The Remarkable Locomotory Ability of Wiseana (Lepidoptera: Hepialidae) Pupae: an Adaptation to Predation and Environmental Conditions?
Total Page：16
File Type：pdf, Size：1020Kb
SR Atijegbe et al. 19 The remarkable locomotory ability of Wiseana (Lepidoptera: Hepialidae) pupae: an adaptation to predation and environmental conditions? SR Atijegbe1, S Mansfield1, 2, M Rostás1, CM Ferguson3 and S Worner1 1 Bio-Protection Research Centre, Lincoln University, PO Box 85084, Lincoln 7647, New Zealand 2 AgResearch Ltd, Lincoln Research Centre, Private Bag 4749, Christchurch 8140, New Zealand 3 AgResearch Ltd, Invermay Agricultural Centre, Private Bag 50034, Mosgiel 9053, New Zealand Abstract The effective management of insect pests requires an understanding of their basic biology and overall life cycle including all life stages. The pupal stage of Wiseana spp., a major pasture pest in New Zealand, has been neglected in terms of investigation in to its biology for almost a century. This study investigated the locomotory ability of the pupae of two Wiseana species (W. cervinata and W. copularis) inside artificial burrows. Descent time and speed were measured from digital images. Both species descended the tunnel in response to applied stimuli but W. cervinata descended slightly faster than W. copularis. This is the first study to quantify the movement of a ground-dwelling hepialid pupa inside a burrow. Keywords: Porina, pupae, movement, burrow, Wiseana 20 The Wētā 54:19-31 Introduction Wiseana spp. (Lepidoptera: Hepialidae), commonly called porina, are endemic to New Zealand and are found in both North and South Islands (Barratt et al. 1990; Dugdale 1994; White 2002). