Modeling Effects of Vertebrate Host Exclosures and Host-Targeted Acaricides on Lone Star Tick (Amblyomma americanum, L.) Infestations

Abstract

We used a spatially explicit model to simulate the potential effects of exclosures and acaricides targeted at medium-sized mammalian hosts on the local distribution and abundance of lone star ticks (Amblyomma americanum) within forestlands of the southeastern United States. Both exclosures and acaricides were successful in markedly reducing the densities of all off-host tick life stages inside the treatment areas. Densities dropped to almost zero immediately inside the edges of the exclosures, with noticeably depressed densities extending outward 30 to 60 m from the exclosures, and the simulated exclosures maintained their effectiveness as their sizes were decreased from 4.5 to 2.25 to 0.8 ha. Densities exhibited a smooth gradient across the edges of the acaricide-treated areas, with depressed densities extending ≈100 m outward from the edges, but with perceptible densities extending ≈60 m inward from the edges; thus, the simulated acaricide areas lost their effectiveness as size was decreased to slightly less than one-half the diameter of the activity range of the targeted host. Our simulation results indicated that off-host nymph densities responded to reductions of medium-sized host densities. These results suggest that targeting acaricides at medium-sized hosts may be an effective, and currently under-utilized, method for tick suppression.

Keywords: disease vectors; simulation models; spatial-temporal dynamics; tick control; tick population dynamics.

Figure 1

Conceptualization of the model used in this study. Yellow boxes represent life stages of the lone star tick [36], blue ovals represent alternative hosts, and yellow lines indicate alternative pathways that can be used to obtain the three blood meals needed to progress through the life stages Adapted from Wang et al. (2012) [33].

Figure 2

Conceptualization of the effects of the physical exclusion of medium-sized hosts and the application of acaricides to medium-sized hosts on the various blood meal pathways illustrated in Figure 1. Ticks within the treatment area (orange area) can obtain blood meals only from small- and large-sized hosts (indicated by orange lines), whereas ticks outside the treatment area (yellow baseline area) can obtain blood meals from host of all three sizes (indicated by yellow lines).

Figure 3

Design of an approximately 0.8 ha (9 cells), 2.25 ha (25 cells), or 4.5 ha (49 cells) treatment area (orange) within the baseline area (yellow). Numbers present the distance (number of cell) from the central grid cell.

Figure 4

Densities of off-host ticks (larvae, nymphs, and adults; individuals/ha) at the indicated distances from the center of 4.5, 2.25, and 0.8 ha exclosures that prevented entry of medium-sized hosts. Bars represent means and standard errors of late-summer/early-fall peak densities (≈week 35 for adults and ≈week 40 for larvae and nymphs) during the 4th (last) year of simulated time.

Figure 5

Densities of off-host ticks (larvae, nymphs, and adults; individuals/ha) at the indicated distances from the center of 4.5, 2.25, and 0.8 ha treatment areas within which acaricides were applied to medium-sized hosts. Bars represent means and standard errors of late-summer/early-fall peak densities (≈week 35 for adults and ≈week 40 for larvae and nymphs) during the 4th (last) year of simulated time.