A 25-year study has been made of the development and loss of resistance to chemicals and, in particular, of the inheritance of resistance in the cattle tick, Boophilus microplus. Various biological effects of pesticidal and other chemicals on ticks have also been investigated. This research is described in 26 papers. Seven supporting publications are appended and a list of six other publications included. These 13 publications deal with other aspects of tick toxicology and with toxic or other oral secretions of ticks.
The first 19 papers represent three phases of research into resistance in B. microplus. In the first phase, methodology was developed which permitted detection and quantification of resistance to the organochlorine compounds. Resistance to toxaphene (Beaudesert strain), dieldrin (Mt. Gravatt strain) and DDT (Rockhampton strain) was detected. No cross resistance to diazinon occurred in the Mt. Gravatt or Rockhampton strains or to dieldrin or toxaphene in the Rockhampton strain. A DDT-resistant strain from Glen Geddes subsequently developed resistance to dieldrin but no cross resistance to coumaphos. However, DDT-resistance could not be detected among 14 strains of B. microplus from quarantine areas of New South Wales at the conclusion of the unsuccessful 1956-7 eradication campaign in which DDT was employed. Parthenogenetic reproduction was demonstrated for the first time in B. microplus by a form of thelytoky which was associated with extended life cycle. A new, simple bioassay method for detecting resistance in ticks, the "packet" method, was developed and later proved to be much more sensitive and convenient than previous methods. It permitted more reliable documentation of resistance and made possible subsequent genetic studies. The method was subsequently adopted by FAO as its standard method for international surveillance of resistance in ticks.
In the second phase, the genetics of resistance to the organochlorine compounds, DDT and dieldrin, was investigated. DDT resistance was due to a single, incompletely recessive autosomal gene; resistance was associated with delayed moulting of nymphs and delayed detachment of engorged females. Dieldrin resistance was due to a single dominant autosomal gene. In dieldrin-free and DDT-free laboratory cultures, the proportion of dieldrin-resistant ticks in one strain declined to an undetectable level after 24 generations while the proportion of DDT-resistant ticks in another strain declined to 55% homozygotes after 13 generations. However, in the DDT-resistant strain, an equilibrium was established, there being no further change for at least another 10 generations, a possible example of "balanced polymorphism".
In the third and most important phase of the study, the inheritance of resistance to the organophosphorus compounds was investigated in some depth. As an aid to this, the assessment of dominance of resistance was placed on a more rational quantitative basis by the development of a formula for the calculation of degree of dominance. Resistance to formothion in the Ridgelands (R) strain, to diazinon and fenthion in the Biarra (B) strain and to dimethoate in the Mackay (M) strain was shown to be due to a single, semidominant autosomal gene in each strain. Dimethoate resistance occurred in the three strains and was almost certainly controlled by a series of closely-linked genes or alleles. Resistance in each strain was shown to be associated with decreased levels of acetylcholinesterase (AChE) activity in the synganglion and both inherited characters were almost certainly pleiotropic effects of the same series of multiple alleles. The biochemical resistance mechanism, decreased AChE sensitivity to inhibitors, is controlled by a single, incompletely dominant autosomal gene. No overdominance of resistance could be demonstrated when strains R, B and M were crossed in all possible ways. The field importance of this finding is discussed. Resistance to chlorpyrifos in the Mt. Afford (A) strain and to diazinon in the Gracemere (G) strain is incompletely dominant and is controlled by two unlinked genes. One of these is almost certainly responsible for the B strain type resistance mechanism of decreased AChE sensitivity and the other for increased detoxication. The genes are complementary in strain A and probably so in strain G.
A fourth area of research was related to more general aspects of pesticidal effects on arthropods. It -was shown that (a) fenthion resistance in larvae of the tropical house mosquito Culex pipiens fatigans was due to decreased absorption coupled with increased detoxification to water soluble metabolites; (b) using a simple laboratory method of evaluation, the N-monomethyl formamidines were the most effective of a group of 19 formamidine and related non-formamidine compounds in causing detachment of B. microplus adults from mice. Monoamine oxidase inhibitors also caused detachment possibly through action on monoaminergic neurones in B. microplus; (c) monoaminergic neurones were present in the synganglion of B. microplus; noradrenaline was identified by microspectrofluorometry as the principal monoamine in an isolated neurone. This was the first record of the presence of noradrenaline in an individual neurone of a invertebrate; (d) aggregation pheromones occur in two Australian ticks, the Australian paralysis tick Ixodes holocyclus and the echidna tick Aponomma concolor.