Veterinary Parasitology, 8 (1981) 327--336 327 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PROTECTION OF PASTURED CATTLE FROM BLACK FLIES (DIPTERA:SIMULIIDAE): IMPROVED WEIGHT GAINS FOLLOWING A DERMAL APPLICATION OF PHOSMET

M.A. KHAN

Research Station, Agriculture Canada, Lethbridge, Alberta TIJ 4B1 (Canada)

(Accepted for publication 23 March 1981)

ABSTRACT

Khan, M.A., 1981. Protection of pastured cattle from black flies (Diptera:Simuliidae): improved weight gains following a dermal application of phosmet. Vet. ParasitoL, 8: 327--336.

In two experiments, a pour-on application of phosmet at 25 mg kg-1 reduced the harassment of cattle by black flies for up to 5--6 weeks posttreatment. During this period, the treated groups either gained more weight or lost less weight than their controls. The untreated groups made compensatory gains when the black fly activity decreased or ceased. No significant differences existed between the average daily gains of a treated group and its control either for the posttreatment-periods of 9 and 15 weeks or for the experimental periods of 15 and 20 weeks in the two experiments.

Black fly toxemia was not seen in indigenous cattle or in naive steers brought into a black fly infested area 1--2 weeks before the onset of black fly activity.

INTRODUCTION

Black flies are a serious pest of cattle and other livestock in northern regions of Canada, including Athabasca County and neighboring districts in Alberta. Of the 10 species of black flies found swarming around cattle in the area, Simulium arcticum Malloch is the most predominant and injurious species to cattle. It breeds in the fast-flowing sections of the large Athabasca River, while most of the other species breed in smaller streams in the area (Fredeen, 1969).

In Athabasca County, the black fly season extends from May to September. During an outbreak of S. arcticum, huge swarms of the black fly at tack cattle, inflict painful bites, suck blood, and inject toxin(s) through the bite. The toxin of ten causes fatal toxemia in newborn calves and naive cattle (cattle exposed to black flies for the first time), particularly bulls (Hutcheon and Chivers-Wilson, 1953; Fredeen, 1973). In 1972, acute or sub- acute toxemia, including 10 deaths, was recorded in 45 naive cattle (2 bulls and 43 heifers) within 3 days of their arrival in the County on 15 June.

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When attacked by swarms of black flies, cattle stop grazing and either leave the pastures to seek refuge in barns and other farm buildings, or crowd together to protect as much of their bodies from black flies as possible. The cattle become severely debilitated and their breeding cycles are often dis- rupted. Young calves are undernourished and stunted as their dams produce little milk and resist nursing because of painful fly bites on teats and udder.

The usual method for controlling black flies has been to kill their larvae developing in the river. In the past, DDT and methoxychlor were used for this purpose (Fredeen, 1975), but their use has been restricted because of environmental concerns. Therefore other methods of protecting cattle were required.

In a preliminary investigation, cattle treated with a pour-on application of phosmet (N-(mercaptomethyl) phthalimide S-(O, O-dimethyl) phosphorodi- thioate) were able to graze during periods of increased black fly activity when the untreated animals either left the pastures or huddled together to escape black fly attacks (Khan, 1971). Consequently, two experiments, one in 1972 and the other in 1973, were conducted to determine the effective- ness of the t reatment for reducing harassment of cattle by black flies. Since the black flies attacking cattle were too numerous to count, the change in the body weight of the treated and untreated cattle, a measure of their ability to graze during periods of black fly activity, was used to determine the effectiveness of the treatment.

MATERIALS AND METHODS

Experiment 1 (19 72)

Three unirrigated pastures, 1, 2, and 3, which were 40, 40, and 20 ha in area, respectively, were located on a farm in the black fly infested part of Athabasca County in northern Alberta. The pastures contained a mixture of brome, creeping red fescue, and t imothy grasses with white Dutch clover.

On 29 May, mature cows and young heifers and steers, 7--18 months old, born and raised on the farm were weighed individually and allotted by con- strained randomization (Haufe and Thomspon, 1964) to two 17-heifer groups, A and B, two 17-steer groups, C and D, and two 18-cow groups, E and F. Groups A and B, C and D, and E and F were comparable in average weight per animal. On 28 June, groups A, C, and E were treated with a pour- on application of Prolate 4 0 S {Stauffer Chemical Company, Mountain View, CA, U.S.A.), an oil soluble formulation containing 4% phosmet, giving a dose of about 25 mg phosmet kg -~ . Grops B, D and F were the untreated controls.

