Comparisons of Boll Weevil (Coleoptera

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statement, and the ILINK and confidence limits op- tions of the LSMEANS statement were used to obtain estimates and 95% confidence interval (CI) on the.
ECOTOXICOLOGY

Comparisons of Boll Weevil (Coleoptera: Curculionidae) Pheromone Traps With and Without Kill Strips C. P.-C. SUH,1 J. S. ARMSTRONG,2 D. W. SPURGEON,3

AND

S. DUKE4

J. Econ. Entomol. 102(1): 183Ð186 (2009)

ABSTRACT Boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), eradication programs typically equip pheromone traps with an insecticide-impregnated kill strip. These strips are intended to kill captured insects, thereby simplifying trap servicing and reducing the loss of weevils from predation and escape. However, the effectiveness of kill strips has not been extensively evaluated. We examined the inßuences of kill strips on weevil captures, trap servicing, and the incidences of weevil predation and trap obstruction (e.g., by spider webs). Evaluations were conducted weekly during three different production periods (pre- to early-, late-, and postseason) of cotton, Gossypium hirsutum L., to represent different environmental conditions and weevil population levels. Within each period, mean weekly captures of weevils in traps with and without kill strips were statistically similar. On average, traps with kill strips took 9 s longer to service than traps without kill strips, but statistical differences were only detected during the late-season period. Overall, the mean weekly proportion of traps with evidence of weevil predation or trap obstruction was signiÞcantly lower for traps with kill strips (0.25) than for traps without kill strips (0.37). However, this reduction in the frequency of weevil predation or trap obstruction was too small to produce a corresponding increase in the numbers of weevils captured. In light of these Þndings, the use of kill strips is likely unnecessary in eradication programs, but may be a consideration in situations when the numbers of deployed traps are reduced and chronic problems with weevil predation or trap obstruction exist. KEY WORDS boll weevil, Anthonomus grandis, pheromone traps, kill strip, dichlorovos

Eradication programs directed against the boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), rely almost exclusively on pheromone traps to detect weevils, assess populations, and indicate the need for insecticide treatment. In addition to a pheromone lure, each trap is typically equipped with an insecticide-impregnated device called a kill strip. These strips are intended to induce mortality in captured insects, subsequently reducing the loss of weevils from escape or predation. At least two types of kill strips are used by eradication programs, and both types contain dichlorvos (2,2-dichlorovinyl dimethyl phosphate) as the killing agent. One type is the Hercon Vaportape II [Hercon Environmental, Emigsville, PA; 10% (wt:wt) dichlorvos] and

Mention of trade names or commercial products in this article is solely for the purpose of providing speciÞc information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. 1 Corresponding author: Areawide Pest Management Research Unit, USDAÐARS, 2771 F&B Rd., College Station, TX, 77845 (e-mail: [email protected]). 2 BeneÞcial Insects Research Unit, USDAÐARS, 2413 East Highway 83, Weslaco, TX 78596. 3 Western Integrated Cropping Systems Research Unit, USDAÐ ARS, 17053 N. Shafter Ave., Shafter, CA 93263. 4 Southern Plains Agriculture Research Center, USDAÐARS, 2772 F&B Road, College Station, TX, 77845.

the other is the Plato Insecticide Strip [Plato Industries, Houston, TX; 6.98% (wt:wt) dichlorvos]. Despite the widespread use of kill strips in eradication programs, rigorous scientiÞc evidence documenting their efÞcacy in the Þeld is not available. Hardee et al. (1996) examined their inßuence on weevil captures, and concluded there was no statistical evidence to justify their use. However, those authors advocated continued use of kill strips because they were inexpensive and simpliÞed operation of traps. Suh et al. (2003) examined the temporal patterns of weevil mortality produced by two types of kill strip and reported that both types produced ⬎90% weevil mortality in traps after 46 h of exposure. However, they also found that neither type reduced the incidence of weevil escape from traps. Armstrong and Greenberg (2008) evaluated extended-life pheromone formulations in traps equipped with and without dichlorovos and reported that the presence of dichlorovos did not signiÞcantly affect the sex ratio of captured boll weevils. Other presumed beneÞts of using kill strips include reduced trap handling times, and reduced incidences of predation on weevils within traps and trap obstruction (e.g., spider webbing, large insects) which can prevent capture of weevils. Whether these beneÞts actually result from the use of kill strips has not been studied.

