Progress 09/01/05 to 02/28/09
Outputs
OUTPUTS: Selective insecticides such as insect growth regulators that kill pests but cause little or no harm to non-target organisms have become increasingly important in crop production systems worldwide. The insect growth regulator pyriproxyfen has been successfully used for the last decade in Arizona as part of an integrated pest management (IPM) program for the sweetpotato whitefly, Bemisia tabaci, a problematic pest in Arizona and other regions of the world throughout the world. Our analysis of the pyriproxyfen-resistant QC02-R strain of the B biotype of B. tabaci indicated no differences in susceptibility between males and females to pyriproxyfen under field conditions, partially-dominant to dominant resistance, and lack of fitness costs associated with resistance. Similar resistance traits in other field populations could threaten the sustainability of pyriproxyfen as part of the IPM program in Arizona. Our analysis represents a proactive approach to understanding the factors, both biological and operational, that could affect the evolution of pyriproxyfen resistance in B. tabaci. Results from our simulation models indicate that the current IPM program for B. tabaci could be improved by curtailing the use of pyriproxyfen in cotton-intensive regions, synchronizing the use of pyriproxyfen with key crop production stages in the field, and applying the lowest pyriproxyfen concentration needed to provide effective control. PARTICIPANTS: David Crowder obtained his PhD working on this project. Christa Kirk, research specialist worked on this project. Mike Sitvarin and Ann Showalter were hired as Research specialist junior to work on this project. They respectively worked 6 months and 1 year. 5 undergraduate students worked on different aspects of this project. TARGET AUDIENCES: IPM specialists, scientists interested in resistance management, crop producers. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Our study of resistance to pyriproxyfen in B. tabaci provides instrumental information to extend the longevity of the IPM program for whitefly control in Arizona and elsewhere in the US. Our novel approaches to examine inheritance of resistance in a haplodiploid pest could also have applications in other systems, such as studies of insecticide resistance in mites, thrips, hymenopterans, and other whiteflies.
Publications
- Crowder, D. W., M. I. Sitvarin, and Y. Carriere. 2009. Plasticity in female mating behavior and sex allocation drive competitive displacement of whitefly biotypes. Behavioral Ecology, submitted
- Crowder, D. W., Carriere, Y. 2009. A general theory for managing the evolution of insect resistance to genetically modified crops. Journal of Theoretical Biology, submitted
- Crowder D. W., Ellsworth P. C., Tabashnik B. E., and Carriere Y.. 2008. Impact of operational factors on the evolution of resistance to pyriproxyfen by the sweetpotato whitefly (Hemiptera: Aleyrodidae). Journal of Economic Entomology. 37: 1514-1524
- Crowder, D.W., Ellers-Kirk, C., Yafuso, C., Dennehy, T.J., Degain, B.A., Harpold, V.S., Tabashnik, B.E., and Carriere, Y. 2008. Inheritance of resistance to pyriproxyfen in Bemisia tabaci (Hemiptera: Aleyrodidae) males and females (B biotype). Journal of Economic Entomology. 101: 927-932.
- Crowder, D. W., T. J. Dennehy, C. Ellers-Kirk, C. M. Yafuso, P.C. Ellsworth, B.E. Tabashnik, and Y. Carriere. 2007. Field evaluation of resistance to pyriproxyfen in Bemisia tabaci (B biotype). Journal of Economic Entomology. 100: 1650-1656.
- Crowder, D. W., Y. Carriere, B. E. Tabashnik, P. C. Ellsworth, and T. J. Dennehy. 2006. Modeling the evolution of resistance to pyriproxifen by the sweet-potato whitefly (Hemiptera: Aleyrodidae). Journal of Economic Entomology. 99: 1396-1406.
