FENVALERATE AND FLUCYTHRINATE RESISTANCE IN A HORN FLY' POPULATION'

D. Craig Sheppard Department of Entomology

Coastal Plain Experiment Station Tifton, GA 31793

Abstract: Horn nics. Haematobia irrilalls (Linnaeus), controlled for 2 yr with 8% fenvalerate catt.le car tags were difficult to control in the third year (1981) at the University of Georgia Alapaha Experimental Range (AER). Field data showed progressively poorer cont.rol over a 4-yr period which stabilized in the fifth year (1983). Vcry poor horn fly control was realized with 7}lz% flucythrinate (another pyrethroid) cattle ear tags when used for the first time in 1983 at AER. Flucythrinate tags gave very good horn fly control at two other sites in 1983 where pyrethroid use had been limited. Suspected flucythrinate-fenvnleratc cross resistance and direct resistance to fenvalerate with resistance ratios of 35 and en. 40, respectively. were confinned in several laboratory tests. Rapid selection of resistant hom nies with pyrethroid ear tags and resistance management are discussed.

Key Words: Hom ny, Haematobia irritans, resistance, fenvalerate, nucythrinate.

J. Agric. Entomol. 1(3), 305·310 (July 1984)

Pyrethroid impregnated cattle ear tags have offered highly effective and convenient long-term horn fly, Haematobia irritans (Linnaeus), control to United States cattlemen for about 4 yr. But many producers and entomologists in the southeastern United States have noted poor horn ny control with these slow release devices, especially after ca. two seasons' use. Pyrethroid resistance has been suspected. One of these horn fly control failures occurred in southern Georgia where fenvalerate impregnated cattle ear tags had been used extensively. The following account documents the progressive loss of control with fenvalerate ear tags and relates it to laboratory determinations of fenvalerate resistance. Flucythrinate cross resistance to fenvalerate also was studied. A secondary objective of this study was development of a simple method of resistance determination to reduce (1) handling of horn nics and (2) equipment necessary for tests.

MATERIALS AND METHODS

Fenvalerate, Icyano (3·phenoxyphenyl) methy]·4-chloro-alpha-(I-methylethyl) benzeneacetatel. 8% impregnated cattle ear tags were used at the University of Georgia Alapaha Experimental Range (AER), Berrien County, GA, from 1979 to 1983. About 250 head of beef cattle are maintained at AER on 120 ha of improved pasture. Other pyrethroid formulations used at AER for horn fly control from 1980·1983 were permethrin, 1(3·phenoxyphenyl) methyl-cis, trans-3·(2,2. dichloroethenyl)-2, 2·dimethylcyclopropane carboxylate] impregnated cattle ear tags, and fenvalerate and permethrin pour-on formulations. Fenvalerate has

I IlWl'ERA; Muaddae 2 Received for publication :I June 1!184; lIccl!pted 28 Jllly 1984. Feu !,/Iid for c"rly publiclltion.

305

306 J. Agric. Entomol. Vol. I, No.3 (1984)

accounted for ca. 80% of the pyrethroid use in the eastern half of AER, where horn flies used in this study were captured. Flucythrinate. [cyano (3-phenoxyphenyl) methyl-4-(dilluoromethoxy)-alpha-(i-methylethyl) benzeneacetatel, 71>% impregnated cattle ear tags were used to tag one herd of 27 cows (2/cQw) in May 1983.

Fenvalerate fonnulations used at AER were supplied by 8DS Biotech, Painesville, OH (formerly Diamond Shamrock Corp., Cleveland, OH). Permethrin formulations were supplied by reI Americas Inc., Wilmington, DE, or Philips Hoxane, Inc., St. Joseph, MO. Flucythrinate-impregnated cattle ear tags were provided by American Cyanamid Co., Princeton, NJ.

Suspected pyrethroid-resistant horn flies at AER were collected by aerial net from corralled cattle. In most tests freshly captured females « 2 d) were used. In one test (12 September 1983) F, suspected resistant horn flies were reared after the method of Schmidt et al. (1976). Pyrethroid-susceptible horn flies were obtained from the laboratory colonies of the University of Florida, Gainesville, FL (UFL), and USDA-ARS, College Station, TX (USDA-CS-TX). All reared flies were used 3 - 6 dafter eelosion. Citrated beef blood was offered to hom flies from eelosion or capture until a test began.