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Although individual kill strips are inexpensive, their cumulative cost can be substantial because of the large number of traps operated by eradication programs. Thus, it is important to determine what, if any, beneÞts are associated with their use. Our objectives were to examine the inßuence of kill strips on weevil captures and trap servicing and to determine whether kill strips reduce the incidence of weevil predation and trap obstruction. Materials and Methods Kill strip evaluations were conducted on the Russell Plantation near San Benito, TX during three time periods in 2004 to facilitate evaluation of kill strips under differing environmental conditions and weevil populations. Periods of study and the corresponding phenology of the cotton crop in the general area were: 17 FebruaryÐ16 March, before planting to presquaring; 1Ð29 July, late-bloom to early crop maturity; and 13 OctoberÐ3 November, postharvest fallow. This resulted in 12 weekly evaluations throughout the year. Traps were established immediately adjacent to prominent vegetation in three distinct habitats typical of the study area: brush-lined canals, woods, and wooded resaca (ox-bow lake). Within each habitat, 10 pairs of Southeastern Eradication Foundation pheromone traps (Technical Precision Plastics, Mebane, NC) were set up in a line roughly perpendicular to prevailing southeasterly winds. Traps were placed on wooden stakes so that the trap tops were ⬇1.2 m above ground level. Pairs of traps within a line were separated by ⱖ50 m, and traps within a pair were separated by ⬇25 m. This orientation was intended to minimize interaction between traps based on the results of Sappington (2002). One week before the initiation of the study, vegetation in a 1.5- to 2-m radius of each trap was removed and the surrounding ground surface was treated with glyphosate (Round-Up Pro, 41% active ingredient [AI], Monsanto Co., St. Louis, MO). These areas were maintained free of interfering vegetation throughout the study periods. At the beginning of each trapping period, each trap was baited with a 10-mg pheromone lure (Scentry Biologicals, Billings, MT), and one trap in each pair was randomly selected and equipped with a Hercon Vaportape II kill strip. This kill strip was selected because it was the same type used by the Texas Boll Weevil Eradication Foundation. The treatment assignment for each trap was maintained throughout the duration of the period. Traps were serviced weekly during each period. Kill strips were replaced only at the beginning of each trapping period, whereas pheromone lures were replaced biweekly. The trap service and lure replacement intervals were consistent with those used in eradication programs (Dickerson et al. 2001). Traps were serviced by the same individual on all dates while an observer recorded the time required to service each trap. In addition to removing weevils from the traps, the capture container of each trap was quickly inspected for the conspicuous presence of