- Crowder D. W., Christa Ellers-Kirk, Bruce E. Tabashnik, and Yves Carriere. 2009. Lack of fitness costs associated with pyriproxyfen resistance in the B biotype of Bemisia tabaci. Pest Management Science, in press
- Crowder D. W., A. Rami Horowitz, Bruce E. Tabashnik, Timothy J. Dennehy, Ian Denholm, Kevin Gorman, and Yves Carriere. 2009. Analyzing haplodiploid inheritance of insecticide resistance in invasive whitefly biotypes. Bulletin of Entomological Research, in press
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Progress 09/01/07 to 08/31/08
Outputs
OUTPUTS: We determined if fitness costs were associated with pyriproxyfen resistance in a laboratory-selected resistant strain (QC02-R) of the B biotype. Fitness costs were not associated with resistance in QC02-R, as mortality caused by pyriproxyfen did not increase over time in unselected hybrid strains and fitness traits were similar in unselected and selected hybrid strains. Using a new method to examine the inheritance of resistance based on data from the fitness cost experiments, we estimated that pyriproxyfen resistance is controlled by two loci in the QC02-R strain. The lack of fitness costs associated with pyriproxyfen resistance could promote the evolution of resistance in field populations with similar traits as QC02-R. We had found that one P450 gene in pyriproxyfen-resistant whiteflies of the B biotype, CYP6_2_2 , was significantly down-regulated. To identify single-nucleotide polymorphisms (SNPs) that discriminate between resistant and susceptible alleles of CYP6_2_2 in the B biotype, a 494 bp genomic fragment from the promoter region of the CYP6_2_2 gene was amplified and sequenced. Comparison of sequences obtained from QC-02 (resistant) and Yuma 04-S (susceptible) individuals revealed the presence of 12 SNPs. These SNPs suggested the presence of at least four alleles: two that are present in both strains and two that are present only in the resistant strain QC-02. We had found that the metabolic gene GST2 was up-regulated in pyriproxyfen-resistant whiteflies from the Q strain. To identify single-nucleotide polymorphisms (SNPs) that discriminate between resistant and susceptible alleles of GST2 in the Q biotype, a ~400 bp genomic fragment from an intron within the GST2 gene was amplified and sequenced. Comparison of sequences obtained from resistant and susceptible individuals revealed the presence of two SNPs at intron position 212 and 290, that allow discrimination between the putative resistant and susceptible alleles of GST2 in the Q biotype. In future work, we plan to use the SNPs identified in the promoter region of CYP6_2_2 and the intron of GST2, to discriminate between the putative resistant and susceptible alleles. PARTICIPANTS: This project was the subject of the PhD thesis of Dave Crowder, who was awarded his PHD in 2008. TARGET AUDIENCES: Farmers and people working in pest control of field crops. PROJECT MODIFICATIONS: None.
Impacts
Our research provides insights about how to better manage the evolution of whitefly resistance to pyriproxyfen and other insecticides in Arizona and elsewhere. Simulations models based on empirical data obtained with resistant and susceptible strains indicate that the evolution of resistance to pyriproxyfen can vary greatly across regions based on operational and environmental factors. Similar results have been documented over the past 11 years in Arizona. Lower action thresholds for using pyriproxyfen, later planting dates for cotton fields, and increased rates of pyriproxyfen decay could delay resistance, but with the potential cost of larger B. tabaci populations, more insecticide sprays, and increased selection pressure for resistance to other insecticides. One promising alternative would be to curtail the use of pyriproxyfen in cotton intensive regions and promote use of buprofezin, an insect growth regulator that has not been affected by increased B. tabaci resistance levels. When pyriproxyfen must be used in cotton intensive regions, increasing the effectiveness of both internal and external refuges could delay resistance. Planting external refuges earlier than pyriproxyfen-treated fields could result in larger refuge populations at the time pyriproxyfen is sprayed and delay resistance. The same result could be achieved by increasing action thresholds for insecticides other than pyriproxyfen in external refuges. Application of lower pyriproxyfen concentrations could also delay resistance while providing effective control of populations. In regions with greater crop diversity, distributing non-cotton refuges and cotton fields uniformly to reduce isolation of pyriproxyfen-treated fields, or synchronizing the harvest of spring non-cotton crops with pyriproxyfen sprays in cotton fields could increase dispersal of susceptible individuals into cotton fields and delay resistance. While these options seem promising for delaying pyriproxyfen resistance and sustaining the whitefly IPM program in Arizona, more field testing will be needed to evaluate their feasibility and impact on cotton production.
