Insecticidal effects of deltamethrin in laboratory and field populations of Culicoides species: how effective are host-contact reduction methods in India?

Abstract

BackgroundBluetongue virus (BTV) is transmitted by Culicoides biting midges and causes bluetongue (BT), a clinical disease observed primarily in sheep. BT has a detrimental effect on subsistence farmers in India, where hyperendemic outbreaks impact on smallholdings in the southern states of the country. In this study, we establish a reliable method for testing the toxic effects of deltamethrin on Culicoides and then compare deltamethrin with traditional control methods used by farmers in India.ResultsEffects of deltamethrin were initially tested using a colonised strain of Culicoides nubeculosus Meigen and a modified World Health Organisation exposure assay. This method was then applied to field populations of Culicoides spp. in India. The field population of C. oxystoma in India had a greater LC50 (0.012 ± 0.009%) for deltamethrin than laboratory-reared C.nubeculosus (0.0013 ± 0.0002%). Exposure of C. nubeculosus to deltamethrin at higher ambient temperatures resulted in greater rates of knockdown but a lower mortality rate at 24 h post-exposure. Behavioural assays with C. nubeculosus in WHO tubes provided evidence for contact irritancy and spatial repellence caused by deltamethrin. The field experiments in India, however, provided no evidence for repellent or toxic effects of deltamethrin. Traditional methods such as the application of neem oil and burning of neem leaves also provided no protection.ConclusionsOur study demonstrates that field-collected Culicoides in India are less susceptible to deltamethrin exposure than laboratory-bred C. nubeculosus and traditional methods of insect control do not provide protection to sheep. These low levels of susceptibility to deltamethrin have not been recorded before in field populations of Culicoides and suggest resistance to synthetic pyrethrioids. Alternative insect control methods, in addition to vaccination, may be needed to protect Indian livestock from BTV transmission.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-017-1992-0) contains supplementary material, which is available to authorized users.

Influences of Exposure Time and Mortality Assessment Interval on Bioassay Results of Insecticide-Resistant Tropical Bed Bugs (Hemiptera: Cimicidae)

“…In a study with Anopheles mosquitoes tested on Mosquitoes Contamination Device (MCD) bottle bioassay, longer exposure times also resulted in significantly higher knockdown rates and 24 h mortalities [ 26 ]. Similarly, De Keyser et al [ 27 ] reported Culicoides nubeculosus (M.) showed substantially lower 24 h mortalities when tested with 10 and 20 min than 60 min on 0.001% deltamethrin-treated paper. According to a study on C. lectularius , the uptake of permethrin by the insects significantly increased with longer exposure time and distance travelled on a permethrin-treated mattress liner [ 25 ].…”

“…[ 26 ], as well as Culicoides spp. [ 27 ], and differences were observed in insecticides uptake, knockdown responses, and mortalities.…”

Field Evaluation of Deltamethrin and Ivermectin Applications to Cattle on Culicoides Host-Alighting, Blood-Feeding, and Emergence

“…In these three trials, there was no effect of ivermectin or deltamethrin on the numbers of Culicoides caught on animal-mounted sticky traps, indicating that these treatments did not deter landing. This is perhaps to be expected as the main actions of deltamethrin and ivermectin are toxicity, although there is also some evidence of contact irritancy and repellence to deltamethrin [18,37,38] and ivermectin has reduced the responses of Culicoides imicola Kieffer, 1913 to host cues in an olfactometer [39]. Crucially, there was also no difference detected in the proportion of Culicoides blood-fed between treatments.…”

“…Yet in India, dipping sheep in a deltamethrin solution had no impact on Culicoides spp. caught in light traps in their pens [18]. Lastly, in the UK a pour-on application of deltamethrin to horses had no effect on light trap catches of Obsoletus and Pulicaris group Culicoides from within mesh enclosures [19].…”

Fungi as Biocontrol Agents of Culicoides Biting Midges, the Putative Vectors of Bluetongue Disease

“…Where disease control by vaccines is not available, midge control by use of fungal biocontrol agents may play an important role in limiting disease outbreaks. Presently, midge control relies predominantly on synthetic pesticides, which pose a risk to humans and the environment (Carpenter et al 2008a, Webb et al 2010, Del Rio et al 2014, Baker et al 2015, De Keyser et al 2017. Climate change models predict warmer and wetter weather, which in turn is expected to lead to larger midge densities (Guis et al 2012, White et al 2017.…”