Distance from the Stable Affects Trapping of Biting Midges (Diptera, Ceratopogonidae)

Several species of Culicoides biting midges (Diptera, Ceratopogonidae) are competent vectors of important veterinary pathogens in Europe. Bluetongue virus (BTV) has emerged in Europe since the late 1990s and is transmitted by several species of the genus Culicoides (Caracappa et al. 2003, De Liberato et al. 2005). African horse sickness (AHS) was observed at the end of the last century on the Iberian Peninsula (Mellor and Hamblin 2004) and since the end of 2011 there has been evidence that biting midges are important vectors of the Schmallenberg Virus (SBV) emerging in Central Europe (Hoffmann et al. 2012). SBV causes considerable congenital damage, premature births, and reproductive disorders in ruminants. Monitoring programs for biting midges are generally based on UV-light/suction trap sampling (Mehlhorn et al. 2009, Nielsen et al. 2010). Results of the sampling and trapping protocols could be significantly affected by uncontrollable environmental factors like wind and temperature (Bishop et al. 2000, Carpenter et al. 2008) and/or even the phase of the moon (Bishop et al. 2000). In contrast, there are controllable factors that could be standardized and would result in more comparable results from entomological surveillance programs. Firstly, there is the standardization of the trapping method, which could have different specificity for abundance, taxa, sex, or feeding stadium (Holbrook 1985, Anderson and Linhares 1989, Venter et al. 2009a, Viennet et al. 2011). Secondly, some authors mentioned that the positions of the traps have a significant impact on the sampling results, such as height (Venter et al. 2009b) or habitat (Bishop et al. 1994, Bishop et al. 1995). In this study, we were interested in the impact of the distance to the stable on the trapping result. Due to a positive correlation between biting midge abundance and the presence and abundance of hosts (Bellis and Reid 1996, Baylis et al. 2010, Garcia-Saenz et al. 2011, Viennet et al. 2011), we expected a decreasing number of biting midges to occur in traps with increasing distance to the stable. The study was conducted between May and June, 2009 at one livestock farm that is surrounded by meadows in northern Germany. This site represents a typical dairy farm with 220 dairy cattle and is located in a typical agricultural region in the marshland of Lower Saxony. The grazing density of cattle was approximately 7.3 individuals per hectare, but the cattle were in the stables during the night. Sampling of Culicoides spp. was conducted with BGSentinelTM light traps fitted with ultraviolet light. Each of the three traps was placed in concentric circles in distances of 25, 50, 100, and 200 m (variable DISTANCE) around one trap in direct proximity to the stable (Figure 1a). All traps were installed at 1.5 m above the ground and operated with one car battery each per sampling period. A photo sensor was installed in the immediate vicinity of each trap. The photo sensor was adjusted to illuminate the ultraviolet light from sunset to dawn, because the activity of Culicoides spp. was expected to be highest in this time period (Kettle et al. 1998). The traps were sampled five times, every seventh day (variable SAMPLINGPERIOD). Insects were collected in 70% ethanol and stored in separate containers. The samples were presorted in order to separate biting midges from other insects and to differentiate members of the C. obsoletus and C. pulicaris group (also C. obsoletus or C. pulicaris in the subsequent text) from other Ceratopogonidae. All data analyses and graphs were made with the program R (R Development Core Team 2011). Due to overdispersion in the data, collection data of females of the Culicoides obsoletus and Culicoides pulicaris group were analyzed with negative binomial generalized linear models through the function glm. nb from the package MASS (Venables and Ripley 2002). The variables DISTANCE, DIRECTION, SAMPLINGPERIOD, and all interactions between the variables were implemented in the model. The value of the variable DIRECTION for each trap tr was calculated as: