Life Cycle

All ticks have four life cycle stages, the embryonated egg and the three active stages, larva, nymph (one or more) and adult. Sexual dimorphism is evident only in the adult stage; thus, reference to a male or female automatically implies that the individual is an adult.

 

Life Cycle Ixodidae

The life cycle of ixodid ticks is remarkably uniform throughout the family. All ixodid ticks are oviparous and have a single nymphal stage.

In most species, each active stage seeks a host, feeds, and drops off to develop in the natural environment (Three-host life cycle). Only in a few species, fed juveniles remain and develop on the host, further shortening the life cycle. Such ticks are termed "two-host" or "one-host" ticks, depending upon whether one or more juvenile stages develop in this manner. Numerous variations in the basic tick life cycle plan occur.

Mating occurs exclusively on the host, i.e. when feeding, in all but species of the genus Ixodes. All ixodids except species of Ixodes require a blood meal to initiate the gonotrophic cycle.

Oviposition of an ixodid female
Oviposition of an ixodid female

Following mating, females feed to repletion, then drop from their hosts and following a brief preoviposition period (except in those with a morphogenetic diapause) commence oviposition. Oviposition occurs in one continuous cycle of ovipositional activity in some sheltered microenvironment, e.g., under leaf litter, rotting vegetation, or some crack or crevice in a natural or man-made shelter, where females lay several thousand eggs.

Egg production accelerates rapidly, reaching a peak within 3-5 days after commencement of oviposition, then again 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. 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.

Females die after oviposition, i.e., there is only one gonotrophic cycle.

 

Three-host life cycle

In a three-host life cycle each active stage repeats the pattern of host-seeking, feeding and off-the-host molting in the environment. It is the most common developmental pattern and characteristic for the vast majority of ixodid species.

Under favorable conditions in the natural environment, the life cycle of such three-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 behavior, development or the onset of oviposition, so that only one life stage can be completed each year. These environmental limitations can extend the duration of the life cycle to as much as 3 years (e.g., Ixodes ricinus).

Illustration of the life cycle of Ixodes ricinus as an example of a three-host tick life cycle
Life cycle of Ixodes ricinus as an example of a three-host tick life cycle. The parasitic phases, i.e., when the different life stages are on the host, are shown in the outer circles. All other phases occur in the natural environment.

Following oviposition, the eggs undergo embryogenesis in the natural environment.

After hatching the emerging unfed larvae disperse into the vegetation or nest environment to seek hosts. After finding appropriate hosts the first parasitic phase of attachment, feeding and off-the-host molting in the environment commences. Larvae that are successful attach and feed slowly; several days are required for a complete blood meal. The repleted 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 similar hosts as those fed upon by the larvae) and the process of host finding, attachment, and repletion is repeated in the second parasitic phase on a second host. The engorged nymphs drop from their hosts and shelter in appropriate niches in the natural environment.

Following the nymphal molt, adult (female and male) emerge and initiate the third parasitic phase, attach to hosts, feed, mate, and the fully engorged females drop to oviposit in some sheltered microenvironment, and dies, completing the life cycle. Except for Idodes spp. Where mating happens in the environment males usually remain to reattach, feed again, and mate with other females.

Larvae, nymphs, or repleted detached females may enter diapause and remain intact for several months (e.g., over winter or, rarely, over summer) until appropriate environmental changes allows them to commence the cycle.

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 of 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

SEM picture of mating Ixodes scapularis
Ixodes scapularis mating

Two- and one-host life cycles

Several variations of this general ixodid developmental pattern exist.

A two-host life cycle is found in a few species of metastriate ixodid ticks, e.g., Hyalomma anatolicum excavatum: Fed larvae remain on the host, molt (ecdyse) in situ, and the unfed nymphs reattach. Development is often rapid, since the fed larvae are in a warm, humid and optimum microenvironment, against the host skin Only following their engorgement, the nymphs do detach. Then, they molt off the host to the adult stage.

The more extreme modification of a one-host life cycle occurs e.g., 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.

Life Cycle Argasidae

Argasid ticks exhibit remarkable diversity in their patterns of development and feeding behavior, in contrast to the much more uniform pattern found in the Ixodidae.

Almost all argasid ticks have a multi-host feeding pattern (Hoogstraal and Aeschlimann, 1982) with multiple parasitic phases and repeated gonotrophic cycles. Although there are some noteworthy exceptions, e.g., Ornithodoros lahorensis, which are two-host ticks or Otobius megnini and O. lagophilus, which are one-host ticks.

Development is gradual, with multiple nymphal stages prior to the adult form. This pattern is similar to that of other Acari, in which the sequence of developmental stages includes a larva, four nymphal stages and the adult.

Following embryogenesis and hatching the larvae attack vertebrate, hosts inhabiting the nest, burrow, or other niche environment. Following host contact, larvae feed rapidly, detach drop off and ecdyse, molting into the first nymphal stage.

In some argasid species, larvae do not feed but molt directly to the first nymphal stage. In others, larvae feed slowly and molt twice (to the second nymphal stage and the first nymph is a non-feeding stage).

The nymphal stages resemble miniature adults in body characteristics, especially the leathery, mammillated body cuticle, but lacking the genital pore and any evidence of sexual dimorphism.

