Evaluation of insecticide-treated cattle as a barrier to re-invasion of tsetse to cleared areas in northeastern Zimbabwe.

A field trial in Zimbabwe investigated the efficacy of insecticide-treated cattle as a barrier to prevent the re-invasion of tsetse, Glossina morsitans and G. pallidipes (Diptera: Glossinidae), into cleared areas. The original tsetse barrier consisted of insecticide-treated odour-baited targets, at an operational density of four to five targets per km2, supported by insecticide-treatments of cattle with either deltamethrin dip (Decatix, Coopers) at two-weekly intervals, or deltamethrin pouron (Spoton, Coopers) at monthly intervals, in a band approximately 20 km wide from the re-invasion front. Tsetse catch, and trypanosomiasis incidence in nine sentinel herds was recorded for 7-8 months, respectively, before the targets were removed, leaving only the insecticide treatment of the local cattle to stem the re-invasion of tsetse. After the removal of the target barrier, the tsetse readily invaded the trial area and the incidence of trypanosomiasis in sentinel herds increased, while their PCVs decreased. After seven months without the targets in place, trypanosomiasis prevalence in the local stock had reached alarmingly high levels; the trial was terminated prematurely and the target barrier re-deployed. Immediately after the re-deployment of the target barrier, the tsetse catch in the trial area reverted to acceptable levels along the re-invasion front, and trypanosomiasis incidence in the sentinel cattle decreased. It is concluded that, under the conditions of the field trial, the insecticidal treatment of local cattle did not in itself form an effective barrier to tsetse re-invasion. By contrast, the target barrier performed as was predicted by mathematical and experimental analysis, and readily cleared the tsetse infestation and reduced trypanosomosis incidence in the trial area.

Field studies on the effect of cattle skin secretion on the behaviour of tsetse.

The effect of ox skin secretions (sebum) on the behaviour of tsetse flies, Glossina spp., was investigated in the field using electrified targets, some of which operated intermittently, and by direct observations of flies landing on treated and untreated cloth. As the off-period of an intermittently operating electrified target increased, the catch decreased both with and without the sebum present. Targets with sebum always caught more flies than targets without sebum, but there was no evidence to suggest that sebum increased the duration of stay on a target. Direct observations of flies on cloth targets revealed that for both species the presence of sebum reduced the duration of contact and for G.pallidipes the number of return contacts was increased. The results from direct observations were used to predict the number of repeat landings that would need to be made by flies in order to account for the catch of tsetse at intermittently electrified targets.

Activation of three species of tsetse (Glossina spp.) in response to host derived stimuli.

Recordings were made of the activation of hungry Glossina morsitans morsitans Westwood, G. pallidipes Austen, and G. austeni Newstead in response to odours from ox breath and ox urine, and a moving visual stimulus, in a wind tunnel. The spontaneous activity of G.m.morsitans was very low (less than 4% of males and 2% of females active per min during control periods). That of G.austeni and G.pallidipes was in the region of 20% except for G.pallidipes females when in excess of 40% were active during control periods. Addition of ox urine odours to the airstream had no effect on activity in any of the species investigated but addition of ox breath odours to the airstream significantly increased activity of G.pallidipes and of G.m.morsitans, although for the latter only approximately 12% of flies were active. For G.austeni the addition of ox breath odours resulted in a significant increase in activity of females but not of males. The moving visual stimulus resulted in a significant increase in the activity of both sexes of G.austeni and G.m.morsitans but no change in the activity of G.pallidipes. The low level of spontaneous activity and the low response to ox breath odours in a strain of G.m.morsitans maintained in the laboratory since 1969 was compared with a new colony of this species which originated from puparia collected in Zimbabwe in 1991. No differences in either spontaneous activity or the response to ox breath odour was recorded, but females from the new colony were significantly more responsive to a moving visual stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of female New World screwworm flies, Cochliomyia hominivorax, to coloured targets in the laboratory.

