High levels of insecticide resistance in introduced horn fly (Diptera: Muscidae) populations and implications for management

The horn fly, Haematobia irritans (L.) (Diptera: Muscidae), was introduced to Chile in the beginning of the 1990s. Since its introduction, farmers have controlled this pest almost exclusively with insecticides. To understand the consequences of different control strategies on the development of insecticide resistance and their persistence, a field survey was conducted at eight farms in the south of Chile to characterize insecticide resistance in field populations and resistance mechanisms. Horn fly samples were assayed to determine levels of resistance to pyrethroids and diazinon, genotyped for kdr and HialphaE7 mutations, and tested for general esterase activity. All field populations, including ones that were not treated with insecticides for the past 5 yr, showed high levels of cypermethrin resistance and high frequencies of the kdr mutation. None of the fly populations demonstrated resistance to diazinon and the HialphaE7 mutation was not detected in any of the fly samples. Esterase activities in all populations were comparable to those found in the susceptible reference strain. The findings of high frequencies of homozygous resistant and heterozygous individuals both in insecticide treated horn fly populations and in the untreated fly populations suggests complex interactions among field populations of the horn fly in Chile.     

Horn fly (Diptera: Muscidae) insecticide resistance in Kentucky and Arkansas

The effect of previous insecticide use patterns for horn fly control on the susceptibility spectrum of horn fly (Haematobia irritans [L.]) populations from Kentucky and Arkansas is described. Populations of horn flies from both states were tested with three pyrethroids (cyhalothrin, cypermethrin, and permethrin), three organophosphates (diazinon, pirimiphos methyl, and tetrachlorvinphos), and a chlorinated hydrocarbon (methoxychlor). Dose-mortality data indicated insecticide resistance in Arkansas and Kentucky. Two permethrin-resistant horn fly populations in Kentucky that did not have a history of exposure to methoxychlor were cross-resistant to this chlorinated hydrocarbon. Horn fly populations from both states with a history of at least three consecutive years of exposure to various pyrethroid ear tags were subsequently exposed to cattle tagged with cyhalothrin-impregnated ear tags for 15-16 wk. Such exposure resulted in a decrease in susceptibility to this pyrethroid (ranging from approximately 30 to greater than 100-fold) when compared with levels before treatment. Horn fly populations from Arkansas resistant to cyhalothrin (as a result of exposure to cyhalothrin ear tags) were cross-resistant to pirimiphos methyl. Seasonal exposure of an Arkansas and Kentucky horn fly population to cattle with ear tags impregnated with pirimiphos methyl resulted in a significant decrease in susceptibility to this organophosphate.     

Dynamics and mechanisms of permethrin resistance in a field population of the horn fly, Haematobia irritans irritans

A study was conducted at the Pressler ranch, near Kerrville, Texas, USA between 2002 and 2006 to determine the dynamics and mechanisms of resistance to permethrin in a field population of the horn fly, Haematobia irritans irritans (L.). Changes of resistance to pyrethroid insecticide associated with use of a pour-on formulation of cyfluthrin in 2002 and use of diazinon ear tags in subsequent years were studied using a filter paper bioassay technique and a polymerase chain reaction assay that detects two sodium channel mutations, kdr and super-kdr resistance alleles. A maximum of 294-fold resistance to permethrin was observed in the summer of 2002. A significant decrease in the resistance level was observed in spring 2003, and resistance continued to decline after animals were treated with diazinon ear tags. In response to pyrethroid treatments, the allelic kdr and super-kdr frequency increased from 56.3% to 93.8% and from 7.5% to 43.8%, respectively in 2002, and decreased significantly in 2003 when the pyrethroid insecticide was no longer used to treat animals. Females were found to have a higher allelic super-kdr frequency than males in 2002, while no difference was detected between males and females in the allelic kdr frequency. There was a significant positive correlation between frequencies of the sodium channel mutations and levels of permethrin resistance, suggesting that the sodium channel mutations, kdr and super-kdr , are the major mechanisms of resistance to pyrethroids in this horn fly population. Results of synergist bioassays also indicated possible contributions of two metabolic detoxification mechanisms, the mixed function oxidases (MFO) and glutathione S-transferases (GST). Compared to a horn fly infestation of an untreated herd, treatments with the pyrethroid pour-on formulation failed to control horn flies at the Pressler ranch in 2002. Sustained control of horn flies was achieved with the use of diazinon ear tags in 2003 and subsequent years.     

