Matthew Cahill

Baseline determination and detection of resistance to imidacloprid in Bemisia tabaci (Homoptera: Aleyrodidae)

Adult Bemisia tabaci (Gennadius) were tested with imidacloprid in a bioassay method using systemically treated cotton leaves. The method was simple, robust and repeatable and provided baseline data for a laboratory and a number of recently collected field strains. The LC 50 for imidacloprid susceptible strains was calculated to be 1.7 ppm and a concentration of 16 ppm determined as diagnostic for imidacloprid resistance. Ten strains of B. tabaci collected from the Almeria region of Spain showed significantly less mortality at the diagnostic dose than the susceptible strains. The intensive use of imidacloprid in the Almeria region is considered the reason for the occurrence of resistance in this locality. Resistance to organophosphates, pyrethroids and endosulfan in B-type and non-B type B. tabaci did not confer resistance to imidacloprid.

Pyrethroid and organophosphate resistance in the tobacco whitefly Bemisia tabaci (Homoptera: Aleyrodidae)

Eleven strains of Bemisia tabaci (Gennadius), including a laboratory susceptible strain, were bioassayed as adults with three organophosphorus (OP) insecticides, three pyrethroids and one OP/pyrethroid combination. The contemporary strains were from diverse geographical areas and hosts and included examples of the A-, B-, and non-B-biotypes. All recent collections were multi-resistant to these insecticides which have been used extensively for their control. The patterns of cross-resistance for the OPs were clear but less so for the pyrethroids. All populations that resisted profenofos and cypermethrin also resisted the combination of profenofos and cypermethrin. Although the importance of selection pressure on levels of resistance was not easily quantifiable the highly selected populations exhibited the highest levels of resistance. The significant within, as well as between, biotype variation in resistance factors clearly indicated that insecticide resistance and biotype were not directly related. The roles of acetylcholinesterase sensitivity and general esterase activity in resistance to OPs and pyrethroids are discussed.

Resolution of baseline responses and documentation of resistance to buprofezin in Bemisia tabaci (Homoptera: Aleyrodidae).

The insect growth regulator buprofezin was tested using a leaf-dip assay on immatures of Bemisia tabaci (Gennadius). The method was robust and repeatable, and baseline data were calculated for a laboratory strain and a number of field collections from Pakistan where the insecticide has not been used. A baseline LC 50 of 0.53 ppm buprofezin was defined and compared to other field collections. Populations known or suspected to have been intensively selected with buprofezin showed substantial tolerance to the insecticide at both the LC 50 and at specific diagnostic doses, with the highest resistance factor of 47 fold in a population from the Netherlands. Reasons for the variation between populations, biotypes and geographical areas are discussed and the dangers of human-assisted movement of resistance genes are highlighted.

The role of B-type esterases in conferring insecticide resistance in the tobacco whitefly, Bemisia tabaci (Genn)

Separation of non-specific esterases on electrophoretic gels has played a key role in distinguishing between races or biotypes of the tobacco whitefly, Bemisia tabaci. One intensively staining esterase in particular (termed E0.14) has assumed significance as a diagnostic of B-type whiteflies (aka Bemisia argentifolii), despite any knowledge of its biological function. In this study, a whitefly strain (B-Null) homozygous for a null allele at the E0.14 locus that had been isolated from a B-type population was used to demonstrate a significant role for E0.14 in resistance of B-type populations to pyrethroids but not to organophosphates (OPs). Bioassays with pyrethroids, following pre-treatment with sub-lethal doses of the OP profenofos (to inhibit esterase activity), coupled with metabolism studies with radiolabelled permethrin, supported the conclusion that pyrethroid resistance in a range of B-type strains expressing E0.14 was primarily due to increased ester hydrolysis. In the same strains, OP resistance appeared to be predominantly conferred by a modification to the target-site enzyme acetylcholinesterase. © 2000 Society of Chemical Industry

Biochemical identification of interbreeding between B-type and non B-type strains of the tobacco whiteflyBemisia tabaci

Polyacrylamide gel electrophoresis and a kinetic microplate assay were used to detect heterozygotes resulting from a cross between B-type and non B-type strains of the whiteflyBemisia tabaci. Both strains were homozygous for different esterase and acetylcholinesterase (AChE) enzymes, and heterozygotes were produced in one of two crosses between B-type females (diploid) and non B-type males (haploid). In reciprocal crosses, however, no female offspring were produced, indicating that fertilization had not occurred. Despite the identification of individuals heterozygous for the esterase and AChE markers, there was clearly a significant degree of reproductive incompatibility between the two strains. The biochemical assays provided a vital component of this study and the advantages of their use are discussed.

A New Group of Plant‐Derived Naphthoquinone Pesticides

A series of compounds with structures based on insecticidal/acaricidal naphthoquinones isolated from Calceolaria andina has been synthesised. A feature of the series is the lack of resistance shown by strains resistant to established classes of pesticides. The importance for activity of the tetra-substituted carbon atom in the side-chain, as observed in the natural products, has been investigated. In analogues with acyclic side-chains the position of the tetra-substituted carbon for optimum activity was dependent on the length of the side-chain. With cyclic side-chains, activity was dependent on the size of the ring, the number and position of the substituents therein. Activity of the compounds examined was particularly high against Bemisia tabaci and Tetranychus urticae in direct-contact tests, but was much lower than expected in leaf-dip tests. A partial improvement based on formulation has been demonstrated.

Managing Resistance to the Chloronicotinyl Insecticides — Rhetoric or Reality?

When the launch of a novel, safe, and highly efficacious insecticide such as imidacloprid (Elbert et al. 1990) provides agricultural producers with a much needed new tool for crop protection, and also heralds a whole new class with the potential to rival the pyrethroids, organophosphates, carbamates and cyclodienes, the optimism and opportunities are vast. For each of these classes, however, the reality has followed a familiar sequence of discovery, development, launch, marketing, deployment, gradual loss of field performance as a result of resistance brought on in most cases by indiscriminate overuse, and finally the urgent need for a new, safe, and efficacious insecticide.