots had damaged tubers.  The number in parentheses is the number of plots used to calculate the mean. 





CM+Fip-In furrow


Seed treatment


3.13 (5)

1.90 (5)

1.73 (5)

4 (1)

1 (1)


2.09 (3)

1 (2)

1.75 (4)



Year 2 Results:

Effects on Wireworms One year later
Effect of wireworm abundance differed among Year 1 treatments (F2,20=5.2, p<0.01, see figure 2). Wireworms were rarely observed in both treatments were Fipronil was applied (mean wireworms per plot<0.5). No wireworms were trapped during the month of May in 70% of these treatments plots. In contrast wireworms were common in other treatments. Wireworms were trapped from all untreated control plots (UTC) with up to 30 larvae trapped per plot in May.  Similar to the results observed in the year one potatoes, Cruiser Maxx alone did not reduce wireworm abundance relative to the two controls (Fungicide and Untreated control).   The fipronil treatments were the only treatment were average wireworm levels were below economic threshold (1 larvae per trap).

Conclusions:   Fipronil is only labeled for use in potatoes, but these results suggest that using this insecticide in this crop will reduce wireworm abundance and damage in subsequent crops. Fipronil was effective at controlling the type of wireworm, common in Montana.  Application, either as seed treatment or furrow, reduced the rates of wireworm damage in potato to less than 5%.  Fipronil kills wireworms, unlike other commonly used insecticides, reducing their numbers in the next year. 

Literature Cited:
Vernon, R.S., W.G. Van Herk, M. Clodius, and C. Harding. 2009. Wireworm management I: stand protection versus wireworm mortality with wheat seed treatments. J. Econ. Entomol. 102: 2126-2136