Life History Traits of a Key Agricultural Pest, Helicoverpa Armigera (Lepidoptera: Noctuidae): Are Laboratory Settings Appropriate?
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Katsikis Christina (Orcid ID: 0000-0003-4585-1127) Life history traits of a key agricultural pest, Helicoverpa armigera (Lepidoptera: Noctuidae): are laboratory settings appropriate? Christina I Katsikis, Peng Wang and Myron P Zalucki1 1School of Biological Science, The University of Queensland, St Lucia, QLD 4072, Australia. Running Title Life history and nutrient self-selection of Helicoverpa armigera Abstract Helicoverpa armigera is intensively researched in laboratory settings, yet developmental rates can vary considerably even under controlled conditions. Here, dietary choice and light spectra were tested as possible factors influencing this variability, a range of fitness indicators were collected, and dietary choice behaviour in early instars was observed. We show that early instars of H. armigera exhibited self-selection of nutrient intake, a novel finding. Larvae given a choice between two artificial diets varying in macronutrient ratios were heavier, exhibited a higher relative growth rate, shorter developmental time and longer eclosion time compared with larvae reared on a single diet. Wing size relative to body mass was higher for larvae on extreme no-choice treatments and smallest for those on the choice diets, indicating a potential adaptation to escape poor nutrient landscapes. Light spectra had an effect on the size of pupae, with H. armigera reared under white LED light having larger pupae than those reared under fluorescent white light. Larvae reared under LED light took longer to emerge from pupation. Giving larvae a choice of diets with a range of nutrients may reduce developmental variability rather than assuming that one diets suits all. Key words This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/aen.12441 This article is protected by copyright. All rights reserved. Lepidopteran bioecology, nutrient choice, geometric framework, light spectrum, developmental variability, polyphagous pests, developmental rate, insect laboratory rearing, wing size, diet deficiency compensation. INTRODUCTION Helicoverpa armigera (Hübner) is a polyphagous insect pest of significant global importance (Kriticos et al. 2015). It is reported as feeding on over three hundred plant species (Cunningham and Zalucki 2014), including many agriculturally and commercially important crops, and accounts for over $2 billion dollars per year in damage to agriculture worldwide (Tay et al. 2013). Its geographic range has been expanding, and in the last decade it has expanded into a range of Latin and Central American countries (Sosa-Gómez et al. 2016). It has also been sporadically detected in the USA though permanent populations have not yet been established (Gonçalves et al. 2019). In addition, it has evolved resistance to a number of key chemical pesticides, making management difficult (Ahmad 2007; Downes et al. 2017). Not surprisingly, H. armigera is the subject of intense study worldwide both in the laboratory and in the field (e.g. Zalucki et al. 1986; Armes et al. 1992; Jallow and Matsumura 2001; Downes and Mahon 2012; Tay et al. 2013; Gregg et al. 2019). Laboratory studies have ranged from chemical and molecular to physiological and behavioural (e.g. Armes et al. 1992; Wei et al. 2002; Xu et al. 2005; Luong et al. 2017). Potential pest control and mitigation strategies have been proposed on the basis of these experiments (Reddy and Manjunatha 2000). Development rate in the laboratory is highly variable at 25 C°, the most common temperature at which H. armigera is raised, with nearly as much variation at this temperature as there was across temperatures (Unpublished data, Puhl de Melo and MP Zalucki, UQ, see Appendix A). Such high variability presents a problem of reproducibility, applicability to natural environments, and raises questions about what factors may affect the development of H. armigera in laboratory settings. While any laboratory experiment runs the risk of abstracting and isolating the subject of study from its natural conditions, the effect this can have on a subject’s life history is often poorly understood (Shields 1989). Causes of high variation in This article is protected by copyright. All rights reserved. developmental rates may include a wide range of factors, such as protozoan, fungal, bacterial(Armes et al. 1992) or viral infections (Hanzlik et al. 1993), inbreeding depression (Cacoyianni et al. 1995) and nutrition (Armes et al. 1992). In laboratory studies H. armigera are often reared from hatching to pupation on artificial diet (Teakle 1991). This is done for a variety of