Humid conditions are essential for survival of the paralysis tick. Dry conditions, relatively high temperatures (32 °C) and low temperatures (7 °C) are lethal for all stages after a few days. An ambient temperature of 27 °C and high relative humidity is thought to be optimal for rapid development (Clunies-Ross, 1935). Very dry or very wet conditions are not ideal.
The tick population in a given year is probably governed by the rainfall in the previous year if the temperature variations have only been moderate. North-easterly weather, with moisture laden sea breezes on the east coast of the continent provide ideal conditions. This pattern usually sets in during the Spring and early Summer. Tick poisoning in animals (especially pets) peaks in Spring to mid-Summer.
Typically, larvae appear late Feb to April/May, followed by nymphs March to September/October and then gradually the adult population August to February, peaking around December. Under favourable conditions a second cycle may result in a second peak in May. However, infestation by adults can occur at any time of the year when conditions are suitable, even in mid winter. In fact it is only in the very hot summer months that they are difficult to find.
The graphs show seasonal occurrence for one species only i.e. Ixodes holocyclus. The extent to which other species found in the area parallel I. holocyclus in this and other aspects of their biology is not known.
The graphs show the average seasonal prevalence of instars and types, observed over an eight year period, which includes six years of detailed field observation, and collection, supported by information and specimens from many people.
I. holocyclus emerged from this survey as the dominant acarine ectoparasite of mammals and avians in the study area, its population obviously dwarfing those of other tick species, and various species of mites.
As distinct from instars (the life cycle stages separated by metamorphosis), only one particular type within an instar has been graphed at this stage - gravid females. It is probable that further separation of types will become possible with additional survey and observation e.g. it appears that at certain times in spring and summer adult males greatly outnumber females, in proportions as high as five to one, These and other indications need further checking to ensure that they are not merely coincidences.
As the graphs show, small numbers of each instar are present throughout the year, with the numbers in each life cycle segment rising to plateau levels at particular times and then falling away again, These pictured results are from observations covering a period when climate in the whole South West Pacific Area was subject to a major weather anomaly. Continued observations may well result in a change in the shape of the curves. One graph line, that of the gravid female type within the adult group, has been shown as incomplete. Positive recordings of winter occurrences of gravids have varied from nil to very small numbers in two of the eight years.
The need for humid conditions largely determines the botanical niche of Australian paralysis ticks. Low, leafy vegetation provides higher humidity levels by reducing the desiccating effects of wind and direct sunlight. This environment also suits the tick's major host, the bandicoot, as it seeks out grubs and worms in the leaf litter. Certain vegetation may be conducive to paralysis ticks. See also Environmental Control of Ticks.
Paralysis ticks can and do climb trees. In fact nymphs and adults climb many meters to the very tops, but descend in windy or dry weather. Larvae appear to remain closer to ground. They reportedly will fall near and onto people sitting underneath (Conroy, 2001 pers com).
The paralysis tick Ixodes holocyclus is found mainly along coastal eastern Australia - from near Cooktown in Far North Queensland to Lakes Entrance in Victoria. In places, it is found more than 100 km inland, particularly in areas of moist escarpments and ranges. Instances of tick paralysis in dogs are being increasingly reported from inland Victoria also.
The following map gives only an approximation of the Paralysis Tick's distribution as the situation is not static. There are reports of paralysis ticks from inland Victoria, including north-eastern suburbs of Melbourne. Additionally, there appears to be a general movement of insects and ticks from equatorial to polar latitudes that may be related to global warming (Bill Conroy, pers com, 2003)
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