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weevil body parts which may indicate predation of captured weevils. The wire cone forming the trap entrance was also examined for any obstruction (e.g., webbing, plugged insects) that could prevent weevils from entering the capture container. Because the inßuence of weevil predation or trap obstruction on weevil captures was unknown, traps exhibiting either incident were recorded as “impeded.” Data for traps that were knocked down or missing parts at the time of servicing were recorded as “missing.” Weevils removed from all traps, except for those that were recorded as missing, were placed in sealable plastic bags and counted in the laboratory. Statistical Analyses. Numbers of captured weevils were examined using mixed model analysis of variance (ANOVA) (PROC MIXED, SAS Institute 2002). Because predation of captured weevils or trap obstructions could have inßuenced observed weevil captures, two separate analyses were performed. In one analysis, counts of weevils from impeded traps were omitted to examine only the direct inßuence of kill strips on weevil captures. An identical analysis was performed with counts from impeded traps included to examine the overall impact of kill strips on weevil captures. In both analyses, Þxed effects in the model contained terms for trapping period, kill strip treatment, and their interaction. Random effects included week nested within trapping period [week(period)], habitat, and trap pair nested within habitat [trap pair (habitat)]. Additionally, because weevil population levels varied considerably among study periods, the data suffered from heterogeneity of variance. We adjusted for this heterogeneity using the GROUP option (group ⫽ period) in the RANDOM statement. Corrected denominator degrees of freedom were obtained using the Kenward-Roger adjustment (DDFM ⫽ KR option of the MODEL statement). Estimates of leastsquare means and corresponding standard errors were obtained using the LSMEANS statement, and differences among levels of Þxed effects were identiÞed using the ADJUST ⫽ TUKEY option of the LSMEANS statement. SigniÞcant interaction terms were further examined using the SLICE option of the LSMEANS statement. The times required to service individual traps with and without kill strips were also compared using mixed model ANOVA (PROC MIXED, SAS Institute 2002). This analysis was identical to those used for weevil captures except no adjustment for heterogeneity was needed. Because the incidence of impeded traps was recorded in a binomial form (yes or no), corresponding data were analyzed using a generalized linear mixed model approach (PROC GLIMMIX, SAS Institute 2006). Initially, the analysis was conducted using the same model speciÞed in the capture analyses. However, this analysis failed to converge. Therefore, a more conservative approach was used to analyze the data. In brief, the 10 trap sets within each habitat were treated as subsamples of the habitat. On each servicing date, the proportion of traps within each habitat that were impeded was calculated for each of the trap

February 2009

SUH ET AL.: EFFICACY OF KILL STRIPS

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Table 1. Mean weekly captures of boll weevils, trap handling times, and combined incidence of weevil predation and trap obstruction in pheromone traps with and without kill strips Trap perioda

Trap treatment

Weevil capturesb (direct impact)

Weevil capturesc (overall impact)

Trap service time (s)

Proportion of impeded trapsd

1

Kill strip No kill strip Kill strip No kill strip Kill strip No kill strip

12.8 ⫾ 5.0a 12.8 ⫾ 5.1a 8.9 ⫾ 3.1a 6.4 ⫾ 3.2a 16.4 ⫾ 12.3a 17.0 ⫾ 12.3a

10.5 ⫾ 7.5a 9.8 ⫾ 7.5a 33.6 ⫾ 28.5a 35.7 ⫾ 28.5a 16.8 ⫾ 13.6a 17.0 ⫾ 12.3a

121 ⫾ 21a 118 ⫾ 21a 129 ⫾ 15b 107 ⫾ 15a 95 ⫾ 13a 93 ⫾ 13a

0.25 (0.15Ð0.39)a 0.42 (0.30Ð0.56)b 0.45 (0.32Ð0.59)a 0.55 (0.42Ð0.68)b 0.11 (0.05Ð0.23)a 0.18 (0.10Ð0.30)b

2 3

Least-square means ⫾ SEM unless noted otherwise; within a trapping period, values within a column followed by different letters are signiÞcantly different (␣ ⫽ 0.05; TukeyÐKramer test). a Period 1 (17 Feb.Ð16 Mar.), before planting to presquaring; period 2 (1Ð29 July), late-bloom to early crop maturity; and period 3 (13 Oct.Ð3 Nov.), postharvest fallow. b Counts of weevils in traps with evidence of weevil predation or trap obstruction were omitted in the analysis. c Counts of weevils in traps with evidence of weevil predation or trap obstruction were included in the analysis. d Traps with evidence of weevil predation or with obstructed entrances were considered impeded; values in parentheses are 95% CI.