The six groups had free access to the three pastures during the pretreat- ment period, 29 May--27 June. During the first 8 weeks posttreatment, which were expected to be the main period of black fly attacks, groups A and B grazed in pasture 3, which was partitioned off from pastures 1 and 2, that were allotted to groups C to F. A partition between pastures 1 and 2

329

during the first 2 of these 8 weeks, separated the treated from the untreated groups. The cattle were exchanged between pastures 1 and 2 after the first week. Eight weeks after t reatment, the partitions be tween all three pastures were removed and all six groups grazed together until the end of the experi- ment on 12 October.

During the experimental period, the black fly activity around the six groups was estimated primarily by sound and sight and ranked as high, medium, and low. The high level of activity was associated with a buzzing sound audible at a distance of 10--15 m. The other two levels were lower gradations.

During the first 8 weeks after t reatment, the black fly activity, when visually estimated at the high or medium level, was also sampled by sweeping around the cattle with an insect net, 38 cm in diameter. Each sampling consisted of 20 sweeps and the flies caught were immobilized with a 0.25% pyrethrum aerosol and counted and identified for their species. The mean number of flies caught in 20 sweeps represented the fly activity at each sampling and the highest of the sampling averages in a day represented the peak daily activity (PDA). The mean of all the samplings within a day represented the average daily activity (ADA). A count of >20 , 10--20, and <10 flies/sweep corresponded to the audio-visual estimate of the high, medium, and low levels of activity, respectively.

Each animal was weighed at 4 weeks and immediately before t reatment and at 1, 2, 4, 6, 8, and 15 weeks after treatment.

Experiment 2 (1973)

Thirty-four yearling Hereford steers, which had never been exposed to black flies, were obtained at Lethbridge and allotted by constrained randomization (Haufe and Thompson, 1964) to two groups, G and H, equal in number and average weight per steer. On 17 May, the steers were taken 270 km by truck to a farm in Athabasca County and placed in an unirrigated 40-ha pasture containing a mixture of t imothy, brome, and fescue grasses with clover. There was no black fly activity when the steers arrived at the farm, although it had a history of heavy infestation with black flies.

On 16 July, nearly 2 weeks after the first appearance of black flies, group G was treated with phosmet used at the same dose and by the same method as in experiment 1. Group H was not treated and used as control. After treat- ment, the two groups were penned separately for 3 days and then returned to the pasture for the duration of experiment. Each steer was weighed at 2, 4, and 6 weeks before treatment, immediately before treatment, and at 1, 2, 3, 4, and 9 weeks after treatment.

The black fly activity around the steers was recorded by the net method used in experiment 1 from 30 May, when the flies made their first appearance, to the end of their activity on 29 September. The activity was

330

recorded daily, in early morning and late afternoon and at other times when the activity appeared to be at the medium or high level.

In both experiments, the safety of phosmet t reatment was judged by clinical examination of animals for signs of organophosphorus toxicosis (Khan, 1973). The effectiveness of the t reatment was determined by comparing average daily gains (ADG) of treated groups with those of their controls during the post t rea tment periods, 29 June--12 October and 17 July--19 September for experiments 1 and 2. The ADGs were compared by the analysis of variance after adjusting the post t reatment ADG of each animal by its weight at the beginning of the experiment and its ADG during the pretreatment period.

RESULTS

Black fly activity

The activity around cattle was variable and intermittent. Swarms of flies would of ten appear in the morning or afternoon, or both, and disappear after attacking cattle for 1 or 2 h. On some days, the activity would continue at the high level (>20 flies/net sweep) throughout the daylight hours, 0500-- 2200 h, but in the following days and even weeks the activity weuld be at the low level (<10 flies/net sweep). The flies would pursue cattle into the shade of trees, but no t into barns and buildings. The fly activity ended after dark.