The Morgan Recharger: a new horn fly (Diptera: Muscidae) control device for beef cattle

A 20% diazinon formulation was evaluated for control efficacy against the horn fly, Haematobia irritans (L.), in the Morgan Recharger (Morgan International Products, College Grove, Tenn.). The Morgan Recharger releases insecticide with a wicking system from an insecticide reservoir and can be attached to an animal's ear or tail. This device was most effective against the horn fly when used as an ear tag with two per head; horn fly counts did not exceed five flies per side through 8 wk. The diazinon formulation tested was released from the Morgan Recharger at a decreasing rate. The problems and potential of the Morgan Recharger as an effective horn fly control device are discussed.     

Influence of permethrin, diazinon and ivermectin treatments on insecticide resistance in the horn fly (Diptera: Muscidae)

The history of insecticide resistance in the horn fly, Haematobia irritans, and the relationship between the characteristics of horn fly biology and insecticide use on resistance development is discussed. Colonies of susceptible horn flies were selected for resistance with six insecticide treatment regimens: continuous single use of permethrin, diazinon and ivermectin: permethrin-diazinon (1:2) mixture; and permethrin-diazinon and permethrin-ivermectin rotation (4-month cycle). Under laboratory conditions, resistance developed during generations 21, 31 and 30 to permethrin, diazinon and ivermectin, respectively. The magnitude of resistance ranged from < 3-fold with ivermectin to 1470-fold with permethrin. Field studies demonstrated that use of a single class of insecticidal ear tag during the horn-fly season resulted in product failure within 3-4 years for pyrethroids and organophosphates, respectively. In laboratory studies, use of alternating insecticides or a mixture of insecticides delayed the onset of resistance for up to 12 generations and reduced the magnitude of pyrethroid resistance. In field studies, yearly alternated use of pyrethroids and organophosphates did not slow or reverse pyrethroid resistance (Barros et al., unpublished data), while a 2-year alternated use with organophosphates resulted in partial reversion of pyrethroid resistance. When pyrethroid and organophosphate ear tags were used in a mosaic strategy at two different locations, efficacy of products did not change during a 3-year period.     

Cyromazine resistance in the house fly (Diptera: Muscidae): genetics and cross-resistance to diflubenzuron

Larvae of a house fly, Musca domestica L., strain collected in a chicken house near Pittsburg, Tex, after a control failure with the poultry feedthrough insecticide cyromazine showed 6.5-fold resistance to cyromazine and 10-fold resistance to diflubenzuron. Adults of the strain showed high levels of resistance to carbaryl, DDT, and diazinon; moderate resistance to cypermethrin and permethrin; and low resistance to dieldrin. In contrast, no resistance to cyromazine was observed in eight laboratory house fly strains with resistance to four groups of conventional insecticides. When the genetics of cyromazine resistance was investigated in crosses to susceptible strains with visible mutant markers, results indicated cyromazine resistance was incompletely dominant over susceptibility and the resistance gene was on chromosome V. The same or a closely linked gene conferred resistance to diflubenzuron. A strain containing only chromosome V from the original resistant strain was resistant to cyromazine and diflubenzuron, but not to other insecticides except for low level resistance to DDT and carbaryl. Resistance to the latter insecticides appeared to be due to a linked, but distinct, gene. Therefore, resistance to cyromazine and probably diflubenzuron appears to be genetically distinct from other types of insecticide resistance.     

