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© 2007 Plant Management Network.
Accepted for publication 2 October 2006. Published 19 July 2007.

Monitoring for Pyrethroid Resistance in Bollworm (Helicoverpa zea) Populations in Texas: Trends from 2003-2005

Patricia V. Pietrantonio, Associate Professor, and Terry A. Junek, Research Assistant, Department of Entomology, Texas A&M University, College Station 77843; Roy Parker, Professor and Extension Entomologist, Texas Cooperative Extension, Corpus Christi 78406; Ed Bynum, Extension Agent IPM, Texas Cooperative Extension, Sweetwater 79556; Greg Cronholm, Extension Agent IPM, Texas Cooperative Extension, Plainview 79072; Glen Moore, Extension Agent IPM, Texas Cooperative Extension, Waxahachie 75165; Dale Mott, Extension Agent IPM, Texas Cooperative Extension, Georgetown 78626; Chris Sansone, Associate Professor and Extension Entomologist, Associate Department Head, Extension Entomology, Texas Cooperative Extension, San Angelo 76901; Kerry Siders, Extension Agent IPM, Texas Cooperative Extension, Levelland 79336; and Noel Troxclair, Assistant Professor and Extension Entomologist, Texas Cooperative Extension, Uvalde 78802

Corresponding author: Patricia V. Pietrantonio.

Pietrantonio, P. V., Junek, T. A., Parker, R., Bynum, E., Cronholm, G., Moore, G., Mott, D., Sansone, C., Siders, K., and Troxclair, N. 2007. Monitoring for pyrethroid resistance in the bollworm (Helicoverpa zea) in Texas: Trends from 2003-2005. Online. Plant Health Progress doi:10.1094/PHP-2007-00719-04-RV.


The purpose of this study was to assess the susceptibility of the bollworm, Helicoverpa zea (Boddie), to the pyrethroid cypermethrin in the primary crop production areas of Texas. Pyrethroid insecticides are widely used in cotton and in other production systems, such as grain sorghum and corn. The statewide monitoring program that evaluated resistance in male H. zea was conducted from April to September 2005, surveying nine Texas counties with a total of 5,041 moths from all areas used for analysis. Data from all areas were sent to Texas A&M University Toxicology Laboratory for analysis. Considerable variability in response to cypermethrin was detected in H. zea across the state. Based on LC50 data, the most resistant populations were from Nueces, Uvalde, and Williamson counties while the most susceptible populations were from Ellis, Fisher and Mitchell, Hockley, and Swisher counties.


Our laboratory has monitored the evolution of resistance to pyrethroids in the bollworm, Helicoverpa zea (Boddie), in Burleson and Nueces counties since 1998 (4,5,8,9). Among the various cypermethrin dosages used for monitoring, two dosages, 3 μg/vial and 10 μg/vial, are considered discriminatory for “susceptible” and “heterozygote resistant” phenotypes, respectively. Kanga et al. (3) recommended a dosage of 2.5 μg cypermethrin/vial as discriminatory, possibly killing all susceptible H. zea. The probit analysis graph shown by these authors suggests the dosage of 5 μg/vial was the ultimate discriminatory concentration for susceptible moths. The IRAC (Insecticide Resistance Action Committee, US) procedure utilized 5 μg/vial for the same discrimination in previous monitoring efforts (7). During the last three years we have more intensively monitored pyrethroid resistance in several counties (10,11) and have detected the widespread presence of individuals with a resistant phenotype, but at different frequencies in different locations. Resistance ratios varied in different counties and in different years, exemplifying the local and dynamic nature of insecticide resistance in general and of cypermethrin resistance in particular. Additionally, immigration of pyrethroid resistant moths from Mexico into Texas (or conversely) and migration from neighboring Texas counties may add to the complexity of addressing pyrethroid resistance management for H. zea in Texas.

Moth Collection and Vial Assays

Male moths of H. zea were trapped using the pheromone Hercon Luretape with Zealure from Great Lakes IPM (Vestaburg, MI). Moths were collected in the morning and placed in vials immediately, except for those from Burleson Co., which were supplied with a 10% sucrose solution for 5 h in the laboratory until placed in vials for bioassays. Only male moths with intact wing scales were used for adult vial tests (AVT) similar to that described by Plapp et al. (13,14). Vials were prepared in the Department of Entomology, Toxicology Laboratory at Texas A&M University, College Station, and shipped as needed to Texas Cooperative Extension collaborators. Stock solutions were prepared by dissolving technical grade (95.2%) cypermethrin in acetone that had been dehydrated for at least 48 h on 4-Ĺ molecular sieves (EM Science) before use. Test vials were prepared by coating the inside with insecticide solutions yielding the following dosages: 0.15, 0.3, 1, 1.5, 2.5, 3, 5, 10, 30, and 60 μg cypermethrin/vial. Control vials were coated with dehydrated acetone only. Vials were prepared by dispensing acetone or cypermethrin solutions and dried on a cold "hot-dog" roller under the hood for at least 15 min until the acetone had evaporated. Vials with 60 μg cypermethrin/vial were only used in Nueces and Burleson counties and the 0.15 μg cypermethrin/vial only in Hockley Co.; all counties received vials with the remaining dosages.

