Lifecycle and seasonal activity patterns of I scapularis

Ticks are obligate parasites. All active stages of I. scapularis must take a blood meal in order to survive, with each stage feeding on a host animal. These ticks begin their life as eggs, which hatch into sexually immature, six-legged larvae that are about the size of a grain of sand, approximately 0.5 mm in length (Fish, 1993; see also Figure 5.3). Larval I. scapularis ticks prefer to feed on small animals like mice or birds. They feed for 3-4 days, taking in blood until they become fully engorged, then drop off the host, usually into the leaf litter. In the northeastern US, larval host-seeking activity peaks during the months of August and September (Fish, 1993; see also Figure 5.4).

Engorged larvae molt into sexually immature, eight-legged nymphs that are about the size of a poppy seed, approximately 1 mm in length (Fish, 1993). The nymphs take a blood meal, usually preferring to feed on small or medium-sized mammals such as mice, chipmunks, and raccoons. Nymphs usually remain attached to their host for 4-5 days while feeding to repletion. Nymphs are most active during the late spring and early summer in the northeastern US, although some nymphs have been collected in the field as late as October (Fish, 1993).

The engorged nymph will drop off the host and eventually molt into a sexually mature, eight-legged adult in the late summer or early fall. The adult female ticks, approximately 2.5 mm in length (Fish, 1993), feed on large mammals, usually white-tailed deer (Odocoileus virginianus) (see pp. 148 for further details on the

Nymph

Female

Male

Nymph

Male

** W fit

CM 1

illlllM!

Mil

Figure 5.3 Active stages of I. scapularis. Photograph courtesy of J. Vellozzi.

Figure 5.3 Active stages of I. scapularis. Photograph courtesy of J. Vellozzi.

JFMAMJJASOND Month

Figure 5.4 Seasonal activity of I. scapularis in Westchester County, NY, based on sampling conducted from 1990 to 2005.

JFMAMJJASOND Month

Figure 5.4 Seasonal activity of I. scapularis in Westchester County, NY, based on sampling conducted from 1990 to 2005.

role of deer in Lyme disease ecology). The smaller adult males are also found on deer, but they feed only intermittently, while females remain attached and will feed for about a week. Mating typically takes place on the host animal, where tick densities are higher than in the environment. Adult ticks in the northeastern and midwestern US have two activity peaks during the year, with a primary peak in October and November and a smaller peak in early spring (Fish, 1993; Figure 5.4). Mark-release-recapture studies demonstrate that these two activity peaks represent the same cohort of adult ticks (Daniels et al., 1989). Those adults that do not find a host in the fall will become active again in the spring, although host-seeking during the winter months can occur on warmer days if a threshold temperature is reached; this has been reported to vary between -0.6°C (Schulze etal,, 2001) and 4.0°C (Duffy and Campbell, 1994).

Engorged adult female ticks will drop off the host and lay approximately 2500 eggs per tick in the spring; these will hatch into larvae which are most active in late summer, and the cycle will begin again (Daniels et al,, 1996). The whole process takes two years in the northeastern US, although the duration of the life-cycle may vary in other parts of the geographic range for I, scapularis (Clark et al,, 1998; Lindsay et al,, 1998).

0 0

Post a comment