Genetics of organophosphate resistance in field populations of the house fly (Diptera: Muscidae)

The genetics of resistance to the organophosphate insecticide diazinon were investigated in four populations of the house fly, Musca domestica L., collected in the southern United States. Crosses were made between individual females of lines derived from each population and males of a susceptible strain with three recessive mutants on chromosome II. Individual F1 females were crossed to mutant males, and the progenies were scored for resistance to diazinon and for the presence of mutant phenotypes. A major chromosome II gene for resistance to diazinon was present in all populations at an overall frequency of 83%. Map distances between the resistance gene and the mutant aristapedia and between the mutants aristapedia and stubby wing were highly variable in all populations. Recombination among the visible mutants was usually reduced in resistant progenies relative to susceptible progenies. The data suggest that a single major gene for resistance to diazinon was present on chromosome II in all test populations at variable map positions and is usually associated with a chromosome rearrangement, probably an inversion. The results are similar to those obtained earlier with house fly populations selected for resistance to insecticides in the laboratory; therefore, they seem to be characteristic of field and laboratory populations of the house fly. Overall, the data offer an explanation for previous results suggesting the existence of multiple, closely linked genes for metabolic resistance to insecticides on house fly chromosome II.     

Variation in general esterase activity within a population of Haematobia irritans (Diptera: Muscidae).

Control of the horn fly, Hematobia irritans (L.), is generally dependent on chemical insecticides. However, the biology and behavior of the horn fly favors rapid development of insecticide resistance. To prolong the effectiveness of the insecticide option, information is required regarding the mechanisms of insecticide resistance. Metabolic hydrolysis of insecticides by esterases is a detoxification mechanism in many insect species. Measurement of general esterase activity within populations of horn flies may provide a diagnostic tool for resistance management. In this study we evaluated the amount of variation in general esterase activity within female and male horn fly samples from a population that had not been exposed to insecticides for 8 yr. We found considerable variation in general esterase activity within samples of each sex, with females demonstrating the greater variation. The observed variation is thought to be the result of age-structure dynamics within the population. The amount of inherent variation makes it difficult to detect small mean differences between populations, thus limiting the utility of general esterase assays. Thus, effective diagnosis of esterase-mediated resistance mechanisms can only be achieved by the identification of specific detoxification esterases and the design of assays, either biochemical or molecular, for their detection and measurement.     

Genotype by environment interaction in Nelore cattle from five Brazilian states

Records from 75,941 Nelore cattle were used to determine the importance of genotype by environment interaction (GEI) in five Brazilian states. (Co)variance components were estimated by single-trait analysis (with yearling weight, W450, considered to be the same trait in all states) and multiple-trait analysis (with the record from each state considered to be a different trait). The direct heritability estimates for yearling weight were 0.51, 0.39, 0.44, 0.37 and 0.41 in the states of Goiás, Mato Grosso, São Paulo, Mato Grosso do Sul and Minas Gerais, respectively. The across-state genetic correlation estimates between Goiás and Mato Grosso, Goiás and Minas Gerais, São Paulo and Minas Gerais, and Mato Grosso do Sul and Minas Gerais ranged from 0.67 to 0.75. These estimates indicate that GEIs are biologically important. No interactions were observed between Goiás and São Paulo, Goiás and Mato Grosso do Sul, Mato Grosso and São Paulo, Mato Grosso and Mato Grosso do Sul, Mato Grosso and Minas Gerais, or São Paulo and Mato Grosso do Sul (0.82 to 0.97). Comparison of single and multiple-trait analyses showed that selection based on the former was less efficient in the presence of GEI, with substantial losses (up to 10%) during selection.     

Field Survey of Insecticide Resistance in Haematobia irritans exigua de Meijere (Diptera: Muscidae)

A survey of farms in northern New South Wales and southeastern, central, western and northern Queensland was conducted to determine levels of insecticide resistance in populations of buffalo fly Haematobia irritans exigua. A field bioassay using discriminating concentrations of 10 insecticides commonly used for buffalo fly control was used. Resistance to all synthetic pyrethroids tested (cypermethrin, deltamethrin, cyhalothrin, flumethrin and cyfluthrin) was common and widespread in coastal zones, but was lower in inland zones. In contrast, there was no resistance to the organophosphate diazinon and only low levels of resistance to ethion and chlorfenvinfos. Synergism between piperonyl butoxide and cypermethrin was demonstrated.     

