Laboratory studies of susceptibility to insecticides
Only two papers have examined the effect of insecticides applied directly to adult C. obsoletus group species. A laboratory-based trial determined the LC90 values of a 1.7% concentration of DDT (dichlorodiphenyltrichloroethane) and a 0.51% concentration of dieldrin using field-collected adults (Service, 1968). A second study examined the effect of black cloths impregnated with lindane (γ-benzene hexachloride [BHC]) and DDT in miscible oil at concentrations ranging from 135–2691 mg/m2 on landing C. obsoletus in the field (Hill & Roberts, 1947). Although initial kill rates exceeded 95% of exposed individuals, after 12 days the mortality rate was reduced to 52–82% across the concentrations tested and by 26 days post-application no significant difference was noted between treated and untreated cloths.
A series of laboratory trials has examined the mortality of adults exposed to insecticide in wind tunnels (Kline et al., 1981; Floore, 1985). The results (Table 1) show that, for adult Culicoides, synthetic pyrethroids (SPs) cause higher knockdown and kill rates than organophosphates (OPs). However, the performance of each insecticide was highly variable across trials as a result, in part, of variations in experimental protocol and possibly differences in susceptibility between species, laboratory colonies and wild populations.
Table 1. The susceptibility of Culicoides midges to selected pyrethroid and organophosphate insecticides in laboratory studies. Midges were exposed in a wind tunnel to solutions of insecticide in acetone, sprayed using an atomizer, except for cyhalothrin, which was presented on filter paper.
|Species||LC90 of active ingredient|
|C. mississippiensis||0.00032%*||0.00177%*||0.01000%*||0.01940%*|| |
|C. sonorensis||0.027%‡||0.025%‡|| ||0.064%‡|| |
|C. imicola|| ||0.46 %§|
The early, and generally unsuccessful, uses of organochlorines against Culicoides were reviewed by Dove et al. (1932) and Kettle (1962); in any case, these insecticides have long since been withdrawn from use in the U.K. and Europe. More recently, various OP and SP insecticides applied by aerial ultra-low-volume (ULV) methods have been used to reduce biting nuisance levels. In most cases, relief from biting activity was transient, lasting only a few days (Table 2). The only exception to this occurred in a study where bifenthrin was used in a peridomestic urban environment against Culicoides ornatus Taylor and Culicoides subimmaculatus Lee & Reye in Queensland, Australia. Application of insecticide to external resting surfaces (houses, fences, vegetation) in the suburbs experiencing biting nuisance resulted in a 65% reduction in mean numbers of these species at 6 weeks post-treatment (Standfast et al., 2003).
Table 2. Effects of selected pyrethroid and organophosphate insecticides upon adult Culicoides midges in field trials.
|Insecticide||Species||Application method (applied at ground level unless stated)||Dose||Mortality (terrain)|
|Bifenthrin||C. subimmaculatus C. ornatus||ULV*||0.1%||> 65% reduction in numbers caught for 6 weeks (urban)|
|Malathion||C. furens||ULV†||100.6 g/ha||90% control up to ∼ 40 m from application area (open)|
|40% control up to ∼ 40 m from application area (vegetated)|
|Resmethrin||C. furens||ULV†||15.6 g/ha||90% control up to ∼ 25 m from application area (open)|
|40% control up to ∼ 94 m from application area (vegetated)|
|Naled||C. furens||ULV†||27.6 g/ha||90% control up to ∼ 105 m from application area (open)|
|40% control up to ∼ 177 m from application area (vegetated)|
|ULV (applied from aircraft)‡||36.5 g/ha||24% reduction in numbers caught at light (open)|
|73 g/ha||> 99% reduction in numbers caught at light for 3 days (open)|
|Thermal fog§||1% at 19–23 L/h||90% control up to ∼ 20 m from application area (open)|
|ULV¶||9.9 g/ha||70% up to 18 m from application area (open)|
During the present epidemic of BTV in the Mediterranean basin, countries such as Greece, Bulgaria, Spain and Italy have used environmental spraying to reduce transmission, at least in the initial stages of virus incursion (M. Patakakis, G. Georgiev, M. A. Miranda Chueca, M. Goffredo, personal communication, 2006). Only one field study, however, has examined the impact of such operations on Culicoides populations. Satta et al. (2004) monitored the abundance of Culicoides on two farms in Sardinia following spraying with a micro-encapsulated formulation of Mycrocrip (Industria Chemica Fine, Cremona, Italy) (including cypermethrin, esbiothrin, piperonyl butoxide and pyrethrin synergists) applied over 1 ha from a vehicle-mounted vaporizer. No decrease in the numbers of Culicoides in treatment over control stations was recorded using light traps following insecticide application, and the authors were not able to detect any significant and unequivocal impact on the midge population.
The small size of Culicoides (1–3 mm in length) means that mesh screens typically used to provide protection from biting flies are not effective. For example, mesh with a pore size of 1.6 mm2, which is effective against mosquitoes, reduced entry rates by only 56% and an even finer mesh of 0.9 mm2 still allowed 5% to pass through (Porter, 1959). Mesh diameters that are small enough to stop midges passing through greatly reduce light and airflow, making them unsuitable for, say, animal housing.