Collaborators conducted their own trapping and bioassay tests in their respective areas, following a single detailed protocol provided by the Toxicology Laboratory [see also (11,12)]. One moth was placed in each vial and the vials were stored at 27°C or room temperature. Equal numbers of moths were tested for each concentration. Mortality was counted after 24 h. Moths were evaluated as alive, dead, or "knocked-down." Moths that were alive but could not fly normally were considered "knocked-down" and were included as dead for calculations of percentage of mortality. In Texas, bioassays conducted in 1988, 1989, and 1993 with H. zea showed significant tolerance to cypermethrin with a range of LC50 values from 0.09 to 0.44 μg/vial, while the laboratory population used for comparison had an LC50 of 0.05 μg/vial and LC90 of 0.57 μg/vial (3). A Burleson Co. H. zea population evaluated in September of 2005 had a LC50 = 0.33 μg/vial and LC90 = 2.44 μg/vial, with no observed mortality above the 3.0 μg/vial dosage. This population was considered susceptible and comparable to the Texas populations evaluated by Kanga et al. (3) (Fig. 1). It was also more susceptible than a laboratory population obtained from the USDA/ARS in Mississippi that we had previously used as a reference (12). The estimated parameters for the Burleson 2005 population support the assessment by Kanga et al. (3) that a dosage of 2.5 μg/vial should kill all susceptible male moths.


Fig. 1. Highest LC50 and LC90 estimated values (µg cypermethrin/vial) in Texas (2005) by county, with corresponding resistance ratios shown above the histogram bars. Significant (P < 0.05) resistance ratios in red. LC90s correspond to the same date when the highest LC50 was detected.


Nine counties in Texas were included in the 2005 monitoring program, as follows: Hockley and Swisher counties in the High Plains; Fisher and Mitchell counties in the Southern Rolling Plains; Ellis and Williamson counties in the Blacklands; Burleson Co. in the Brazos River Bottom; Uvalde Co. in the Winter Garden area; and Nueces Co. in the Coastal Bend. In 2005, a total of 5,041 moths from all areas were utilized for bioassays. Bioassays in some areas were conducted over several nights and the data pooled if probit lines from these populations were determined to be parallel and equal, according to methods described in Robertson and Preisler (15).

Data from all areas of Texas were sent to the Toxicology Laboratory at Texas A&M University for analysis. Data were analyzed using POLO-PC, Probit and Logit Analysis program, and dosage-mortality regressions were plotted using SigmaPlot software. Data were corrected for mortality using Abbott's formula (1). The LC50 and LC90 values from the susceptible Burleson Co. September 2005 population were used to calculate resistance ratios. Confidence intervals for resistance ratios were calculated as described by Robertson and Preisler (15). The lethal concentration resistant ratios of different populations were considered not significantly different if the 95% confidence intervals included one (15).

2005 Season Description of H. zea Resistance Monitoring, by County

Monitoring for H. zea resistance to cypermethrin was conducted from April to September 2005. Monitoring was more intensive in Nueces and Burleson counties due to an increase in control problems reported with pyrethroid use in 2003, 2004, and in previous years (10).

Nueces. In Nueces Co., a total of 1,590 moths were tested using two traps spaced one mile apart. Dates of bioassays were: 7-12 April, 23-25 May, 9-16 June, 20 June-3 July, and 29 September-6 October. In 2005, there were approximately 146,000 acres of cotton and 235,000 acres in the nearby region including the Nueces Co. figures. An estimated 80% of the grain sorghum acreage was treated with pyrethroids for H. zea and fall armyworm (headworm: Spodoptera frugiperda) infestations. No problems were experienced controlling H. zea in sorghum with medium to high rates of pyrethroids, nor at that time were problems experienced in cotton. However, in June, about 2 to 3 weeks following the grain sorghum treatments, growers did experience problems controlling H. zea in cotton with pyrethroids, or alternate materials were used in anticipation of the problem. Two treatments were applied to cotton for H. zea control on about 60% of the acres. There were control problems with pyrethroids during the mid-season, with high label rates being used. Peak numbers of larvae were present during this period. Pyrethroid use consisted primarily of zeta-cypermethrin (Mustang Max), used at the mid-label rate of 0.0195 lb a.i./acre, up to the high label rate of 0.0225 lb a.i./acre; lambda cyhalothrin (Karate), used at the mid-label rate of 0.0325 up to the high label rate of 0.04 lb a.i./acre; and deltamethrin (Decis) used also at mid-label of 0.0245 lb a.i./acre up to the high label rate of 0.03 lb a.i./acre. The higher rates of pyrethroid were used on grain sorghum and from early to mid-June on cotton. Emamectin benzoate (Denim) at 0.01 lb a.i./acre, and indoxacarb (Steward) at 0.11 lb a.i./acre, were also used. One of the mixtures used was the labeled rate of a pyrethroid (mid- to high rate) plus emamectin benzoate (Denim) 0.16 EC at 0.007619 lb a.i./acre which is one gallon of the emamectin benzoate (Denim) per 21 acres. During the mid-season period of pyrethroid failure, control was achieved with emamectin benzoate or indoxacarb. Several producers were concerned about the difficulty in controlling H. zea and the high cost of the alternatives. Vial bioassays during mid-season confirmed that indeed pyrethroid resistance was the cause for field control failures.

