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Ticks: Tick control
Ticks
Tick Control
Authors: Dr Reginald De Deken, Prof Ivan Horak, Prof Maxime Madder, Dr Hein Stoltsz
Licensed under a Creative Commons Attribution license.
ACARICIDE RESISTANCE
Resistance to acaricides is the ability in a strain of ticks to tolerate doses of acaricides that would prove lethal
to the majority of individuals in a normal population of the same species.
Some species have a natural resistance to insecticides. The reasons for this might be impermeability of the
cuticle; behaviour traits - they may be repelled by the insecticide; and physiological processes make them
not susceptible to the effect of the particular insecticide.
Natural resistance is present at the outset in all individuals of the species and does not develop as a result of
acaricidal pressure. In a population of ticks there are those individuals that are more robust and are better
equipped biochemically to counter the effects of acaricides. This ability is not necessarily passed on to their
offspring.
Genetic resistance of ticks spreads by the reproduction of resistant individuals that have survived acaricidal
concentrations insufficient to ensure the death of all. The acaricide acts as a selective screening process
which concentrates resistant individuals already present in a population. The resistance is hereditary and the
original heterogenous population is selected by acaricidal pressure to become a homogenous resistant
population.
Resistance is not an adaptation that develops because of exposure to acaricides, but spreads because
resistant genotypes already present in the population are favoured. The population carries the factors for
resistance in low frequency before the acaricide was applied and resistant individuals are therefore selected
by the application of the acaricide.
Truly sublethal concentrations of an acaricide, which do not kill a single individual, cannot select for
resistance because they do not favour the genetic material available. Conversely acaricides used at a
concentration that will kill each exposed individual cannot select for resistance as no individuals will escape.
The selection of resistant strains is a function of acaricide concentrations between these two points. An
entire population of arthropods will not become resistant in one generation - this is a process that requires
several generations.
The more heterogenous a population is initially, the greater the possiblity of finding resistant individuals. The
higher the selection pressure the more rapid the spread of resistance in the population.
Acaricidal pressure is also the number of times an individual or its offspring are exposed to the acaricide
during their lifetimes or particular portions thereof. The blue tick, which spends 23 days on the host animal, is
Ticks: Tick control
thus exposed to a higher acaricide pressure with regular weekly dipping than is the brown ear tick which only
spends approximately 5 days on the host in each of its three developmental stages.
When resistance against a particular acaricide has become established in an arthropod strain it can only
persist provided homozygosity exists throughout the strain. Resistance may be conferred by a single gene or
may be polygenic and one or more chromosomes may be involved.
The rapidity with which resistance spreads through a population is dependent upon whether it is conferred by
an incompletely recessive or a dominant gene.
Acaricide resistance is a function of gene-modified enzymes. This may involve:

Altered cuticular biochemistry which retards the entry of acaricides.


More rapid enzymatic degradation of the acaricide before it reaches the target area.
Modification of cholinesterase, which renders it less sensitive to inhibition by the acaricide.
Cross-resistance to acaricides that act in the same way does occur. Thus ticks resistant to DDT may also be
resistant to the pyrethrins and the synthetic pyrethroids. There is also cross-resistance between the various
organophosphates.
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