Publications
- Crowder D. W., Christa Ellers-Kirk, Bruce E. Tabashnik, and Yves Carriere. 2009. Lack of fitness costs associated with pyriproxyfen resistance in the B biotype of Bemisia tabaci. Pest Management Science, in press
- Crowder D. W., A. Rami Horowitz, Bruce E. Tabashnik, Timothy J. Dennehy, Ian Denholm, Kevin Gorman, and Yves Carriere. 2009. Analyzing haplodiploid inheritance of insecticide resistance in invasive whitefly biotypes. Bulletin of Entomological Research, in press
- Crowder D. W., Peter C. Ellsworth, Bruce E. Tabashnik, and Yves Carriere. 2009. Impact of operational factors on the evolution of resistance to pyriproxyfen by the sweetpotato whitefly (Hemiptera: Aleyrodidae). Journal of Economic Entomology, in press
- Crowder, D.W., Ellers-Kirk, C., Yafuso, C., Dennehy, T.J., Degain, B.A., Harpold, V.S., Tabashnik, B.E., and Carriere, Y. 2008. Inheritance of resistance to pyriproxyfen in Bemisia tabaci (Hemiptera: Aleyrodidae) males and females (B biotype). Journal of Economic Entomology. 101: 927-932.
- Crowder, D. W., T. J. Dennehy, C. Ellers-Kirk, C. M. Yafuso, P.C. Ellsworth, B.E. Tabashnik, and Y. Carriere. 2007. Field evaluation of resistance to pyriproxyfen in Bemisia tabaci (B biotype). Journal of Economic Entomology. 100: 1650-1656.
- Crowder, D. W., Y. Carriere, B. E. Tabashnik, P. C. Ellsworth, and T. J. Dennehy. 2006. Modeling the evolution of resistance to pyriproxifen by the sweet-potato whitefly (Hemiptera: Aleyrodidae). Journal of Economic Entomology. 99: 1396-1406
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Progress 09/01/06 to 08/31/07
Outputs
OUTPUTS: Inheritance of resistance to pyriproxyfen in B and Q biotype: The insect growth regulator pyriproxyfen is a selective insecticide that has been used for over a decade in Arizona and Israel to control the sweetpotato whitefly. Resistance to pyriproxyfen has evolved in strains of both the B and Q biotypes of B. tabaci. We incorporated data from laboratory bioassays into models to estimate the number of loci affecting pyriproxyfen resistance in both biotypes. In the B biotype, results suggest that resistance is controlled by more than one locus, most likely two. In the Q biotype, results suggest that resistance is affected by one or a few loci. Molecular basis of resistance to pyriproxyfen in B and Q biotype: The relative fold-change of metabolic gene RNAs in 4th instars from pyriproxyfen-resistant and susceptible strains of the B and Q biotypes was evaluated with real-time PCR analyses focusing on 17 metabolic genes. Results were normalized to the actin mRNA level. In the B
biotype, one P450 gene was significantly down-regulated (~10 fold) in the resistant strain compared to its susceptible counterpart. The relative expression of all other 16 metabolic genes was not significantly different from 1. In the Q biotype,one P450 gene was significantly down-regulated (~10 fold), while GST2 was up-regulated in the resistant compared to susceptible strain. No difference in expression levels were observed in the other 15 metabolic genes. Resistance to pyriproxyfen in both biotypes may be associated with down-regulation of specific P450 detoxification genes. The two biotypes do not down-regulate the same P450 gene, suggesting that evolution of resistance accrued in parallel and that gene flow between biotype B and Q is low or nil. Up-regulation of GST2 in the resistant Q strains may provide an additional detoxification component to pyriproxyfen resistance. Modelling the evolution of resistance to pyriproxyfen in the B biotype of the whitefly: We used computer
simulations to examine the impact of several operational factors on the evolution of resistance to the insect growth regulator pyriproxyfen in the B biotype. The model was parameterized using results from laboratory and field experiments. Lower action thresholds and/or higher action thresholds for insecticides other than pyriproxyfen slowed the evolution of resistance. However, lower action thresholds resulted in more insecticide applications per year. Resistance to pyriproxyfen evolved fastest in cotton-intensive regions and slowest in regions with multiple crops throughout the year. In regions with non-cotton crops present during part or all of each year, increasing dispersal from non-cotton crops into cotton slowed the evolution of resistance. Planting cotton earlier in the season increased the rate of resistance evolution, although fewer insecticide applications were needed compared to fields planted later in the year. Faster rates of pesticide decay slowed the evolution of
resistance. Results suggest that modification of operational factors, which can be controlled by growers, could prolong the sustainability of pyriproxyfen.