First nymphal stages again attack hosts that enter the niche, repeating the cycle. This cycle of host contact, rapid feeding (15-30 minutes), engorgement, detachment and ecdysis in the niche can occur several times. The precise number of nymphal stages is indeterminate and as many as 8 nymphal instars have been recorded in some species (Hoogstraal,1985). An important factor enabling the nymphs to feed rapidly is their ability to eliminate excess blood meal water in the form of coxal fluid, a clear, colorless liquid excreted from the coxal glands during or shortly after feeding. The fed nymphs molt again to yet another nymphal stage, and the cycle of host seeking, feeding and molting is repeated

Following emergence, adults may first mate or attack hosts. However, in the Argasidae, adults become sexually active after emergence from the last nymphal molt, and they do not require a blood meal to initiate gametogenesis. Mating occurs before, as well as after, blood feeding, but rarely if ever on the host itself. However, the gonotrophic cycle is completely dependent upon the blood meal (except in those rare instances in which mated females oviposit autogenously, i.e., without feeding).

Adults seek hosts, feed rapidly and engorge several times. Fed mated females oviposit small egg clutches (< 500 eggs/cycle) after each blood meal, whereupon they return to attack hosts and feed again, i.e., there are multiple gonotrophic cycles.

Nonfeeding males also occur in the life cycles of some species, similar to the pattern observed in some species of Ixodes.

The number of gonotrophic cycles varies considerably between individuals within the species, as well as between species, although it rarely exceeds 6. This pattern of repeated gonotrophic cycles, often following long periods of waiting between blood meals, enables argasid ticks to disperse their progeny gradually over time, often across a span of many years.

The argasid strategy is very different from that of the ixodids, where all of the progeny of a fecund female is produced in one massive surge of ovipositional activity.

Diapause is a major factor regulating the time of development of many of the argasid species, which must survive in empty burrows or nests for periods of many months until their hosts return or new hosts arrive.

The time to completion of the entire life cycle is generally much longer than that of hard ticks, lasting over several years. Additionally, many soft ticks have an uncanny resistance to starvation, and can survive for many years without a blood meal.

Egg

After mating and the completion of the blood meal, female ticks seek sheltered environments, similar to those used by the immatures for molting, to lay their eggs.

The period between engorgement and the commencement of oviposition is the pre-oviposition period; its length varies, depending upon the species, temperature, diapause and, possibly, other factors.

Maximum egg yields normally are produced only by females that have mated and engorged to repletion.

The number of eggs produced by mated, blood-fed ticks is dependent upon many factors, the most important of which are the blood meal volume, the egg size and the species.

Blood meal volume is the major determinant affecting the number of eggs deposited. In general, females consuming larger blood meals lay more eggs.

In the Argasidae, small egg batches are deposited after each blood meal. These ticks are capable of feeding and ovipositing many times, i.e., they have multiple gonotrophic cycles. In Argas persicus, batches contained from 47 to 646 eggs.

In the Ixodidae, the mated females lay thousands of eggs in a single gonotrophic cycle. Ixodid ticks exhibit phenomenal fecundity, perhaps the highest of any hematophagous arthropod. In Dermacentor variabilis, a typical example, oviposition begins within 3-6 days after females feeding. Egg production surges rapidly, reaching a peak on the 4th day, and continuing for an additional 16-17 days, for a total of 20-21 day egg laying period. The American Dog tick lays an average of 5.380 eggs/female (Sonenshine and Tignor, 1969).

The largest egg mass ever recorded for a single tick was 22.891, from a single Amblyomma nuttalli female.

Larva

The first immature stage, the larvae (which are often called seed ticks), only have six legs. Unfed larvae hatch from an egg batch of some hundreds (Argasidae, soft ticks) to several thousands (Ixodidae, hard ticks).

Because of the difficulty of finding a suitable host, larvae can withstand long periods without feeding. Following host contact, larvae attach, insert their mouthparts into the host skin and feed. Engorgement is completed within several days, depending upon the species and host. In some argasid species, larvae do not feed.

The engorged larva molts into an unfed eight-legged nymph.

Nymph

Nymphs resemble the adult tick in that they have eight legs. They do not, however, have a genital opening. Nymphs must be able to live without feeding for long periods of time until it finds a suitable host.

Hard ticks have only one nymphal instar while soft ticks may have several.

Soft ticks often have many nymphal molts in the life cycle and the number of nymphal stages is not consistent, even within the same species. Nutritional factors, especially blood volume taken in previous stages, is believed to be an important indicator of the number of nymphal stages. Moreover, males usually emerge sooner than females, i.e., males require 1 or 2 fewer nymphal stages than do females.

In the Argasidae, the passage of so many nymphal stages contributes to a much longer life cycle than in the Ixodidae. In addition, many argasid ticks can resist long periods of starvation during their development, so that the life cycle can be extended for many years.

Adult

Following the nymphal molt, ixodid adults emerge to attack hosts, feed, mate, and the fed females drop to oviposit in some sheltered microenvironment, completing the life cycle.

Male hard ticks usually die soon after mating, and females die soon after laying their eggs.

Following emergence, argasid adults may first mate in the niche or attack hosts. Following host contact, adults feed rapidly, engorge and drop off. Mated females oviposit small egg clutches, whereupon they return to attack hosts and feed again. Adults seek hosts, feed and engorge several times and fed mated females oviposit after each blood meal. Adult soft ticks are generally longer-lived, and egg-laying is a periodic activity of the female.

References

Life Cycle Ixodidae

Sonenshine DE: Biology of Ticks. Part 1, 1991, Oxford University Press, New York 

 

Life Cycle Argasidae

Hoogstraal H: Argasids and nuttalliellid ticks as parasites and vectors. Adv Parasitol. 1985, 24, 135-238 

Sonenshine DE: Biology of Ticks. Part 1, 1991, Oxford University Press, New York 

 

Egg

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 

Further Reading

Hoogstraal H:Argasids and nuttalliellid ticks as parasites and vectors. Adv Parasitol. 1985, 24, 135-238

Sonenshine DE: Biology of Ticks. Part 1, 1991, Oxford University Press, New York

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