A test method was developed in which hom flies were exposed to pyrethroid residues, much as they would be in a field situation. Handling of flies also was minimized. Flies were exposed to several concentrations of fenvalerate or flucythrinate residue expressed as Ilg/cm 2 to develop dosage mortality data. Technical fenvalerate (92.3%) from Shell Development Co., Modesto, CA, and flucythrinate (75.3%) from American Cyanamid Co., Princeton, NJ, were used to produce the residues in these tests. In all but one test reported here, four to seven residue levels were applied to 100-rom glass petri dish bottoms by evenly drying 1 ml of a pyrethroid in acetone solution on its inside plane surface (62.2 cm 2). In one test fenvalerate residues on 9-cm filter paper disks were prepared in a similar manner and placed in lOO-mm plastic petri dishes. Residues were allowed to dry thoroughly and used within 2 d. Residue levels were chosen to produce near 0 to near 100% mortalily. In each test an appropriate number of flies was placed on acetone-treated surfaces as a check treatment. Suspected resistant (AER) and susceptible (UFL or USDA-CS-TX) horn flies in a given test always were exposed to residues produced from the same dilution scheme. Ten females were placed in each petri dish with one to three dishes for each level in a given trial. Wild-strain (resistant) hom flies typically survive and feed poorly under laboratory conditions when compared to laboratory adapted susceptible strains. A short test interval with no food was chosen to minimize this difference and achieve a better measure of the response to the pyrethroid being tested. Schnitzerling et al. (1982) while studying fenvalerate resistance in Haematobia irrilans exigua (DeMeyere) used a posttreatment period of 12 min to determine mortality. In studying resistance at AE R, flies unable to stand at 2 h were considered dead since preliminary tests showed this interval was adequate for determination of dosage-mortality response in horn flies.

In some cases results from small trials producing nonsignificant regressions individually were formed into larger composite tests for a more powerful analysis. Daum's computer program for probit analysis (Daum 1970) was used to determine LC values and 95% confidence limits, and slopes of regression lines.

307 SHEPPARD: Pyrethroid Resistance in Hom Flies

RESULTS AND DISCUSSION

Fenvalerate-impregnated cattle ear tags gave very good horn fly control for the first 2 yr but control decreased considerably the third and fourth years (Table 1). One tag per cow gave practically 100% control I-d posttreatment in 1979 and 1980. By 1983 a higher rate of two tags per cow gave only 68% control at 1 d. Duration of good control (~50/side of cow) was also greatly reduced between 1979 and 1983, decreasing over 69% (Table 1). Longevity of control by fenvalerate tags was not available in 1981 and 1982 due to other control measures necessitated by other studies. These other treatments kept horn flies wen below 50/side.

Table 1. Efficacy of pyrethroid-impregnated cattle ear tags for horn fly control at the Alapaha Experimental Range, Berrien County, GA, 1979 - 1983.

Short term Posttreat time to Year Tags/cow control > 50 hom flies/side of cow

8% Fenualerate tags 1979 2 (I> cows) 100% @ 1 d 13 wk + 1980 1981 1982

1 1 1

96% @

88% @

59% @

1 d 1 d 1 d

12 wk - . - .

1983 2 68% @ 1 d 4 wk

7.5% Flucythrinate tags 1983 2 70% @ 2 d 4 wk • Dlta "'rn not taken.

Flucythrinate had not been previously used at AER but control also was poor with this tag in 1983. Horn fly control was similar to that with the fenvalerate tag (Table 1). This poor control, coupled with good control with fiucythrinate tags at two other sites in Georgia (~ 15 wk) where pyrethroid resistance had not been observed, suggests cross resistance between fenvalerate and flucythrinate.

Results of laboratory tests in 1982 and 1983 repeatedly showed significant resistance ratios (R/S = resistant LC :-.oIsusceptible LC so) of ca. 40 for AER hom flies when compared to UFL or USDA-CS-TX strain hom flies (Table 2). Some tests (not shown) did not produce significant regressions for both populations, but all tests indicated resistance levels similar to those in Table 2. Typically, the level of pyrethraid that killed 100% of the susceptible population produced little or no mortality in the AER flies. This level of resistance was exhibited against fenvalerate which had been used at AER since 1979, and against flucythrinate, to which the AER horn flies exhibited resistance on fIrst exposure. Fenvalerate resistance developed over two or more years, and a control problem was very evident in early 1982 (Table 1).