Several studies have assessed whether treating mesh with various OP insecticides can enhance performance. Laboratory studies showed that aluminium mesh treated with malathion (7.7% w/v dissolved in 80% ethyl ethanol) provided rapid (< 1 h) knockdown and 100% mortality for 21 days post-treatment (Jamnback, 1961). In subsequent laboratory studies, treatment of identical screens with 5% propoxur or 6% malathion killed 100% of midges within 60 min for up to 27 days post-treatment, even when the insecticide-treated mesh was exposed to weathering (Jamnback, 1963). Dukes & Axtell (1976) assessed the performance of a range of emulsifiable concentrate formulations on commercial aluminium screens, and showed that propoxur and malathion were more effective than identical w/v applications of dichlovos, stirofos and dimethoate against Culicoides furens (Poey). In studies of the susceptibility of Culicoides mississippiensis Hoffman to OP-treated aluminium mesh, Kline & Roberts (1981) showed that an 8% formulation of propoxur produced 73–92% knockdown at 30 min post-exposure and a mortality of > 97% for 35 days after treatment, whereas formulations of chlorpyriphos, malathion and fenthion were less effective. Surprisingly, the performance of pyrethroid-treated mesh for controlling Culicoides has not been tested despite the widespread use of pyrethroid-treated nets and curtains for controlling mosquitoes.
The treatment of livestock with SPs is an important means of controlling various arthropod-borne diseases (Eisler et al., 2003). The insecticide kills vectors that contact the animal and, in some cases, may also reduce feeding probability or the duration of contact (Habtewold et al., 2004). In general, studies of the effect of these formulations on Culicoides have focused on the performance of pour-on rather than dip-wash formulations.
In laboratory studies, Mullens (1993) showed that hair from permethrin-treated goats caused knockdown and reduced the feeding rate of Culicoides variipennis (Coquillet) (= sonorensis Wirth & Jones); hair from the backline, where the insecticide was applied, reduced the feeding rate and produced 100% knockdown for up to 69 days, whereas hair from the belly was less effective. Studies of C. sonorensis exposed to hair from cattle treated with permethrin or pirimiphos methyl (Mullens et al., 2000) and of Culicoides nubeculosus Meigen exposed to hair from pyrethroid-treated (deltamethrin, cypermethrin) sheep and cattle (S. J. Torr, S. Carpenter, J. Barber, P. S. Mellor, D. I. Farman, unpublished data, 2007; Carpenter et al., 2007) have also shown that hair from the belly, where midges are known to feed (Nielsen et al., 1988), caused lower mortality than hair from the backline. In a field trial, however, Mullens et al. (2001) found that applying 0.2% w/v permethrin to the bellies of a herd of 200 cattle had no significant effect on BTV transmission. It is unclear whether the absence of any effect reflected the poor performance of the insecticide and/or whether the intervention itself was applied on too small a scale to have any significant impact on the local midge population.
Similar trials were also carried out in Australia, at Beatrice Hill Farm, 60 km southeast of Darwin (Melville et al., 2001, 2004; Doherty et al., 2004). In one study, four different treatments were tested for their ability to reduce seroconversion to BTV: (a) diazinon-impregnated ear tags; (b) deltamethrin pour-on (25 g/L) applied at 4 mL/100 kg bodyweight; (c) flumethrin pour-on (75 g /L) applied at 10 mL/100 kg bodyweight, and (d) a fenvalerate dip-wash (200 g/L) applied using a hand-operated sprayer (Melville et al., 2004). No significant differences in BTV seroconversion rates were found, either between treated groups of cattle or in comparison with untreated controls, although the lowest rate was recorded with the deltamethrin product. It was subsequently demonstrated that the deltamethrin-, flumethrin- and 4% permethrin-based spray treatments all significantly reduced numbers of both fed and unfed Culicoides collected on the cattle, demonstrating at least some kind of an effect of the insecticides. As expected from the seroconversion study, deltamethrin had the greatest effect, although it was noticeable that small numbers of blood-fed Culicoides (< 25 specimens/test day) continued to be collected from these treated individuals.
Subcutaneous injectable avermectins are widely used to protect farm stock from internal parasites, especially gastrointestinal worms, and they are also used increasingly as agents against ectoparasites (e.g. Jess et al., 2007). In addition, these compounds have been employed successfully in Australia against Culicoides brevitarsis Keiffer, causing > 99% mortality in laboratory-reared adult insects fed on cattle up to 7 days after injection of 200 μg Ivermectin/kg bodyweight (Standfast et al., 1984). In the U.S.A., a laboratory-based assay on membrane-fed C. sonorensis, using sheep blood containing 0–1.0 μg Ivermectin, found no significant effect on vector mortality over this range of doses (Holbrook & Mullens, 1994). The authors compared their results with data on the concentrations of Ivermectin circulating in cattle that had been successfully treated against C. brevitarsis in Australia (Standfast et al., 1984), and calculated that the lethal dose for C. sonorensis was > 100-times greater than that for C. brevitarsis. To reach a lethal concentration for C. sonorensis, these authors concluded that cattle would have to be inoculated with doses of Ivermectin greater than allowed under existing legislation. At present there are no data on the efficacy of avermectins against Culicoides spp. in Europe.