Williamson. In Williamson Co., a total of 324 moths were collected from three traps spaced a quarter of a mile apart. Dates of bioassays were: 21 June and 7 July 2005. Populations of H. zea were light in cotton and moderate in corn and grain sorghum; however, these populations exhibited the highest resistance ratios for both the LC50 and the LC90 among all counties surveyed in 2005 (Fig. 1) and the level of resistance has increased in the last three years (Fig. 2). Pyrethroids were avoided in cotton in 2005 because of failures in previous years. No control problems or field failures were reported when spinosad (Tracer) was used at a rate of approximately 0.0682 to 0.078 a.i./acre (2.2 to 2.5 fl oz/acre).

Uvalde. In Uvalde Co., a total of 270 moths were collected from five traps, spaced 300 yards to 0.75 mile apart. Moths were tested on 13 July 2005. In 2005 there were no known control problems or field failures although the resistance ratios were high (Fig. 1). Producers did not communicate concerns, and growers and consultants indicated that there was no pyrethroid use in the area this year in cotton, corn, or grain sorghum. A lack of local pyrethroid use and bioassay results showing a significant resistant ratio suggest that pyrethroid resistant moths collected in Uvalde immigrated from south Texas (17).

Burleson. In Burleson Co., 1,480 moths were collected from seven traps spaced over 4 miles. Dates of bioassays in 2005 were: 5 May, 26 May, 23 June, 7 July, 26 July, 7 September, and 20 September. There were 11,000 acres planted in cotton, most of which was transgenic Bt. There was one major flight of H. zea in July during which time the cotton field in the trapping area was treated with full rate 0.025 lb a.i./acre of zeta-cypermethrin (Mustang Max). Grain sorghum in the area was treated in early June for H. zea (headworms) with the maximum rate of zeta-cypermethrin, and in late June with methomyl (Lannate), to alternate chemistries. Chlorpyrifos (Lorsban) was used to control sorghum midge. Burleson Co. showed an improving situation, with a progressive return towards susceptibility from 2003 to 2005 (Fig. 2). No survivorship was observed beyond the 5 μg/vial concentration in July of 2005.


Fig. 2. LC50 resistance ratios in Texas (2003-2005) by county (BC = Burleson Co. 2005).


Ellis. In Ellis Co., 2 traps spaced 1 mile apart were used to collect a total of 193 moths. Bioassays were conducted on moths collected from 20-23 June. Crop acreage for this county was 30,000 acres of cotton (80% transgenic Bt), 48,000 acres of corn, and 10,000 acres of sorghum. Cotton yield was above average. H. zea density in cotton and other crops was much lighter than in 2004 and as compared to past 3 years. A hot and dry spring and early summer, and the absence of wild host plants for H. zea could have been contributing factors for reduced populations, making insecticidal control unnecessary. There were no reports of control failures. The only pyrethroid treatments were applications made against boll weevil during midseason; 3000 acres were treated with lambda cyhalothrin (Karate) at 0.04 lb a.i./acre. Bioassays confirmed that the population was susceptible even during June (Fig. 1).

Fisher and Mitchell. In Fisher and Mitchell counties, 180 moths were collected from 6 traps spaced 0.5 miles apart, and tested on 3 and 7 September 2005. There were 114,500 acres planted in cotton (35% transgenic Bt), 7,500 in sorghum, and none in corn. Cotton yield for 2005 was above average. H. zea density was moderate in 2005 but higher than in the past 3 years. Fifty percent of non-Bt cotton fields were treated twice and 10% of Bollgard cotton fields were treated once. H. zea control was achieved with the pyrethroid lambda cyhalothrin (Karate) 0.04 lb a.i./acre, applied the third week in July. The only bioassay available, in late season, showed no resistance to pyrethroids (Fig. 1).