The responses of unmated female New World screwworm flies, Cochliomyia hominivorax Coquerel, to visual targets were studied in a wind-tunnel. Both activity and frequency of contacts with targets increased greatly when the screwworm attractant mixture swormlure-4 was added to the airstream. Target-orientated responses depended on target colour, with red and black targets being preferred over blue, white and yellow ones; this preference was much greater in the presence of odour than in its absence. No preference was detected for different shapes and orientations of red targets, all of equivalent surface area. Omitting different components from swormlure-4 generally resulted in a large reduction in activation and target contacts. Attempts to substitute 1-octen-3-ol for the butanol fraction were unsuccessful, but skatole may substitute to some extent for indole; the two isomers of butanol normally present in swormlure-4 may substitute partly or completely for each other. This type of measurement forms a suitable bioassay in the development of attractive targets for monitoring and control of wild adult screwworm populations.

Responses of female New World screwworm flies, Cochliomyia hominivorax, to swormlure-4 in the laboratory.

The behaviour of 4-day-old virgin female New World screwworm flies Cochliomyia hominivorax Coquerel was investigated in a wind tunnel using video. Addition of the screwworm attractant swormlure-4 to the airstream resulted in an increase in the number of take-offs and in the duration of flight. In the presence of swormlure-4 the flight speed decreased and the rate of turning increased. Take-off was significantly orientated upwind during the period when swormlure-4 was added to the airstream, and significantly orientated downwind in the period after the addition of swormlure-4. The possible means by which this species finds a point source of odour in the field are discussed and compared with the mechanisms used by other flies.

Responses of tsetse to ox sebum: a video study in the field.

The behaviour of tsetse (mainly Glossina pallidipes Austen) around odour-baited targets, with or without a coating of ox sebum, was recorded in the field using video. The addition of sebum increased the total time a fly was in contact with the target, as well as the time spent flying around and landing on it. When carbon dioxide was released as part of the attractant odour plume, the presence of sebum on the target increased the number of landings made by each fly, but did not significantly affect the duration of each contact. When carbon dioxide was absent from the odour plume, sebum did not affect the number of landings made by flies but the duration of each contact with the target did increase. Evidence for an interactive effect of sebum and carbon dioxide was obtained. In addition, the presence of sebum on the target increased the percentage of landed flies which walked on its surface; such behaviour may represent an 'inspection' of the artificial host. The potential tsetse control application of the current findings are discussed.

Responses of tsetse flies (Glossina spp.) to compounds on the skin surface of an ox: a laboratory study.

The behaviour of male Glossina morsitans morsitans Westwood and Glossina pallidipes Austen (Diptera: Glossinidae) alighting on targets with or without ox sebum was compared. The presence of ox sebum did not increase significantly the number of flies alighting on the target in either species. However, after contact with the sebum coated target, both species showed an increase in flight activity, and G. m.morsitans showed a greater tendency to return to the target. This behaviour resulted in a number of short flights which may reflect the search for a feeding site on a host. The duration of each visit to the target was significantly reduced when sebum was present for G. m. morsitans but not for G. pallidipes. This is explained by documented differences in the resting behaviour of the two species which shows that G. m. morsitans normally rests for longer periods on the surface of an untreated black target than does G. pallidipes. Other experiments showed that the presence of sebum elicited a probing response in G. m. morsitans and G. pallidipes. The results are discussed with reference to the possible use of host sebum to improve trap catches in the field.

Effect of host behaviour on host preference in Stomoxys calcitrans.

Field observations suggest that, in the U.K., cattle are the preferred host of Stomoxys calcitrans (L.), followed by horses. Differences were observed in the numbers of flies feeding on individual animals both in the field and under controlled conditions. Analysis of the behaviour of four Friesian calves under attack from S. calcitrans in controlled conditions revealed that the differences in the levels of attack between individual hosts are dependent on the reactions of the host when under attack. Those hosts which respond vigorously by tail flicks, foot stamps and head-swings suffer less from attack by S. calcitrans than their more placid contemporaries.