reasons, such as taking potential plant- invertebrate interaction out of equation, and yielding specimens of higher fitness and fecundity, and general ease and convenience (Wu and Gong 1997). However, most diets have not been optimized to correspond to the diet a caterpillar would naturally encounter or choose in the field, which may be quite different in macronutrient content (Behmer 2009). Diets may also differ between research facilities (Ritter and Nes 1981; Wu and Gong 1997; Hamed and Nadeem 2008). Lepidoptera in natural settings, including H. armigera, regulate their intake of a wide range of nutrients by changing food source and the amount eaten (Raubenheimer et al. 2009). The nutrient landscape that Lepidoptera find themselves in is highly heterogeneous, even in crop monocultures (Deans et al. 2015). Many caterpillars show a natural preference for a specific ratio of different nutrients, termed their intake target (IT), of which carbohydrate and protein composition in particular have been intensely studied (Behmer 2009). A self-selected intake target can be different depending on the life stage of the caterpillar, and is hypothesized to reflect an evolutionary compromise between food availability, maximum fitness, avoidance of secondary plant metabolites and minimization of danger due to predation (Lee et al. 2002; Despland and Noseworthy 2006; Behmer 2009). Food intake preferences can shift even over the course of a single instar, due to changing energetic and developmental needs of the caterpillar (Cohen et al. 1987). Larvae of H. armigera raised on an artificial diet with a single defined protein:carbohydrate composition are therefore unable to optimize their food intake, which may alter their development compared to their wild counterparts, or to larvae raised on an artificial diet with a choice of different macronutrient composition (Deans et al. 2015). Further, a range of behaviours (such as shelter seeking or relocating in order to pupate) may interact with nutrient self-selection and further modify the natural diet of H. armigera compared to their stationary laboratory-raised counterparts (Perkins et al. 2009). This article is protected by copyright. All rights reserved. Remarkably, all nutrient choice experiments documented for Lepidoptera have been conducted exclusively on late or final instar caterpillars (Lee et al. 2002; Lee et al. 2004; Lee et al. 2006; Behmer 2009; Deans et al. 2015) including H. armigera (Raubenheimer and Browne 2000; Browne and Raubenheimer 2003; Tessnow et al. 2018). It is not known whether nutrient choice differs between instars, or whether early instars are capable of self- selecting their food. Early instars show different feeding behaviour to other stages (Johnson & Zalucki 2007). In Helicoverpa zea (Boddie), first and second instar caterpillars prefer different plant parts to later instars (Cohen et al. 1988). Early instars of H. armigera have been shown to exhibit specialized feeding patterns, preferring soft terminal leaves and buds (Perkins et al. 2008), and location of larvae on reproductive plant parts have been linked to adaptive differential survival (Bahar et al. 2019)First instars have a higher relative growth rate, and distribute their time between feeding, resting and moving differently compared to later instars (Johnson and Zalucki 2007). Fourth and fifth-instar H. armigera also exhibit dynamic changes in feeding behaviour as they grow (Raubenheimer and Browne 2000; Browne and Raubenheimer 2003). The ability to choose between different nutritional options may therefore affect caterpillar development compared to their non-optimizing conspecifics restricted to a single diet. Some of the variation found in the laboratory may be explained by the lack of choice provided to larvae, combined with variability in ingredients such as type of flour (Naseri et al. 2010) and macronutrient content (Behmer 2009). As H. armigera are generally reared in constant environment cabinets on artificial diet and under artificial light (Armes et al. 1992), the type of light used may also be pertinent. Artificial lighting spectra depend strongly on the type of illumination used. Incandescent, fluorescent and LED lights all have highly distinct spectra, and all differ from natural light, including in their effects on Lepidoptera (Shields 1989; Johansen et al. 2011; Longcore et al. 2015; Degen et al. 2016). Light plays a key role in regulating insect development, circadian rhythms and diapause initiation/termination, flight and ovipositing (Shimoda and Honda 2013). Photoperiod, the This article is protected by copyright. All rights reserved. specific spectrum (wavelength profile) of the light, the intensity of light and its polarization have been shown to