Resistance Selection of Tetranychus cinnabarinus to Three Acaricides and Its Management Strategy

The effects of retarding resistance evolution of mixing and sequencing acaricides were studied based on the resistance selection of three acaricides to Tetranychus cinnabarinus in the laboratory. Tetrany-chus cinnabarinus was continuously selected with fenpropathrin, abamectin and pyridaban for 28, 30 and 31 generations, respectively, and resistance factor increased to 65.6, 7.2 and 1.2 time, accordingly. Results of cross-resistance determination suggested that there were no cross-resistance existed among the three acaricides. Analysis of resistance selection and estimation of realized heritability (h2) of resistance to three single and two mixed acaricides showed that a mixture of fenpropathrin+abamectin would slow resistance development compared with a sequence of fenpropathrin followed by abamectin while the mixture of pyridaben+abamectin and the sequence of pyridaben followed by abamectin would effectively retard resistance evolution.

Selection,effective dominance,and completeness of Cry1A.105/Cry2Ab2 dual-protein resistance in Helicoverpa zea(Boddie)(Lepidoptera:Noctuidae)

In the U.S.,Helicoverpa zea(Boddie)is a major pest targeted by both transgenic maize and cotton expressing Bacillus thuringiensis(Bt)proteins.Resistance of insect to Bt maize and cotton containing cry1A and cry2A genes has widely occurred in the U.S.In this study,two trials were performed to investigate larval survival and development of a Cry1A.105/Cry2Ab2 dual-protein resistant(VT2P-RR),a susceptible,and an F1 heterozygous(VT2P-RS)populations of H.zea on ears of nine Bt and three non-Bt maize hybrids.The Bt maize hybrids evaluated represent five common pyramided traits expressing two or three of the Cry1A.105,Cry1Ab,Cry1F,Cry2Ab2,and Vip3Aa20 proteins.In the laboratory,neonates of the three H.zea populations were inoculated on silks of ears collected from maize at R1-R2 plant stages;and larval survivorship was checked 10 d after neonate release.All three insect populations survived normally on non-Bt maize ears.Varied numbers of VT2P-RR and VT2P-RS survived on ears of Cry1A.105/Cry2Ab2 maize,while all larvae of the three populations died or could not develop on ears of Vip3Aa20-expressing maize.The results demonstrated that the dual-protein resistant H.zea was not cross-resistant to Vip3Aa20-expressing maize,and thus traits with vip3Aa20 gene should be effective to manage Cry1A.105/Cry2Ab2-resistant H.zea.The resistance in VT2P-RR was determined to be incomplete on Cry1A.105/Cry2Ab2 maize.The effective dominance levels varied greatly,from recessive to incompletely dominant,depending on maize hybrids and trials,suggesting that proper selection of maize hybrids could be important for mitigating the Cry1A.105/Cry2Ab2 resistance.The data generated should aid in modeling multiple-protein Bt resistance in H.zea.

Risk of control failure to insecticides malathion,profenophos+cypermethrin mixture,and fipronil in boll weevil(Coleoptera:Curculionidae)populations from Bahia,Brazil

Background To control the boll weevil Anthonomus grandis grandis(Coleoptera:Curculionidae),a key pest of cotton in the Americas,insecticides have been intensively used to manage their populations,increasing selection pressure for resistant populations.Thus,this study aimed to detect insecticide resistance and assess insecticide control failure likelihood of boll weevil populations exposed to malathion,profenophos+cypermethrin,and fipronil insecticides.Results Twelve populations of the boll weevil were collected from commercial cotton fileds of the state of Bahia,northeastern Brazil.These populations were exposed to malathion,profenophos+cypermethrin mixture,and fipronil,at their respective maximum label dose for field applications.Three replicates of 10 adult beetles were exposed to the insecticides and mortality was recorded after 24 h treatment.The control failure likelihood was determined after 48 h.Highest median lethal times(LT_(50))were observed for malathion and the profenophos+cypermethrin mixture.Resistance to at least one insecticide was detected in 11 populations;three populations were resistant to malathion and profenophos+cypermethrin;seven were resistant to all insecticides tested.The resistance levels were low(<10-fold)for the three insecticides.Among 12 populations tested,58%of them exhibited significant risk of control failure for the insecticides malathion and profenophos+cypermethrin.The insecticide fipronil was efficient for the control of the boll weevil in 83%of the populations.Conclusions The results confirm the significant risk of insecticide control failure in the boll weevil populations to the main compounds used in the region.Thus,proper insecticide resistance management plans are necessary for the boll weevil in the region,particularly for malathion and profenophos+cypermethrin insecticides.