Exposure of field-collected resistant and laboratory-reared susceptible female horn flies to serial levels of pyrethroid residues for 2 h provided data suitable to detennine valid dosage mortality responses. The method was convenient and gave reproducible results. Mortality in checks was usually zero. LC :-.ovalucs determined with this technique were very consistent from test to test when similar horn flies were used. Four tests with fenvalerate residues on glass are represented in Table 2, and in three of these tests there is no significant difference (indicated by

Table 2. LC 50 values, slopes of lines estimated by probit regression, and resistance ratios of resistant strain (AER) and susceptible strain female horn flies to residues of two pyrethroids expressed as ,Ltg/cm 2, 1982 and 1983.

Test date(s) Compound Strain­ Flies/dose Substrate

14 Aug. 82 Fenvalerate AER USDA·CS·TX

20 20

Paper

6 Jul. 83 Fenvalerate AER UFL

10 10

Glass

2, 3 Aug. 83 Fenvalerate AER USDA·CS·TX

20 20

Glass

15, 30, 31 Aug. 83 Fenvalerate AER UFL

40 50

Glass

12, 14 Sept. 83 Fenvalerate AER UFL

30 30

Glass

23 Sept. and 5, 6 Oct. 83

Flucythrinate AER UFL

50 50

Glass

LC 50t (95% confidence limits)

10.983 (15.298·7.847) 0.277 (0.360 . 0.212)

0.187 (0.257·0.130) 0.00542 (0.00767.0.00381)

0.100 (0.128 . 0.0780) 0.00178 (0.00225·0.00137)

0.0287 (0.0351 . 0.0227) 0.00162 (0.00188·0.00141)

0.0615 (0.0749·0.0488) 0.00146 (0.00171· 0.00124)

0.0219 (0.0258·0.0183) 0.000618 (0.000705·0.000541)

Slope (SE) RIS;

1.873 (0.26) ,..39.6

2.920 (0.4 7) > p'

4.365 (1.20) '" 34.5 3.543 (0.82)

SO"'" 3

3.135 (0.50) :<­56.2 3.342 (0.58) <

:<­:­2.741 (0.37)

3.458 (0.36) 17.7 z

?

'"' 3.772 (0.64) -::42.1 5.046 (0.81) 00'"

~

2.832 (0.33) 35.4 4.121 (0.46)

• AER - Georgia Ahlpaha E_periment.1 Ranre; USDA-CS-TX - USDAlARS. CoUege St.tion, TX; m-L = Unh-usity of nunda_ Gainesville. FLo t J!g/cm 2 :;: Ruistance /'8Itlo - I.e ~o relilltant populationILC ~O IUlceptible population.

309 SHEPPARD: Pyrethroid Resislance in Horn Flies

overlap in 95% confidence intervals} in the LC flO values for the susceptible strain. The somewhat larger variation in LC ,,0 values for field-collected resistant horn Dies is at least partially due to the proximity of recent pyrethroid treatments to collection dates. LC 50 values taken in tests with fenvalerate residues on paper were higher t.han in tests with residues on glass. Residue's on glass were more available to the flies than residues on and within a filter paper disk. This lowered the apparent LC 50 value for the residues on glass.

AER horn flies with a resistance ratio of ca. 40 to fenvalerate and flucythrinate were controlled very poorly with cattle ear tags impregnated with these materials. Duration of control was reduced from ca. 4 months to 4 wk. Interestingly this population has remained around 40-fold resistant to fenvalerate for a year or more (Table 2) and field control remained about the same from 1982 to 1983 (Table 1). A large increase in the resistance ratio which was anticipated in 1983 did not occur. This was in spite of steady selection with fenvalerate tags and pour-on treatments necessary to moderat.e horn f1y numbers. Immigration of susceptible individuals can slow the development of resistance (Taylor et a1. 1983) and immigration at AER may have been in a rough equilibrium with selection pressures.

Previous to widespread pyrethroid cattle ear t.ag use, horn flies were considered to be readily controllable. Only (;\'10 previous cases of field resistance in horn nies have been documented; resistance to ronnel in Louisiana (Bums and Wilson 1963) and to stirofos in Georgia (Sheppard 1983). The report of Burns and Wilson was confirmed by Harris et al. (1966) who developed a resistance ratio of ca. 50 in a series of laboratory selections. Reports of horn fly resistance to pyrethroid ear tags became widespread in the southeastern United States about 2 yr after their introduction.