Genetic characterization of cassava (Manihot esculenta) landraces in Brazil assessed with simple sequence repeats

Based on nine microsatellite loci, the aim of this study was to appraise the genetic diversity of 42 cassava (Manihot esculenta) landraces from selected regions in Brazil, and examine how this variety is distributed according to origin in several municipalities in the states of Minas Gerais, São Paulo, Mato Grosso do Sul, Amazonas and Mato Grosso. High diversity values were found among the five above-mentioned regions, with 3.3 alleles per locus on an average, a high percentage of polymorphic loci varying from 88.8% to 100%, an average of 0.265 for observed heterozygosity and 0.570 for gene diversity. Most genetic diversity was concentrated within the regions themselves (H_S = 0.52). Cluster analysis and principal component based scatter plotting showed greater similarity among landraces from São Paulo, Mato Grosso do Sul and Amazonas, whereas those from Minas Gerais were clustered into a sub-group within this group. The plants from Mato Grosso, mostly collected in the municipality of General Carneiro, provided the highest differentiation. The migration of human populations is one among the possible reasons for this closer resemblance or greater disparity among plants from the various regions.     

Insecticide resistance in populations of Tuta absoluta (Lepidoptera: Gelechiidae)

1 Control failures of insecticides used against the tomato leafminer Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) in Brazil led to the investigation of the possible occurrence of resistance of this insect pest to abamectin, cartap, methamidophos and permethrin. 2 The insect populations were collected from seven sites in the states of Minas Gerais, Rio de Janeiro, and São Paulo. These populations were subjected to concentration–mortality bioassays using insecticide‐impregnated filter papers. 3 We were unable to obtain a single population which provided a susceptibility standard for all insecticides tested. Therefore, the resistance levels were estimated in relation to the most susceptible population to each insecticide. Resistance to abamectin and cartap were observed in all populations when compared with the susceptible standard population, with resistance ratios ranging from 5.2‐ to 9.4‐fold and from 2.2‐ to 21.9‐fold for abamectin and cartap, respectively. Resistance to permethrin was observed in five populations with resistance ratios ranging from 1.9‐ to 6.6‐fold, whereas resistance to methamidophos was observed in four populations with resistance ratios ranging from 2.6‐ to 4.2‐fold. 4 The long period and high frequency of use of these insecticides against this insect pest suggest that the evolution of insecticide resistance on them has been relatively slow. Alternatively, the phenomenon might be widespread among Brazilian populations of T. absoluta making the finding of suitable standard susceptible populations difficult and leading to an underestimation of the insecticide resistance levels in this pest. 5 Higher levels of resistance to abamectin, cartap and permethrin are correlated with greater use of these compounds by growers. This finding suggests that local variation in insecticide use was an important cause of variation in susceptibility.     

Evaluation of alternative tactics for management of insecticide-resistant horn flies (Diptera: Muscidae)

A 3-yr study was conducted to determine the efficacy of tactics that could be used to manage populations of insecticide-resistant horn flies, Hematobia irritans irritans (L.). Insecticide spray, spot-on or pour-on formulations and two IGRs in bolus formulation, 1.3- and 3.2-ha pasture rotations on different rotation schedules, 0-50% Brahman breeding, selected fly-resistant cows, and a mechanical trap were evaluated singly and in combination. Concentration-mortality tests indicated that horn flies collected from cows used in the current study were significantly less susceptible to diazinon, coumaphos, and methoxychlor than horn flies from cows at the same locations previously used to determine baseline susceptibility. During the 3-yr study at the Southeast Research and Extension Center (SEREC), the IGR-bolus significantly reduced (P < 0.05) horn fly numbers on both the continuous and rotational graze regimens, resulting in significantly (P < 0.05) greater calf weaning weights (average of 24 kg). Horn fly numbers were significantly greater on untreated cows during the 3-yr study at the Southwest Research and Extension Center (SWREC) compared with the mean fly numbers on cows that received fly-management treatments. All tactics and tactic-combinations used at SWREC on cattle having no Brahman breeding failed to significantly reduce insecticide-resistant horn fly numbers. However, the combination of Brahman breeding with the IGR-Bolus and mechanical trap significantly reduced horn fly numbers and resulted in significant increases in calf weaning weight. In addition, mean horn fly numbers decreased significantly as the percentage Brahman breeding increased with 50% Brahman breeding reducing horn fly numbers by 140 flies per cow. No significant difference was found between the mean fly numbers on the fly-resistant purebred group and the cows that had no Brahman breeding but received the IGR-Bolus or used the mechanical trap. The use of synergized zeta-cypermethrin pour-on treatment successfully complimented the use of IGR-bolus and mechanical traps in reducing insecticide-resistant horn fly numbers. Neither 1.3- nor 3.2-ha size paddocks and stocking rates used in the rotation graze regimens at SEREC and SWREC, respectively, significantly reduced horn fly numbers when compared with continuously grazed paddocks. Data indicated the importance of using tactics that reduce horn fly numbers to approximately 150 horn flies per cow. These data demonstrated the efficacy of using tactic combinations to manage insecticide-resistant horn fly populations.     