treatments. Thus, there was one value (a proportion) for each combination of treatment, habitat, and week of trapping. Those proportions were analyzed with weeks serving as replications. Because this type of analysis cannot accommodate proportional values of 0 or 1, a value of 0.0001 was added to the 0-values and subtracted from the 1-values before analysis. Fixed effects in the new model contained terms for trapping period, kill strip treatment, and their interaction. Random effects included only a term for habitat. To ensure the G-matrix remained positive deÞnite, the TYPE ⫽ CHOL option was used in the random statement to specify a Cholesky root parameterization for unstructured blocks in the covariance matrix. Additionally, the LOWERB option of the PARMS statement was used to specify the lower boundary limits for the covariance parameters by constraining diagonal terms of the matrix to be greater than or equal to 1E-4 (SAS Institute 2006). A Beta error distribution and the default logit link function were speciÞed in the model statement, and the ILINK and conÞdence limits options of the LSMEANS statement were used to obtain estimates and 95% conÞdence interval (CI) on the scale of the means (inverse log scale). Asymmetric conÞdence intervals (95% CI) are reported instead of standard errors because standard errors on the logit scale cannot be back-transformed to a single value on the original scale. Differences among levels of Þxed effects and their interaction were identiÞed using the ADJUST ⫽ TUKEY option. Results and Discussion Of the 720 trap observations, seven observations were omitted from all analyses because the traps were down or missing parts at the time of servicing. Of the remaining 713 trap observations, 230 were classiÞed as impeded as result of weevil predation or trap obstruction. Therefore, 483 trap observations were available to assess the direct impact of kill strips on weevil captures, whereas 713 trap observations were used to assess their overall (direct and indirect) inßuence on captures. Neither analysis detected signiÞcant differences in the mean numbers of weevils captured trap⫺1

week⫺1 among trapping periods (direct, F ⫽ 0.78; df ⫽ 2, 3.79; P ⫽ 0.521; overall, F ⫽ 0.42; df ⫽ 2, 3.62; P ⫽ 0.684). Both analyses also indicated the mean numbers of weevils captured trap⫺1 week⫺1 were similar between traps with and without kill strips (direct, F ⫽ 0.42; df ⫽ 1, 448; P ⫽ 0.517; overall, F ⫽ 0.08; df ⫽ 1, 669; P ⫽ 0.777). The treatment-by-trapping period interaction in each analysis was not signiÞcant (direct, F ⫽ 0.79; df ⫽ 2, 447; P ⫽ 0.455; overall, F ⫽ 0.09; df ⫽ 2, 669; P ⫽ 0.912), indicating the absence of a treatment effect on weevil captures was consistent among trapping periods (Table 1). These results indicate the presence of kill strips in traps had no or minimal inßuence on the observed numbers of boll weevils captured in traps. Consequently, our results suggest traps without kill strips would be as effective as those with kill strips for monitoring weevil population levels. In this respect, our results were similar to those of Hardee et al. (1996) and Armstrong and Greenberg (2008), who also found that use of the kill strip did not signiÞcantly increase the numbers of weevils captured in traps. In our study, signiÞcant differences in mean trap handling times were not observed among trapping periods (F ⫽ 0.82; df ⫽ 2, 9; P ⫽ 0.470), but differences were detected between trap treatments (F ⫽ 10.56; df ⫽ 1, 670; P ⫽ 0.001). Overall, the mean ⫾ SE amount of time required to service an individual trap equipped with a kill strip was 115 ⫾ 10 s whereas traps without kill strips averaged 106 ⫾ 10 s. However, a signiÞcant interaction between trap treatment and trapping period indicated the effect of kill strips on trap servicing was not consistent among trapping periods (F ⫽ 4.71; df ⫽ 2, 670; P ⫽ 0.009). Examination of effect slices (i.e., treatment effects within each period) revealed that traps with kill strips took signiÞcantly longer to service than those without kill strips only during the second period (F ⫽ 18.96; df ⫽ 1, 670; P ⬍ 0.001; Table 1). During the other two periods, mean trap handling times were statistically similar between treatments (period 1, F ⫽ 0.54; df ⫽ 1, 669; P ⫽ 0.464; period 3, F ⫽ 0.26; df ⫽ 1, 669; P ⫽ 0.609). The reason for this inconsistency among study periods is probably related to the magnitude of captures in the respective periods.