In their present fonn, insecticide-impregnated cattle ear tags rapidly select for resistance in horn nies. ft is well established that persistent insecticides and formulations select more rapidly for resistance (Taylor and Georghiou 1982; Taylor et aJ. 1983; Georghiou 1980; Denholm et al. 1983; Georghiou and Taylor 1977b). Also, each adult horn fly in a t.reated herd will almost certainly contact pyrethroid residues (no "refugia"), thus hastening the development of resistance (Georghiou and Taylor 1977a).

A third factor that favors selection for resistance by current insecticide­impregnated cattle ear tags is their relatively low release rates. Calculations with figures reported by Miller et a1. (1983) show an average daily release rate of ca. 2.0 mg/d for an 8% fenvalerate tag over a 6-month period. Two liters of a 0.0] % fenvalerate spray would deliver 200 mg, or 100 times as much as the average daily release rate for the tag. After 140 d the 8% fenvalerate tag releases less than 1 mg/d (Miller et a1. 1983). These low doses presumably select for heterozygous resistant individuals and cause resistance to be functionally dominant and evolve rapidly (Georghiou 1980).

Persons planning for hom fly control should be aware of the possibility of strong cross resistance between fenvalerate and Ducyt.hrinate. A slow-release device which provides high doses throughout the season could I'educe selection for resist.ance. A resist.ance management plan currently pract.icable is tagging cattle as late as possible with slow release devices to reduce selection for resistant horn flies. Early spring generations will not be selected, and late summer and fall

310 J. Agric. Entemo!. Vol. I, No.3 (1984)

generat.ions will receive higher doses. Removal of tags afler they become apparently ineffective will allow survival of immigrating susceptibles.

ACKNOWLEDGMENTS

The author thanks Robert L. Harris USDAIARS, CoUege Station, TX, and Jerry Butler, University of Florida. Gainesville, FL, for graciously supplying susceptible horn flies. Sincere thanks are expressed to William V. Miller, Modesto, CA. for advice in developing the residual exposure technique for monitoring hom fly resistance.

REFERENCES CITED

Bums, E. C., and B. H. Wilson. 1963. Field resistance of hom nies to the organic phosphate insecticide ronnel. J. Ecan. Entomol. 56: 718.

Daum, R. J. 1970. Revision of two computer programs Cor probit analysis. Bull. Entomo!. Soc. Amer. 16: 10·J2.

Denholm, I., A. W. Farnham, Kate O'Dell, and R. M. Sawicki. 1983. Factors aHecting resistance to insecticides in house-flies, Musca domestica L. (Diptera: Muscidae). I. Long­term control with bioresmethrin of nies with strong pyrethroid-resistance potential. Bull. Entomol. Res. 73: 481·489.

Georghiou, G. P., and C. E. Taylor. 1977a. Genetic and biological influences in the evolution of insecticide resistance. J. Econ. Entomol. 70: 319-323.

Georghiou, G. P., and C. E. Taylor. 1977b. Operational influences in the evolution of insecticide resistance. J. Econ. Entomol. 70: 653-658.

Georghiou, G. P. 1980. Management of insecticide resistance. Residue Reviews. 76: 131­145.

Harris, R. L., E. D. Frazar, and O. H. Graham. 1966. Resistance to ronnel in a strain of horn flies. J. Econ. Entomol. 59: 387-390.

Miller, J. A., D. D. Oehler, and S. E. Kunz. 1983. Release of pyrethroids from insecticidal ear tags. J. Econ. Entomol. 76: 1335-1340.

Schmidt, C. D., J. M. Dreiss, J. L. Eschle, R. L. Harris, and M. O. Pickens. 1976. Horn fly: modified laboratory rearing methods. Southwest. Entomol. 1: 49-51.

Schnitzerling, H. J., P. J. Noble, A. Macqueen, and R. J. Dunham. 1982. Resistance of the buffalo fly, Haematobia irritans exigua, to two synthetic pyrethroids Bnd DDT. J. Aust. Entomol. Soc. 21: 77-80.

Sheppard, D. C. 1983. Shrofos resistance in a population of horn flies. J. Ga. Entomol. Soc. 18: 370-376.

Taylor, C. E., F. Guaglia, and G. P. Georghiou. 1983. Evolution of resistance to insecticides: 8 cage study on the influence of migration and insecticide decay rates. J. Econ. Entomol. 76: 704-707.

Taylor, C. E., and G. P. Georghiou. 1982. Influence of pesticide persistence in evolution of resistance. Environ. Entomol. II: 746·750.