In the first experiment, black flies were active from 29 May to 19 Septem- ber. Their activity, beginning at the high level on 29 May, dropped to the low level on the following day and stayed at that level until 14 June. There- after, three periods of increased activity, 15--22 June (P1 }, 29 June--12 July (P2), and 26 July--18 August (P3), could be identified, when the PDA remained at the medium or high level with an occasional drop to the low level. During the rest of the season, the PDA remained at the low level.

During P2 and P3, when the activity was sampled by the net method, the PDA was recorded at the medium and high levels for a total of 6 and 9 days, respectively, and at the low level on all other days. During these periods, the ADA was recorded at the medium and high levels for a total of 2 and 3 days, respectively, and at the low level on all other days. S. arcticum S. vittatum, S. venustum, and S. decorum comprised 95.7, 4.0, 0.2, and 0.1%, respectively, of the black flies collected, with 97.8% of them being without a blood meal.

In the second experiment, the black flies were seen attacking cattle from 30 May to 29 September. During this period, the PDA was recorded at the high and medium levels for 14 days each and at the low level for 94 days; and the ADA was recorded at the high level for 2 days, at the medium level for 15 days, and at the low level for 105 days. The black fly activity varied widely even on days when PDA was recorded at the high level. For example,

331

the activity ranged between 1 .8--55.9 , 2 .2--40.1 , and 1 .8- -51 .9 flies per sweep on 1, 18 and 26 July (Fig. 1).

Kappa statistics (Fleiss, 1973) showed good agreement (P < 0 .001) between the visual estimates o f black fly activity and the sweep-net counts of the flies.

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The species of black flies collected were the same as those in experiment 1. S. arcticum, S. venustum, S. vittatum, and S. decorum comprised ca. 89.9, 5.9, 3.7, and 0.5% of the total catch. The first specimens of S. venustum and S. vi t tatum were collected on 30 May, S. decorum on 2 June, and S. arcticum on 11 June, and the last specimens of these species were collected on 29, 28, 29, and 19 September, respectively.

Clinical effects o f black fly attacks on cattle

At the first appearance of black flies at the medium or high level, the animals became restless and were seen to shake their heads continuously,

332

flip their ears, and switch their tails. They walked and t rot ted along the fences as if trying to break out of the pastures. As the black fly season progressed, the animals' behavior changed and they tended to huddle together, whether they were standing or lying down. The animals generally rested on high ground in the pastures, where the wind, if strong enough, would reduce the black fly activity.

Numerous fly bites, o f t e n associated with edema, were seen around the eyes, abdomen, udder, and teats or scrotum. None of the animals, however, showed signs of systemic black fly toxemia.

Weight gains in cattle

Experiment 1 The first significant difference be tween weight changes in the two groups

of heifers occurred during weeks 5 and 6 post t reatment , when the weight loss of the treated group A was lower (P < 0.01) than that of the control group B. However, during the next 2 weeks, group B outgained (P < 0.01) group A, and no significant difference existed between the ADGs of the two groups for the rest of the l l - w e e k post t rea tment period {Table I).

Both the treated steers (group C) and cows (group E) outgained (P < 0.01) their controls (groups D and F) during the first 2 weeks after t reatment but were outgained (P < 0.01) by these groups during the next 2 weeks. During weeks 5 and 6 post t reatment , no significant difference existed between weight losses for the two groups of steers, but the weight loss was lower (P < 0.05) for the treated than for the untreated cows. During the remaining 9 weeks of the post t rea tment period, the weight changes in the treated steers and cows were not significantly different from those in their respective controls {Table I).

There were no significant differences be tween the weight gains of the three treated groups and their respective controls during the entire posttreat- ment period. There were also no significant differences between the three treated groups and their respective controls in their final weights recorded on 12 October at the end of the experiment (Table I).

Experiment 2 No significant differences existed between the total ADGs of groups G

and H for the entire pretreatment period of 6 weeks. During the posttreat- ment period, both groups lost weight during the 1st week but the loss was lower (P < 0.05) for the treated group, G, than for the untreated group, H. Later, both groups gained weight during the remaining 8 weeks of the post- t reatment period, with group G outgaining (P < 0.05) group H during weeks 2--5 post treatment. No significant differences existed between the ADGs of the two groups during weeks 6--9 post t reatment , the entire post t reatment period, or the pre- and post t rea tment periods combined {Table II).