Dieldrin-resistant cabbage root fly in England

The existence of dieldrin resistance in Erioischia brassicae has been investigated in the main areas in England and Wales where brassicas are grown for human consumption. Dieldrin-susceptible and dieldrin-resistant flies were collected and bred as separate strains in the laboratory. In toxicity tests adult flies were treated with topically applied doses of insecticide in microdrops of cellosolve, and larvae were dipped in solutions of insecticide in 70% acetone and 30% water. Adults of the laboratory resistant strain showed a high level of resistance to dieldrin and aldrin and a lower level of resistance to γ-BHC. There was no cross-resistance with chlorfenvinphos or diazinon. Larvae of the resistant strain were shown to be resistant to dieldrin. Dieldrin resistance has been found in four areas in southern England, but not in a fifth major area, and resistant flies did not increase as rapidly in some areas as others. Sufficient migration of flies appeared to take place to produce similar percentages of resistant flies over areas of several square miles, but considerable differences were found in fly populations about 10 miles apart. It is suggested that the speed of development of insecticide resistance is affected by the proportion of brassica crops which are treated in an area, the amounts of insecticide applied to individual crops, and hence the insecticidal pressure applied to the E. brassicae population.     

High levels of pyrethroid resistance in horn flies (Diptera: Muscidae) selected with cyhalothrin.

Lambdacyhalothrin cattle ear tags controlled horn fly, Haematobia irritans (L.), for 14 wks or longer during 1986-1988 in Georgia, USA. In 1989 and 1990, control of < 50 horn flies per side of cow was achieved for < or = 4 wk because of high levels of pyrethroid resistance in horn flies selected with lambdacyhalothrin. The highest resistance ratios (RRs) were seen in 1989. These were 498 for lambdacyhalothrin; 92,000 for fenvalerate; and 54 for permethrin. RRs for cypermethrin as high as 8,800 were estimated in 1990 when the RR for fenvalerate was only 1,060. No cross-resistance to diazinon was detected. These high levels of pyrethroid resistance seem to have a large component of metabolic resistance. Synergistic coefficients as high as 3,600 were determined by addition of nonlethal amounts of piperonyl butoxide. Resistance development in a no-pyrethroid-use area indicates movements of > or = 3km by sufficient numbers of horn flies can significantly change the RR.     

Pyrethroid and organophosphate pesticide resistance in field populations of horn fly in Brazil

Pesticides are used worldwide to control arthropod parasites in cattle herds. The indiscriminate and/or inappropriate use of pesticides without veterinary guidance is a reality in several countries of South America. Improper pesticide use increases the chances of contamination of food and the environment with chemical pesticides and their metabolites. Reduction of these contamination events is an increasing challenge for those involved in livestock production. The horn fly, Haematobia irritans (Linnaeus) (Diptera: Muscidae), is one of the most economically important parasites affecting cattle herds around the world. As such, horn fly control efforts are often required to promote the best productive performance of herds. Pesticide susceptibility bioassays revealed that pyrethroid resistance was widespread and reached high levels in horn fly populations in the Brazilian state of Rondônia. The knockdown resistance (kdr) sodium channel gene mutation was detected in all horn fly populations studied (n = 48), and the super kdr sodium channel gene mutation was found in all homozygous resistant kdr individuals (n = 204). Organophosphate resistance was not identified in any of the fly populations evaluated.     