Resistance of the fall armyworm,Spodoptera frugiperda,to transgenic Bacillus thuringiensis Cry1 F corn in the Americas:lessons and implications for Bt corn IRM in China

The fall armyworm,Spodoptera frugiperda(J.E.Smith),is a major target pest of Bt crops(e.g.,corn,cotton,and soybean)in North and South America.This pest has recently invaded Africa and Asia including China and the invasion has placed a great threat to the food security in many countries of these two continents.Due to the extensive use of Bt crops,practical resistance of S.frugiperda to Cry IF corn(TC 1507)with field control problems has widely occurred in Puerto Rico,Brazil,Argentina,and the mainland United States.Analyzing data generated from decade-long studies showed that several factors might have contributed to the wide development of the resistance.These factors include(1)limited modes of action of Bt proteins used in Bt crops;(2)cross-resistance among Cryl proteins;(3)use of nonhigh dose Bt crop traits;(4)that the resistance is complete on Bt corn plants;(5)abundant in initial Cry IF resistance alleles;and(6)lack of fitness costs/recessive fitness costs of the resistance.The long-term use of Bt crop technology in the Americas suggests that Bt corn can be an effective tool for controlling S.frugiperda in China.IRM programs for Bt corn in China should be as simple as possible to be easily adopted by small-scale growers.The following aspects may be considered in its Bt corn IRM programs:(1)use of only"high dose^traits for both S.frugiperda and stalk borers;(2)developing and implementing a combined resistance monitoring program;(3)use“gene pyramiding”as a primary IRM strategy;and(4)if possible,Bt corn may not be planted in the areas where S.frugiperda overwinters.Lessons and experience gained from the global long-term use of Bt crops should have values in improving IRM programs in the Americas,as well as for a sustainable use of Bt corn technology in China.

Detection and monitoring of insect resistance to transgenic Bt crops

Transgenic crops expressing Bacillus thuringiensis （Bt） endotoxins havebecome one of the most important tools for managing corn and cotton insect pests in the US and other countries. The widespread adoption of transgenic Bt crops could place a high degree of selection pressure on the target insect populations and accelerate development of resistance, raising concerns about the long-term durability of Bt plants as an effective pest management tool. Conservation of Bt susceptibility in insects has become one of the most active research areas in modern agriculture. One of the key factors for a successful Bt resistance management plan is to have a cost-effective monitoring system that can provide information on. （i） the initial Bt resistance allele frequencies at low levels in field insect populations; and （ii） early shifts in Bt resistance allele frequencies so that proactive measures for managing resistance can be deployed well before field control failures. Developing such a monitoring program has been difficult because： （i） resistance traits that occur at very low frequencies are hard to detect; （ii） many factors affect the sensitivity and accuracy of a Bt resistance monitoring program; and （iii） monitoring resistance is costly. Several novel methods for detecting Bt resistance alleles developed during the last decade have made a cost-effective monitoring system possible. Future studies should focus on how to improve and standardize the methodologies for insect sampling and Bt resistance detection.

Management of diamondback moth, Plutella xylostella （Lepidoptera： Plutellidae） by mating disruption

Field trials were conducted in China in 2008 and 2009 to evaluate the efficacy of mating disruption （MD） on diamondback moth, Plutella xylostella, in cabbage, Brassica oleracea var. capitata. Effectiveness was positively correlated with the MD dispenser density in the field. A density of 167 MD dispensers per ha produced an average population decrease of about 50% compared to the conventional-practice field. Significant fewer males were captured in pheromone-treated and conventional-practice fields than in the blank control field, but the difference was not significant between the pheromone-treated and conventional-practice fields. In addition, fewer eggs and larvae were observed in pheromone-treated fields. Our results suggest mating disruption coupled with minimal insecticidal supplements is a promising solution for resistance management and control of diamondback moth infestation.

