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Locomotion activity meter for quality assessment of mass-reared sterile male moths (Lepidoptera)

Abstract

Irradiation is used to provide sterile insects from mass-rearing facilities, but irradiation can degrade insect quality. A system is described that uses repeatable pheromone stimuli to activate male moths housed with clean airflow in a commercially available insect activity meter, for potential use in quality assessment of mass-reared moths in sterile insect programs. We tested sexually mature wild and sterile light brown apple moth (LBAM), Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), males at the commencement of scotophase in a simple assay for increased mate-seeking activity after a 2 s stimulus with a 4-component synthetic pheromone source for 2 s, after a 5 s baseline of activity. Male activity at the upwind end of the glass tube was assessed by infrared beam. Next, we tested if a temperature shock at 30 °C in the pupal stage for various durations (0, 1, 2 or 4 h) would have an effect on male moth response to the sex pheromone. The results indicated similar baseline activity in clean airflow, but a significantly greater response after pheromone stimulation from non-irradiated males, compared with irradiated males. Responses from irradiated males averaged 78% of the control response (n = 320). The temperature shock did not appear to change the response of the non-irradiated moths (n = 64), but there was a slight decline in response by irradiated moths at the 2 and 4 h temperature shock durations. The system could be readily modified to be suitable for factory scale quality assurance. Pest management programs targeting Lepidoptera have increasingly looked to the sterile insect technique (SIT) for solutions because of its selectivity and proven track record against several species (Bloem et al. 2005; Bloem et al. 2007). The SIT relies on mass-rearing and release of sterilized but competitive males to locate and mate with wild female insects, to provide population suppression. Improvements in mass-rearing techniques, knowledge of genetics, trapping systems and other factors may account for increased interest in the SIT (Klassen & Curtis 2005; Vreysen et al. 2013). In moths, inherited sterility (IS) offers considerably greater suppressive potential from the release of partially sterile insects with a lower quality deficit due to the lower irradiation dose. Any F1 offspring resulting from a sterile male: wild female cross produces completely sterile F2 progeny, which increases the cost effectiveness of the IS approach over fully sterile releases (Carpenter et al. 2005). Lower radiation doses required to induce IS enable the release of moths that live longer, are stronger fliers, and mate more frequently than fully sterile insects (Carpenter 2000; Bloem et al. 2006; Stringer et al. 2013). Suppression or eradication was achieved using the SIT within area-wide programs for the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae) (Bloem et al. 2005; Bloem et al. 2007) and the Australian painted apple moth, Teia anartoides Walker (Lepidoptera: Lymantriidae) (Suckling et al. 2007b). The SIT has been investigated with this goal for the European grape vine moth, Lobesia botrana (Denis & Schiffermüller) (Lepidoptera: Tortricidae) (Bloem et al. 2005), the false codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) (Carpenter et al. 2007) and the light brown apple moth (LBAM), Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae) (Kean et al. 2011; Soopaya et al. 2011; Jang et al. 2012). LBAM is an invasive tortricid from southeastern Australia, and its current known distribution outside of its natural range includes New Zealand, the USA, and some European countries (Suckling & Brockerhoff 2010). Sterility has been examined in the context of the SIT and modeled with competitiveness parameters to estimate the over-flooding ratio required for population suppression (Kean et al. 2011). The SIT requires assessment of the effects of irradiation and other factors on insect quality (Vreysen 2005), and considerable effort has been expended on this for fruit flies (Cáceres et al. 2007), and more recently moths (Simmons et al. 2010). Many authors have used simple activity measures for flies, such as flight ability out of a cylinder, but this approach may have limitations for moths. Carpenter et al. (2012) described a very cost-effective and simple bioassay for assessing the quality of sterilized codling moths for both field and laboratory by counting the number of released moths that have flown from a cylinder over a period of around 3 days. While this assay is effective and affordable, it does come at the expense of time. Wind tunnel assessment has been used to determine the effect of irradiation and other factors on male Australian painted apple moth arrival at a female (Suckling et al. 2004; Stephens et al. 2006) with quality assessment performed weekly during the painted apple moth eradication program in New Zealand (Suckling et al. 2004; Simmons et al. 2010). The evaluation proved to be valuable at detecting and improving insect quality by altering the handling process (Stephens et al. 2006). The reduction in quality caused by irradiation was used in a model designed to estimate the over-flooding ratio (Kean et al. 2007) during the male-only release program. Digital tracking of LBAM flight behavior in a wind tunnel also found some effect of irradiation on male quality of individuals, which was evident in the field with recaptures in hedgerows and vineyards (Suckling et al. 2011). The need for specialized facilities or release into the environment (which adds variables) reduces the practicality of this approach for routine assessment of quality. Other assays of male behaviors in response to pheromone stimulus have been conducted, including a wing fanning assay in glassware (Bartell & Shorey 1969). We have previously assessed several methods for their suitability at detecting the effects of irradiation on male moth competitiveness (Suckling et al. 2011; Stringer et al. 2013). The close correlation between proclivity of an individual for activation and wing fanning and then arrival after a sustained zig-zag flight (Suckling et al. 2011) led us to hypothesize that direct assessment of wing fanning after pheromone stimulus might be a suitable measure of insect quality. We tested the suitability of a commercially available locomotor activity meter (LAM), originally designed for vinegar flies. This project followed earlier use of locomotor activity meters for assessing Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) quality and competitiveness after irradiation (Dominiak et al. 2014). Here, we sought to understand whether impacts of irradiation on laboratory-reared male LBAM could be detected using observations of behavior obtained during pheromone stimulus. The focus was on upwind walking and wing fanning, because these behaviors were correlated with arrival and they effectively showed a dose response with irradiation (Suckling et al. 2011). We also tested whether heat shocks of various durations would have an effect on the quality of male moths in conjunction with irradiation.

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