Hockley. In Hockley Co., a total of 684 moths were collected from two traps spaced 880 yards apart. Bioassays were conducted on moths collected from 11 July to 1 August and 22-29 August. Crop acreage for the area was 230,000 acres of cotton (15% transgenic Bt), 12,000 acres of sorghum, and no corn. Cotton yield for the area was 425 lb of lint per acre for dryland and 725 lb of lint per acre for irrigated. The past two years experienced unusually light bollworm density, with 2005 having generally low activity in most crops. There were no reported control problems and no pyrethroids were applied. This was perhaps a consequence of no applications targeting boll weevils and beneficial insects being abundant. Pyrethroid resistance appeared to occur at a higher frequency in 2003 than in 2005 (Fig. 2), although the resistance ratio of 4.4 for the LC90 was significant for August (Fig. 1).

Swisher. In Swisher Co., a total of 320 moths were collected from three traps spaced 1.25 miles apart. Bioassays were conducted from moths collected on 15 June, 19 and 27 July, and 24 August. Crop acreage for the Hale and Swisher Co. area was: 350,000 acres of cotton (20 to 25% transgenic Bt), 150,000 acres of sorghum, and 75,000 acres of corn. H. zea populations were more widespread on cotton and densities were generally higher than in the past three years. Limited pyrethroid applications were made to sorghum and corn. About 90% of non-Bt-cotton and several Bollgard I fields were treated for H. zea. Treatments for a given crop included one or more applications of the following pyrethroids: cypermethrin (generic) 0.08 to 0.1 lb a.i./acre; cyfluthrin (Baythroid) 0.03 to .04 lb a.i./acre; and cyhalothrin (Karate) 0.03 to .04 lb a.i./acre. Cypermethrin was the most commonly used pyrethroid. No significant control failures were observed in nearby fields. Bioassay results also confirmed these field observations (Fig. 1).


Analyses of the 2005 monitoring season, together with those in 2003 (4,134 moths tested) and 2004 (3,377 moths tested), justify the increasing concern about the effectiveness of pyrethroids in Texas. Resistant populations in south Texas (Nueces Co.) appear to have stabilized in the last three years, with continued high resistant ratios (Fig. 2). The situation in Williamson Co. deteriorated in 2005, with a rapid loss of pyrethroid efficacy in these populations, occurring over the last three years. Improvement in the Burleson Co. populations was surprising, but it possibly shows that alternating chemistries with different modes of action, and/or using higher rates of pyrethroids when resistance is high, can aid in minimizing the frequency of pyrethroid-resistant heterozygotes in the population. Use of high pyrethroid rates, however, is not routinely recommended for counties with low LC50 values such as those in northern Texas (Fig. 2). Hockley and Uvalde Co. populations should be more closely monitored in 2006, considering that individuals surviving the 10μg/vial dosage were detected in both areas. We speculate that high resistance ratios (Fig. 1) at Uvalde could be due to immigrating resistant moths from south Texas, since preliminary analysis of wind trajectories at 500 m, the altitude at which H. zea is known to migrate in Texas (17), show overnight trajectories originating in Nueces Co. and ending in Uvalde Co. at the same time the bioassays were conducted (J. Westbrook, personal communication).

In both Nueces and Burleson counties, the LC50 of the populations decreased toward the end of the season after the high insecticide pressure subsided; this may reflect a fitness cost of the resistance allele(s) in the absence of pyrethroid selection and in competition with susceptible individuals in the field, or emergence of susceptible moths from non-treated hosts (2). This may also explain why the LC50 at the beginning of the season for both counties is low. In general, migration of moths according to predominant wind patterns also must be overlaid in the analysis of resistance evolution, and for the development of resistance management strategies since immigration of resistant moths from other neighboring states or counties with higher frequencies of resistant individuals can result in lower pyrethroid effectiveness even in populations without previous local pyrethroid use or exposure. This can be particularly useful for north Texas where the first moths captured in the season are believed to be migrants (16). It is also critical to preserve pyrethroid susceptibility in north Texas since there is evidence of a fall "reverse migration" of bollworms towards south Texas (6). This may aid in diluting the resistance pool and contribute to pyrethroid use sustainability.

International collaboration on resistance monitoring of H. zea populations began in 2006 for Rio Bravo (Tamaulipas), Mexico. Collaboration with meteorologists will also aid in determining predominant wind trajectories that may favor moth migration and may help explain part of the variation in resistance and susceptibility in different counties.


We are grateful to Cotton Incorporated and Dr. Patricia O'Leary as well as the Texas Department of Agriculture for funding this project. Dr. V. Vassiliou assisted with vial preparation, field work, and data analysis. Christopher Marsh and Amar Trivedi helped with vial preparation and moth rearing. Mark Nemec and Dusty Tittle provided field information for Burleson Co.

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