Research to realisation: the challenging path for novel pest management products in Australia

Bisexual Attract-and-Kill: A Novel Component of Resistance Management for Transgenic Cotton in Australia

In Australia, destruction of overwintering pupae of Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) (Lepidoptera: Noctuidae) has been a key component of mandatory resistance management schemes to constrain development of resistance to Bt toxins in transgenic cotton. This has been accomplished by tillage ('pupae busting'), but it is expensive and can interfere with farming operations. Bisexual attract-and-kill technology based on plant volatile formulations offers a potential alternative in some circumstances. We discuss strategies for using such products and describe two trials in which three applications of an attract-and-kill formulation substantially reduced the numbers of Helicoverpa spp. moths and the numbers of potentially overwintering eggs they laid. One trial tested a curative strategy in which the last generation of moths emerging from transgenic cotton was targeted. The other tested a preventive strategy which aimed to reduce the numbers of eggs in the last generation. The preventive strategy reduced egg numbers by about 90% and is now included as an optional alternative to pupae busting in resistance management strategies for Australian cotton. It is limited to fields which have not been defoliated prior to 31 March and was developed to be used primarily in southern New South Wales. In the 2020-2021 cotton season, it was adopted on approximately 60% of the eligible cotton area. We describe the process whereby the strategy was developed in collaboration with the transgenic technology provider, supported by the cotton industry, and approved by the regulatory authority.

Development of a chimeric odour blend for attracting gravid malaria vectors

Background

Odour-based tools targeting gravid malaria vectors may complement existing intervention strategies. Anopheles arabiensis are attracted to, and stimulated to oviposit by, natural and synthetic odours of wild and domesticated grasses associated with mosquito breeding sites. While such synthetic odour lures may be used for vector control, these may have limited efficacy when placed in direct competition with the natural source. In this study, workflows developed for plant-feeding pests was used to design and evaluate a chimeric odour blend based on shared attractive compounds found in domesticated grass odours.

Methods

Variants of a synthetic odour blend, composed of shared bioactive compounds previously identified in domesticated grasses, was evaluated sequentially in a two-choice olfactometer to identify a ratio-optimized attractive blend for malaria vectors. During this process, blends with ratios that were significantly more attractive than the previously identified synthetic rice blend were compared to determine which was most attractive in the two-choice olfactometer. To determine whether all volatile components of the most attractive blend were necessary for maximal attraction, subtractive assays were then conducted, in which individual components were removed for the most attractive blend, to define the final composition of the chimeric blend. Binary logistic regression models were used to determine significance in all two-choice assays. The chimeric blend was then assessed under field conditions in malaria endemic villages in Ethiopia, to assess the effect of dose, trap type, and placement relative to ground level. Field data were analyzed both descriptively and using a Welch-corrected t-test.

Results

A ratio-optimized chimeric blend was identified that significantly attracted gravid An. arabiensis under laboratory conditions. In the field, trap captures of An. arabiensis and Anopheles pharoensis were dependent on the presence of the lure, trap type (CDC, BG Sentinel and Suna traps), placement relevant to ground level, with low release rates generally luring more mosquitoes.

Conclusions

The workflow designed for the development of chimeric lures provides an innovative strategy to target odour-mediated behaviours. The chimeric lure identified here can be used in existing trapping systems, and be customized to increase sustainability, in line with goals of the Global Vector Control Response Group.

Advances in Attract-and-Kill for Agricultural Pests: Beyond Pheromones

Attract-and-kill has considerable potential as a tactic in integrated management of pests of agricultural crops, but the use of sex pheromones as attractants is limited by male multiple mating and immigration of mated females into treated areas. Attractants for both sexes, and particularly females, would minimize these difficulties. Volatile compounds derived from plants or fermentation of plant products can attract females and have been used in traps for monitoring and control, and in sprayable attract-and-kill formulations or bait stations. Recent advances in fundamental understanding of insect responses to plant volatiles should contribute to the development of products that can help manage a wide range of pests with few impacts on nontarget organisms, but theory must be tempered with pragmatism in the selection of volatiles and toxicants and in defining their roles in formulations. Market requirements and regulatory factors must be considered in parallel with scientific constraints if successful products are to be developed.