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335

DISCUSSION

The high level of black fly activity reduced weight gains in pastured cattle as indicated by weight changes in naive cattle during weeks 3 and 4 of pre- t reatment period in experiment 2 and in untreated cattle in both experiments. Phosmet t rea tment reduced weight loss in cattle up to 6 weeks after treat- ment in experiment 1 and up to 5 weeks in experiment 2 (Tables I and II).

Weight gain data suggest that, during the periods of increased black fly activity, the treated animals made bet ter use of pastures than the untreated ones. However, the untreated animals made compensatory gains (Hironaka and Kozub, 1973) when black fly activity dropped to the low level. Consequently, at the end of the black fly season, no significant differences existed between the weight gains or body weights of the treated and untreated animals (Tables I and II).

In experiment 1, all groups lost weight during weeks 5 and 6 posttreat- ment (26 July--8 August), which coincided with the greater part of the third period of increased black fly activity (26 July--18 August). The weight loss was due partly to temporary deterioration of pastures, caused by lack of rains.

Cattle seem to develop a tolerance to black fly toxemia by gradual exposure to black flies. None of the indigenous cattle used in experiment 1 showed signs of black fly toxemia, bu t acute or subacute toxemia occurred in naive cattle brought into the area during a black fly outbreak. Similarly, black fly toxemia was not seen in the steers used in experiment 2, as they were gradually exposed to increasing levels of black fly activity.

The mode of action of phosmet against black flies was not clear. It is a persistent chemical as residues of 1510--21660 and 6--126 ppm were found on the hair from the back and underline, respectively, of two cows 21 days after a pour-on application of phosmet at approximately the rates used in this experiment (Mann, 1972). These residues indicated the long-term potential of phosmet as an insecticide against black flies landing on treated cattle and crawling through their hair to reach the skin.

Another compound, permethrin ( 3 -phenoxyphenyl) -methyl (+) -cis, t rans - 3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate), was found effective for preventing black flies (S. a r c t i c u m ) from feeding on steers. One t reatment with 1 mg permethrin kg-1 body weight applied to the entire body surface reduced the number of flies feeding on the steers by at least 70% for 8 days after t reatment (Shemanchuk, 1981}.

Black flies were seen to leave their hosts immediately after a pour-on application of phosmet. The duration of this repellent effect is not known. The combinat ion of repellent and insecticidal effects may reduce the number of flies feeding on cattle and thus may help the treated animals to develop a tolerance to black fly attacks.

336

REFERENCES

Fleiss, J.L., 1973. Statistical methods for rates and proportions. Wiley, Toronto, pp. 143--147.

Fredeen, F.J.H., 1969. Outbreaks of the black fly Simulium arcticum Malloch in Alberta. Quaest. Entomol., 5: 341--372.

Fredeen, F.J.H., 1973. Black flies. Can. Dep. Agric., Pub]. 1499, 19 pp. Fredeen, F.J.H., 1975. Controlling black fly outbreaks. Can. Agric., 20(4): 15--17. Haufe, W.O. and Thompson, C.O.M., 1964. Weight changes in catt le on dry range in rela-

t ion to chemical t reatment for fly control. Can. J. Anita. Sci., 44: 272--280. Hironaka, R. and Kozub, G.C., 1973. Compensatory growth of beef catt le restricted at

two energy levels for two periods. Can. J. Anita. Sci., 53: 709--715. Hutcheon, D.E. and Chivers-Wilson, V.S., 1953. The histaminic and anticoagulant activity

of extracts of the black fly (Simulium vittatum and Simulium venustum). Rev. Can. Biol., 12: 77--85.

Khan, M.A., 1971. Protection of catt le from black flies in the County of Athabasca. Canada Commit tee on Pesticide Use in Agriculture, Agric. Can., Ottawa, Ont., Pestic. Res. Rep., pp. 197--198.

Khan, M.A., 1973. Toxici ty of systemic insecticdes. Vet. Rec., 92: 411--419. Mann, J.J., 1972. Stauffer Chemical Company, Mountain View, CA, U.S.A. Personal

communicat ion. Shemanchuk, J . A , 1981. Repellent action of permethrin, cypermethrin, and resmethrin

against black flies (Diptera:Simuliidae) at tacking cattle. Pestic. Res., in press.