Evolution of insecticide resistance in non-target black flies (Diptera: Simuliidae) from Argentina

Black flies, a non-target species of the insecticides used in fruit production, represent a severe medical and veterinary problem. Large increases in the level of resistance to the pyrethroids fenvalerate (more than 355-fold) and deltamethrin (162-fold) and a small increase in resistance to the organophosphate azinphos methyl (2-fold) were observed between 1996-2008 in black fly larvae under insecticide pressure. Eventually, no change or a slight variation in insecticide resistance was followed by a subsequent increase in resistance. The evolution of pesticide resistance in a field population is a complex and stepwise process that is influenced by several factors, the most significant of which is the insecticide selection pressure, such as the dose and frequency of application. The variation in insecticide susceptibility within a black fly population in the productive area may be related to changes in fruit-pest control. The frequency of individuals with esterase activities higher than the maximum value determined in the susceptible population increased consistently over the sampling period. However, the insecticide resistance was not attributed to glutathione S-transferase activity. In conclusion, esterase activity in black flies from the productive area is one mechanism underlying the high levels of resistance to pyrethroids, which have been recently used infrequently. These enzymes may be reselected by currently used pesticides and enhance the resistance to these insecticides.     

Toxicity of organophosphorus insecticides to diazinon-reversely-selected,pesticide-pressure-relaxed and field-resistant strains of bulb miteRhizoglyphus robini (Acari: Acaridae)

Some field strains ofRhizoglyphus robini in Taiwan, developed high multi-resistance to organophosphorus insecticides including diazinon, dimethoate, fenitrothion, malathion, methyl parathion, omethoate, parathion and prothiophos. The organophosphorus resistance of bulb mites remained stable when the pesticide pressure was relaxed. Changes of the susceptibility to several organophosphorus insecticides for a strain after relaxation of selection pressure for 3 and 27 generations were compared. Since susceptible bulb mites could not be found in the field, a reverse selection for susceptibility to diazinon was designed and conducted. Although heterogeneity was observed in the progeny of the mixed populations, the susceptibility of these reversely-selected mites increased slowly. A stable and susceptible strain was obtained after 26 generations of selection.Contribution No. 1412 from Taiwan Agricultural Research Institute.     

Predicting insecticide resistance: mutagenesis, selection and response

Strategies to manage resistance to a particular insecticide have usually been devised after resistance has evolved. If it were possible to predict likely resistance mechanisms to novel insecticides before they evolved in the field, it might be feasible to have programmes that manage susceptibility. With this approach in mind, single-gene variants of the Australian sheep blowfly, Lucilia cuprina, resistant to dieldrin, diazinon and malathion, were selected in the laboratory after mutagenesis of susceptible strains. The genetic and molecular bases of resistance in these variants were identical to those that had previously evolved in natural populations. Given this predictive capacity for known resistances, the approach was extended to anticipate possible mechanisms of resistance to cyromazine, an insecticide to which L. cuprina populations remain susceptible after almost 20 years of exposure. Analysis of the laboratory-generated resistant variants provides an explanation for this observation. The variants show low levels of resistance and a selective advantage over susceptibles for only a limited concentration range. These results are discussed in the context of the choice of insecticides for control purposes and of delivery strategies to minimize the evolution of resistance.     

A new sand fly, Lutzomyia campograndensis sp. n. (Diptera: Psychodidae: Phlebotominae) from the State of Mato Grosso do Sul, Brazil

During studies of the phlebotomine sand fly fauna of Campo Grande, State of Mato Grosso do Sul, Brazil, a new species was captured with CDC light trap and is described here. The new species resembles Lutzomyia lutziana (Costa Lima) and is named Lutzomyia campograndensis sp. n. after the type locality.