Life Cycles Ixodidae
In the Ixodidae, development is accelerated, with a single nymphal stage that matures directly into the adult, i.e., the life cycle is truncated. In a few species, fed juveniles remain and develop on the host, shortening the life cycle even further. Such ticks are termed "2-host" or "1-host" ticks, depending upon whether one or more juvenile stages develop in this manner. Following mating, females imbibe enormous blood meals, lay thousands of eggs and die, i.e., there is only one gonotrophic cycle. All species are oviparous. Numerous variations in the basic tick life cycle plan occur.
The life cycle of ixodid ticks is remarkably uniform throughout the family. All ixodid ticks have a single nymphal stage. Females swell greatly during feeding, occasionally in excess of 100 times their unfed body weight. Following mating, which occurs on the host in all but species of the genus Ixodes, females feed to repletion, drop from their hosts and commence oviposition in some sheltered microenvironment, e.g., under leaf litter, rotting vegetation, or some crack or crevice in a natural or man-made shelter. Adults of all ixodids except species of Ixodes require a blood meal to initiate the gonotrophic cycle.
Metastriate ticks mate exclusively on the host, i.e., while feeding. Following a brief preoviposition period (except in those females that have a morphogenetic diapause), the females lay several thousand eggs in one continuous cycle of ovipositional activity. The American Dog tick, Dermacentor variabilis, lays an average of 5,380 eggs/female (Sonenshine and Tignor, 1969).
Egg production accelerates rapidly, reaching a peak within 3-5 days after commencement of oviposition, then declines gradually; 90 % of the egg mass is deposited within the first 10 days, but small numbers continue to be passed for an additional 5-10 days, whereupon the exhausted female dies.
Although proportions vary among different species, more than 50% of the engorged, mated female body weight is converted to eggs during this process.
Clearly, ixodid ticks are among the most prolific of all arthropods with regard to their ovipositional capacity.
Three-host Life Cycle
The life cycle of a typical ixodid tick may be appreciated by examining the following flow diagram:
The parasitic phases, i.e., when the different life stages are on the host, are shown in the exploded circles. All other phases occur in the natural environment. The different events in the life cycle are indicated by the key words followed by a number in parentheses.
Egg (1), undergoing embryogenesis (Emb) in the natural environment. Hatching (H) (2) follows embryogenesis, yielding unfed larvae. Questing behavior (= host seeking behavior) (Q, item 3) by hatched larvae. The larvae climb the vegetation or other surfaces in their natural environment and commence questing behavior. Alternatively, larvae may enter diapause (3 A) (specifically, behavioral diapause) (D) and over winter (or, rarely, over the summer) until appropriate environmental changes terminate that state. Post diapause larvae commence questing behavior. Host contact (Hc) (4). Some larvae cling to passing hosts. This is the beginning of the first parasitic phase on the first host (I). Attachment (A, item 5). Ticks commence exploratory behavior and, if on a suitable host, they commence attachment behavior and insert their mouthparts into the host skin. Feeding (F, item 6) follows successful attachment. Engorgement (Eng, item 7) is completed within several days, depending upon the species and host. Detachment (Dt, item 8) of the engorged larva follows. Drop off (9). The detached larva drops off and undergoes ecdysis (DO/E) in the natural environment, whereupon the engorged larva (E/L) molts into an unfed nymph (U/N). Nymphs (10) commence questing and the entire cycle of host contact, attachment, feeding, engorgement, and detachment (11-15) is repeated on the second host (II). Detached engorged nymphs drop off (16), ecdyse in the natural environment, and the engorged nymphs (E/N) molt into unfed adults, males and females (M and F). The emerging adults harden (M, F) climb to a suitable surface (e.g., tips of vegetation) and commence questing (17). The cycle is repeated when the ticks contact their third host (III). Host contact (18) is followed by attachment and feeding (19, 20). Further development, however, is dependent upon mating (M, item 21), which takes place on the host in all metastriate ixodid ticks. Mated females (22) engorge rapidly to repletion, the fed females detach (FDt, item 23), and drop off (DO, item 24). Males usually remain to reattach, feed again, and mate with other females. Replete females (R/F) that have detached seek suitable sheltered sites in the natural habitat. Following a brief period of preovipositional (PrO) development (25), they commence oviposition (OV, item 26). Alternatively, in some species, replete detached females may enter ovipositional diapause (OvD, item 25 A) (= development diapause) and remain inactive for several months (e.g., overwinter) before they commence oviposition. The oviposition female (O/F) lays numerous eggs and dies (Dth, item 27).