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Numbers of captured weevils were numerically much higher during the second study period compared with other periods. Because many weevils in both trap treatments were still alive when traps were serviced, the larger number of weevils captured during the second study period would have necessitated that extra care be taken in removing the weevils while either ensuring the pheromone lure and kill strip remained or were replaced in the capture container. The overall trap servicing times of the respective periods do not reßect the magnitude of captures because inclement weather during the Þrst study period likely extended trap service times, and servicing times probably decreased somewhat as the trap operator gained experience in servicing traps. Also, the use of a single combination dispenser (pheromone lure and kill strip combined), which is currently available, would likely have eliminated the time differences observed in our study. Regardless, our results demonstrate the use of kill strips does not simplify trap servicing. The mean weekly proportion of traps that were classiÞed as impeded as a result of weevil predation or trap obstruction varied among all trapping periods (F ⫽ 14.83; df ⫽ 2, 64; P ⬍ 0.001). The highest proportion of impeded traps (weekly mean; 95%CI) was observed during the second trapping period (0.50; 0.41Ð 0.60), followed by the Þrst period (0.33; 0.25Ð 0.43), and was lowest during the third period (0.14; 0.09 Ð 0.22). Overall, the mean weekly proportion of impeded traps was signiÞcantly lower for traps with kill strips (0.25; 0.18 Ð 0.33) than for traps without kill strips (0.37; 0.29 Ð 0.45; F ⫽ 4.85; df ⫽ 1, 64; P ⫽ 0.031). The treatment-by-trapping period interaction was not signiÞcant (F ⫽ 0.21; df ⫽ 2, 64; P ⫽ 0.811), indicating the observed effect of kill strips on the combined incidence of weevil predation and trap obstruction was consistent among trapping periods (Table 1). These results indicate the use of kill strips consistently reduced the combined incidence of weevil predation and trap obstructions, although the extent of those reductions was relatively small. In conclusion, our results demonstrate the use of kill strips in traps has no or minimal inßuence on weevil captures and does not simplify trap servicing as reported previously. Although kill strips provided a statistically signiÞcant reduction in the combined incidence of weevil predation and trap obstruction, the overall level of reduction we observed (12%) was too

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small to produce corresponding increases in the numbers of weevils captured in traps. These results, combined with those of Suh et al. (2003) that kill strips do not reduce the incidence of weevil escape from traps, suggest the elimination of kill strips from traps would have little impact on the effectiveness of traps for monitoring boll weevil population levels. In this regard, the use of kill strips in active eradication programs is likely unnecessary. However, the small reduction in interference of traps by predators or obstructions that was associated with the kill strip may be a consideration in situations when the numbers of deployed traps are reduced and chronic problems with weevil predation or trap obstruction exist. Acknowledgments We thank Frank and James Russell for the use of their farm and Charles Allen and the Texas Boll Weevil Eradication Foundation for providing kill strips.

References Cited Armstrong, J. S., and S. M. Greenberg. 2008. Evaluation of extended-life pheromone formulations used with and without dichlorovos for boll weevil (Coleoptera: Curculionidae) trapping. J. Econ. Entomol. 101: 399 Ð 403. Dickerson, W. A., A. L. Brashear, J. T. Brumley, F. L. Carter, W. J. Grefenstette, and F. A. Harris. 2001. Boll weevil eradication in the United States through 1999. The Cotton Foundation Publisher, Memphis, TN. Hardee, D. D., A. A. Weathersbee, III, J. M. Gillespie, G. L. Snodgrass, and A. R. Quisumbing. 1996. Performance of trap designs, lures, and kill strips for the boll weevil (Coleoptera: Curculionidae). J. Econ. Entomol. 89: 170 Ð 174. Sappington, T. W. 2002. Mutual interference of pheromone traps within trap lines on captures of boll weevils (Coleoptera: Curculionidae). Environ. Entomol. 31: 1128 Ð 1134. SAS Institute. 2002. SAS/STAT userÕs guide, release 9.1 ed. SAS Institute, Cary, NC. SAS Institute. 2006. The GLIMMIX procedure. (http:// support.sas.com/rnd/app/papers/glimmix.pdf). Suh, C. P.-C., D. W. Spurgeon, and S. Hagood. 2003. Evaluation of kill strips on boll weevil (Coleoptera: Curculionidae) mortality in pheromone traps and impact on weevil escape. J. Econ. Entomol. 96: 348 Ð351. Received 17 April 2008; accepted 1 October 2008.