Relative fitness of susceptible and Cry1A.105/Cry2Ab2-single-/dual-protein-resistant Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) on non-Bt diet and a diet containing a low concentration of two proteins

Helicoverpa zea(Boddie)is a destructive agricultural pest species that is targeted by both Bacillus thuringiensis(Bt)maize and cotton in the United States.Cry1A.105 and Cry2Ab2 are two Bt proteins expressed in a widely planted maize event MON 89034.In this study,two tests(Test-Ⅰ and Test-Ⅱ)were conducted to evaluate the relative fitness of Bt-susceptible and-resistant H.zea on non-Bt diet(Test-Ⅰ and Test-Ⅱ)and a diet containing a mix of Cry1A.105 and Cry2Ab2 at a low concentration(Test-Ⅱ only).Insect populations evaluated in Test-Ⅰ were two Bt-susceptible strains and three Bt-resistant strains(a single-protein Cry1A.105-,a single-protein Cry2Ab2-,and a dual-protein Cry1A.105/Cry2Ab2-resistant strains).Test-Ⅱ analyzed the same two susceptible strains,three backcrossed-and-reselected Cry1A.105/Cry2Ab2-single-/dual-protein-resistant strains,and three F1 heterozygous strains.Measurements of life table parameters showed that neither the single-nor dual-protein Cry1A.105/Cry2Ab2 resistance in H.zea was associated with fitness costs under the test conditions.The single Cry protein resistances at a concentration of a mix of Cry1A.105 and Cry2Ab2 that resulted in a zero net reproductive rate for the two susceptible strains were functionally incomplete recessive or codominant,and the dual-protein resistance was completely dominant.The lack of fitness costs could be a factor contributing to the rapid revolution of resistance to the Cry proteins in this species.Data generated from this study should aid our understanding of Cry protein resistance evolution and help in refining IRM programs for H.zea.

Performance of Bt-susceptible and -heterozygous dual-gene resistant genotypes of Spodoptera frugiperda (J.E.Smith) (Lepidoptera:Noctuidae) in seed blends of non-Bt and pyramided Bt maize

A seed blend refuge has been implemented in the U.S.Corn Belt for Bt maize resistance management.The fall armyworm,Spodoptera frugiperda(J.E.Smith),is a target pest of Bt maize in the Americas.The larvae of this pest are mobile,which may affect the efficacy of seed blend refuges.In this study,field and greenhouse trials were conducted to determine the performance of Bt-susceptible(aabb)and-heterozygous dual-gene-resistant(AaBb)genotypes of S.frugiperda in seed blends of non-Bt and pyramided Bt maize.Three field trials evaluated larval survival,larval growth,and plant injury with aabb in seed blends of Bt maize expressing Cry1A.105/Cry2Ab2/Vip3A with 0–30%non-Bt seeds.Greenhouse tests investigated the performance of aabb and AaBb in seed blends of Cry1A.105/Cry2Ab2 with 0–30% non-Bt seeds.In pure non-Bt maize plots,after 9–13 d of neonates being released on the plants,0.39 and 0.65 larvae/plant survived with leaf injury ratings of 4.7 and 5.9(Davis's 1–9 scale)in the field and greenhouse,respectively.In contrast,live larvae and plant injury were virtually not observed on Bt plants across all planting patterns.Larval occurrence and plant injury by aabb on non-Bt plants were similar between seed blends and pure non-Bt plantings,suggesting that the blended refuges could provide an equivalent susceptible population as structured refuge under the test conditions.In the greenhouse,the two insect genotypes in seed blends performed similarly,indicating that the seed blends did not provide more favorable conditions for AaBb over aabb.The information generated from this study should be useful in managing S.frugiperda and evaluating if send blends could be suitable refuge options for Bt resistance management in the regions where the insect is a primary target pest.