Following oviposition, hatching begins and the emerging larvae disperse into the vegetation or nest environment to seek hosts. Those that are successful attack and feed slowly; several days are required for a complete blood meal. In most species, the replete larvae drop from their hosts, find a sheltered microenvironment and undergo ecdysis (= molting). After emerging from the larval molt, the unfed nymphs seek hosts again (often the same hosts as those fed upon by the larvae) and the process of host finding, attachment, and repletion is repeated. The engorged nymphs drop from their hosts and shelter in appropriate niches in the natural environment. Following the nymphal molt, the adults emerge to attack hosts, feed, mate, and the fed females drop to oviposit in some sheltered microenvironment, completing the life cycle. This pattern of host-seeking, feeding and off-the-host molting in each life stage constitute the 3-host life cycle. This is the most common developmental pattern and is characteristic of the vast majority of the species. It is the least highly evolved of the various life cycle patterns. Under favorable conditions in the natural environment, the life cycle of such 3-host ticks, from the time of hatching of the larvae to the hatching of the next generation, can be completed in less than 1 year
However, climatic conditions and diapause may delay host seeking behaviour, development or the onset of oviposition, so that only one life stage can be completed each year. These environmental limitations tend to extend the duration of the life cycle to as much as 3 years (e.g., Ixodes ricinus).
Two- and One-host Life Cycles
Several variations of this general ixodid developmental pattern exist. In some species, e.g., Hyalomma anatolicum excavatum, fed larvae remain on the host, molt in situ, and the unfed nymphs reattach. Only following their engorgement do the nymphs detach. Then, they molt off the host to the adult stage. These ticks are known as 2-host ticks.
Symbols and abbreviations as in Fig. 1. The parasitic phases are also the same as in the 3-host cycle illustrated in Fig. 1. However, following completion of embryogenesis, Emb (1), hatching, H (2), questing, Q (3) and the first cycle of host contact, Hc (4), attachment, A (5), feeding, F (6, 10) and engorgement (7) accomplished on the first host (I), the replete larvae, shown in the oval labeled E/L, do not detach. Rather, the fed larvae remain attached and ecdyse, E (8) in situ, molting to unfed nymphs, shown in the second oval labeled U/N. Development is rapid, since the fed larvae are in a warm, humid and optimum microenvironment, against the host skin. Reattachment – The emerging nymphs reattach, Re (9) to the same host, feed again (10), and engorge (11). The engorged nymphs shown in the upper oval labeled E/N now detach (12), drop off and ecdyse in the natural environment (DO/§, 13) and molt into adults. Males and females (labeled M and F). The life cycle continues as the adults quest (Q) for hosts (14). Male and female (M, F) ticks that contact hosts (15) will attach (16) to these new hosts, which represent the second host (II) or second parasitic phase in the life cycle. The remaining events, i.e., feeding, F (17), mating, M (18), female engorgement, Eng (19), detaching of the fed females, Fdt (20), drop off, DO (21), preoviposition, PrO (22) and oviposition by the fed females, O/F (23), are the same as in the 3-host life cycle. Following oviposition, the spent female dies, Dth (24).
A more extreme modification occurs in the winter tick Dermacentor albipictus or the cattle tick Boophilus microplus and other Boophilus species.
In these species, all stages remain on the host after the larvae attach. Larvae and nymphs feed and remain in situ. Following molting to the adult stage, the males and females remain to feed and mate, and only the fed, mated females drop to oviposit in the natural environment. These ticks are termed 1-host ticks.
A very different pattern of development and mating behavior occurs in the ixodid genus Ixodes, the only genus in the Prostriata. In these ticks, gametogenesis begins with the nymphal to adult molt, and the young, unfed adults are sexually active soon after molting. Mating can and often does occur before feeding, as well as on the host.
However, in many nest or barrow inhabiting species (nidicoles) in this genus, the males are not found on the hosts. In these species, the males have hypostomes with only vestigial denticles and are incapable of feeding. A noteworthy exception occurs in the subgenus
- Sonenshine DE: Biology of Ticks. Part 1, 1991, Oxford University Press, New York
- Sonenshine DE, Tignor JA: Oviposition and hatching in 2 species of ticks in relation to saturation deficit (Acarina: Ixodidae). Ann Entomol Soc Am. 1969, 62, 628-40