Non-Target Impacts of an Attract-and-Kill Formulation Based on Plant Volatiles: Responses of some Generalist Predators

Responses of non-target insects to a blend of plant volatiles used as components in an attract-and-kill formulation for Helicoverpa spp. (Lepidoptera: Noctuidae) were studied in an Australian cotton field. Two experiments, one involving suction sampling during the day and the other at night, were conducted. Rows that had been treated with the volatile blend, with no added insecticide, were sampled with a large suction sampler 18, 42, and 85 h (day experiment) and 6, 30, and 78 h (night experiment) after treatment. Rows located 5, 10, 20, and 300 m away from the treated row were similarly sampled. Of seven generalist predators, only one accumulated on the treated rows compared to the untreated rows. Of the other six, five were found in lower numbers on the treated rows, and for one no significant effects were detected. Compared to pre-spray baseline levels, numbers of several taxa increased across the whole field after spraying, suggesting area-wide attraction, but localized responses to the treated rows were weak, and apparent repellence was more common than attraction. We suggest that attract-and-kill with plant volatiles should have minimal effects on populations of these predators, and is likely to be compatible with integrated pest management.

Developing Bisexual Attract-and-Kill for Polyphagous Insects: Ecological Rationale versus Pragmatics

We discuss the principles of bisexual attract-and-kill, in which females as well as males are targeted with an attractant, such as a blend of plant volatiles, combined with a toxicant. While the advantages of this strategy have been apparent for over a century, there are few products available to farmers for inclusion in integrated pest management schemes. We describe the development, registration, and commercialization of one such product, Magnet(®), which was targeted against Helicoverpa armigera and H. punctigera in Australian cotton. We advocate an empirical rather than theoretical approach to selecting and blending plant volatiles for such products, and emphasise the importance of field studies on ecologically realistic scales of time and space. The properties required of insecticide partners also are discussed. We describe the studies that were necessary to provide data for registration of the Magnet(®) product. These included evidence of efficacy, including local and area-wide impacts on the target pest, non-target impacts, and safety for consumers and applicators. In the decade required for commercial development, the target market for Magnet(®) has been greatly reduced by the widespread adoption of transgenic insect-resistant cotton in Australia. We discuss potential applications in resistance management for transgenic cotton, and for other pests in cotton and other crops.

Proof of concept for a novel insecticide bioassay based on sugar feeding by adult Aedes aegypti (Stegomyia aegypti)

Aedes aegypti L. (Stegomyia aegypti) (Diptera: Culicidae) is the principal vector of dengue and yellow fever viruses in tropical and subtropical regions of the world. Disease management is largely based on mosquito control achieved by insecticides applied to interior resting surfaces and through space sprays. Population monitoring to detect insecticide resistance is a significant component of integrated disease management programmes. We developed a bioassay method for assessing insecticide susceptibility based on the feeding activity of mosquitoes on plant sugars. Our prototype sugar-insecticide feeding bioassay system was composed of inexpensive, disposable components, contained minimal volumes of insecticide, and was compact and highly transportable. Individual mosquitoes were assayed in a plastic cup that contained a sucrose-permethrin solution. Trypan blue dye was added to create a visual marker in the mosquito's abdomen for ingested sucrose-permethrin solution. Blue faecal spots provided further evidence of solution ingestion. With the sugar-insecticide feeding bioassay, the permethrin susceptibility of Ae. aegypti females from two field-collected strains was characterized by probit analysis of dosage-response data. The field strains were also tested by forced contact of females with permethrin residues on filter paper. Dosage-response patterns were similar, indicating that the sugar-insecticide feeding bioassay had appropriately characterized the permethrin susceptibility of the two strains.

Adaptive management of pest resistance by Helicoverpa species (Noctuidae) in Australia to the Cry2Ab Bt toxin in Bollgard II® cotton

In Australia, monitoring Helicoverpa species for resistance to the Cry2Ab toxin in second generation Bacillus thuringiensis (Bt) cotton has precisely fulfilled its intended function: to warn of increases in resistance frequencies that may lead to field failures of the technology. Prior to the widespread adoption of two-gene Bt cotton, the frequency of Cry2Ab resistance alleles was at least 0.001 in H. armigera and H. punctigera. In the 5 years hence, there has been a significant and apparently exponential increase in the frequency of alleles conferring Cry2Ab resistance in field populations of H. punctigera. Herein we review the history of deploying and managing resistance to Bt cotton in Australia, outline the characteristics of the isolated resistance that likely impact on resistance evolution, and use a simple model to predict likely imminent resistance frequencies. We then discuss potential strategies to mitigate further increases in resistance frequencies, until the release of a third generation product. These include mandating larger structured refuges, applying insecticide to crops late in the season, and restricting the area of Bollgard II® cotton. The area planted to Bt-crops is anticipated to continue to rise worldwide; therefore the strategies being considered in Australia are likely to relate to other situations.