Mixed-sex clusters on grass blades: breeding strategy of the ornate dog tick, Dermacentor reticulatus

Abstract

Background: The ornate dog tick Dermacentor reticulatus is second only to the hard tick Ixodes ricinus in terms of importance as a vector of infectious organisms, especially of Babesia canis, the agent of canine babesiosis. Both the geographical range and local densities of D. reticulatus are steadily increasing in many regions of Europe. In the present study, we tested the hypothesis that D. reticulatus possesses an efficient breeding strategy that allows for a rapid increase in tick numbers and densities through the formation of mixed-sex clusters/aggregations while questing in the environment.

Methods: An observational study was carried out in the spring of 2023, at three sites in two regions in Central and North-Eastern Poland, both characterised by high tick densities. At each site, a 400-m-long transect was inspected for questing ticks. All noted ticks were collected, and tick numbers and sexes per stem were recorded. Differences in tick distribution by site and sex were analysed statistically.

Results: A total of 371 D. reticulatus (219 females, 152 males) ticks were collected from 270 grass stems over a combined 1200 m of transect. The majority of grass stems (74.4%) were occupied by just a single individual, with two-tick clusters the second most common category. The maximum number of D. reticulatus individuals observed on a single grass stem was six. Mixed-sex clusters were significantly more common than single-sex clusters at all three sites. With study sites combined, mixed-sex clusters accounted for 17.4% (95% confidence limit [95% CL] 13.9-21.6%) of observations, while for multiple males and multiple females, the values were 2.6% (95% CL: 1.4-4.7%) and 5.6% (95% CL: 3.7-8.3%), respectively.

Conclusions: Mixed-sex clusters of D. reticulatus ticks were significantly more common than single-sex clusters, which we hypothesise reflects an efficient, likely pheromone-mediated breeding strategy of this expansive tick species.

Keywords: Aggregations; Assembly; Behaviour; Dermacentor reticulatus; Mixed-sex clusters; Single-sex clusters; Ticks.

Fig. 1

Map of Poland showing tick sighting locations

Fig. 2

Clusters of adult questing Dermacentor reticulatus ticks on grass stems. a Two females, b 2 males, c 1 male and 2 females, d 1 female and 2 males, e, f 1 male and 1 female, g 4 males and 1 female, h 1 plant with 8 ticks on stems

Fig. 3

a Frequency distribution of single and multiple D. reticulatus tick occupancy on grass stems. The percentages of grass stems with 1-, 2-, 3-, 4-, 5- and 6-tick occupancy were 74.4%, 17.4%, 5.9%, 1.1%, 0.7% and and 0.4%, respectively (see also Table 1). b Percentage of grass stems occupied by the various combinations of D. reticulatus ticks: single male (m), single female (f), multiple tick occupancy–only males (mm), multiple tick occupancy–only females (ff) and multiple tick occupancy–mixed sexes (mf). The number of records at each of the 3 collection sites for m, f, mm, ff and mf assemblages was 33, 42, 3, 5 and 17, respectively, at Kury; 29, 45, 4, 3 and 19, respectively, at Siekierki; and 16, 36, 0, 7 and 11, respectively, at Urwitałt. CI₉₅, 95% Confidence interval