PARTICIPANTS: Yves Carriere. Professor of Entomology, University of Arizona; Dave Crowder. PhD candidate, University of Arizona; Christa Ellers-Kirk, Research specialist, University of Arizona; Tim Dennehy. Professor of Entomology, University of Arizona; Bruce Tabashnik, Professor of Entomology and Department Head, The University of Arizona; Rami Horowitz. Director of Gilat Research Center, Israel; Shai Moring. Lecturer, Hebrew University of Jerusalem. This project is allowing PhD training of David Crowder in the field of resistance management and Entomology.
TARGET AUDIENCES: US farmers, PCAs, and Entomologists: farmers, PCAs and Entomologists in many countries where the whitefly is a key pest of crops grown for food and fiber.
PROJECT MODIFICATIONS: None
Impacts
These findings will improve our ability to develop efficient resistance management strategies for the sweetpotato whitefly, a key pest of many crops inthe US and worldwide.
Publications
- Crowder, D. W., C. Ellers-Kirk, C. M. Yafuso, B. A. Degain, V. S. Harpold, T. J. Dennehy, B. E. Tabashnik, and Y. Carriere. 2008. Association between genotype and survival to the insect growth regulator pyriproxyfen in Bemisia tabaci males and females of the B biotype. Journal of Economic Entomology. In press.
- Crowder, D. W., T. J. Dennehy, C. Ellers-Kirk, C. M. Yafuso, P.C. Ellsworth, B.E. Tabashnik, and Y. Carriere. 2007. Field evaluation of resistance to pyriproxyfen in Bemisia tabaci (B biotype). Journal of Economic Entomology. 100: 1650-1656.
- Crowder, D. W., Y. Carriere, B. E. Tabashnik, P. C. Ellsworth, and T. J. Dennehy. 2006. Modeling the evolution of resistance to pyriproxifen by the sweet-potato whitefly (Hemiptera: Aleyrodidae). Journal of Economic Entomology. 99: 1396-1406.
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Progress 09/01/05 to 09/01/06
Outputs
Arizona: We used simulation models to identify factors that may contribute in delaying the evolution of resistance in the whitefly. In laboratory experiments, the dominance of resistance decreased as the dose of pyriproxyfen increased. Resistance was nearly additive with low doses but recessive with high doses. Susceptibility of susceptible male and female eggs was not significantly different. However, susceptibility of nymphs of resistant males was significantly higher than susceptibility of nymphs of resistant females. Pyriproxyfen applied from airplanes resulted in incompletely recessive resistance in the B strain. We found no difference between the susceptibility of susceptible males and females or resistant males and females in the field. We have created hybrid strains of the B biotype. Two hybrid strains are not exposed to pyriproxyfen, while two related hybrid strains are selected for resistance. After 14 generations, resistance to pyriproxifen has increased in
the two selected hybrid strains, but resistance to pyriproxyfen has not declined in the unselected strains, suggesting an absence of fitness costs associated with resistance to pyriproxyfen. Israel: In general, no differences in resistance were observed between male and female eggs of the Q biotype in the laboratory. Amplified fragment length polymorphisms (AFLP) and cDNA microarrays are used for molecular characterization of pyriproxyfen resistance in the Q biotype. Resistant and susceptible individuals derived from a susceptible strain by recurrent selections with 50 ppm pyriproxyfen (so far eight generations) will be compared. In selective AFLP amplifications, we generated 239 polymorphic markers. Eight markers (3%) were present in 75-100% of the individuals from the resistant strain but in only 0-25% of the individuals from the susceptible strain. We plan to use a recently developed cDNA microarray that contains 8,000 spots representing 6,000 unique contigs and singletons to
compare resistant and susceptible nymphs of the Q biotype.
Impacts
Preserving the efficacy of pyriproxyfen for whitefly control in cotton and other crops will contribute in reducing use of broad-spectrum insecticide, help conserving natural enemies, and maintain farmer profits.
Publications
- Crowder, D. W., Y. Carriere, B. E. Tabashnik, P. C. Ellsworth, and T. J. Dennehy. 2006. Modeling the evolution of resistance to pyriproxifen by the sweet-potato whitefly (Hemiptera: Aleyrodidae). Journal of Economic Entomology. 99: 1396-1406.
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