Host Preference and Thermal Biology of Helicoverpa punctigera (Wallengren) (Lepidoptera: Noctuidae)

Abstract

<p>The native budworm <i>Helicoverpa punctigera</i> (Wallengren) and cotton bollworm <i>Helicoverpa armigera</i> (Hübner) are important insect pests of field and horticultural crops in Australia. <i>H. armigera</i> is a well-studied, cosmopolitan species, whereas <i>H. punctigera</i> is a widely distributed Australian native with little knowledge of its general ecology. It is believed that native noncrop host plants in the inland regions of the continent could be possible sources of <i>H. punctigera</i> moths migrating to the south-eastern cropping regions as larvae have been recorded on these native non-crop hosts in the inland regions. Plants with the C4 photosynthetic pathway are among the dominant native non-crop hosts in inland regions of Australia. However, there has been little work on their occurrence and successful development on these non-crop hosts, especially on plants with the C4 photosynthetic pathway.</p> <p>My thesis reports aspects of <i>H. punctigera</i> ecology to improve our understanding of its population dynamics especially in the inland regions. I assessed adult oviposition preference, larval survival and adult oviposition response of <i>H. punctigera</i> on some plants with the C4 photosynthetic pathway, compared with C3 plants under glasshouse conditions. In addition, because inland Australia is characterised by high and extreme temperatures in summer, I assessed the physiological responses of the insect to exposure to both high and extreme temperatures.</p> <p>For oviposition studies, I conducted two-way and multi-choice tests on two C4 plants (<i>Atriplex nummularia</i> Lindl. and <i>Atriplex vesicaria</i> Heward ex Benth), against two C3 plants (<i>Medicago polymorpha</i> L. and <i>Cullen cinereum</i> (Lindl.) J.W.Grimes). Although oviposition occurred on both the C3 and C4 plants, ovipositing females showed preference for the C3 plants vs the C4 plants. In addition, females preferred <i>Atriplex nummularia</i> against <i>Atriplex vesicaria</i> in both a multi-and two-choice test, and preferred the adaxial and abaxial of <i>Atriplex nummularia</i> and <i>Atriplex vesicaria</i> respectively as their oviposition site. </p> <p>There are few records of <i>H. punctigera</i> survival on non-crop hosts especially on plants with the C4 photosynthetic pathway. Larval performance was investigated on four C4 plant species (<i>Atriplex numularia</i> Lindl. <i>Atriplex vesicaria</i> Heward ex Benth., <i>Portulaca oleraceae</i> L. and <i>Tribulus terrestris</i> L.) and two C3 plant species (<i>Medicago polymorpha</i> L. and <i>Cullen cinereum</i> (Lindl.)). Larvae survived on all the C4 plants, with high survival (32.2±6.9 and 31.7±6.9 percent survival) on both <i>Atriplex</i> species and lowest survival (15.9±7.9 percent survival) was on P. oleraceae. Larval survival on the C4 plant <i>A. nummularia</i>, was significantly higher than on both the C4 plant A. vesicaria and the C3 plant <i>M. polymorpha</i>.</p> <p>Ovipositing plant-feeding insect species use volatile organic compounds released by their host plants to recognise their hosts and select oviposition sites for egg deposition. By assessing the oviposition response of ovipositing <i>H. punctigera</i> females to leaf extracts of two C4 plants, I found in both two-way and multi-way oviposition response bioassays, substrates treated with extracts of the C4 plants attracted more females than untreated controls, suggesting that the plants may contain a compounds that can influence gravid <i>H. punctigera</i>. Leaf extracts of <i>Atriplex nummularia</i> received significantly more eggs compared with extracts of <i>Atriplex vesicaria</i>.</p> <p>A warmer and drier climate is predicted to become more extreme into the future exposing terrestrial insects to further thermal stress. On this premise, I assessed the thermal physiology (critical thermal maximum) of <i>H. punctigera</i> across three life stages (larvae, pupae and adults) using thermolimit respirometry assays to gain an understanding of the impact of rising temperature on the insect. Critical thermal maximum significantly differed among the life stages with larvae having the highest thermal maximum (49.1°C±0.3°C) compared with pupae (47.3°C ±0.2°C) and adults (46.9°C±0.2°C).</p> <p>High temperatures in the field, which occur as single, prolonged, or repeated small-scale events, can affect the population performance of insects. Assessing the impact of different heat stress regimes can provide important information on the biological performance of <i>H. </i>punctigera under climate change. Here, I subjected <i>H. punctigera</i> pupae to two extreme temperatures – 44.2°C and 43°C which are just below the CT_max and two exposure treatments (single prolonged exposure and repeated short exposure), to assess the biological traits of individuals. Heat stress on pupae resulted in reduced fecundity by 50%, nil egg hatchability, extended pre-oviposition period by one day, and adult deformities.</p> <p>This thesis has broadened our understanding that <i>Helicoverpa punctigera</i> could utilize plants with the C4 photosynthetic pathways that are common in inland regions of Australia. This is a significant contribution to our understanding of the ecology and biology of <i>H. punctigera</i>. </p>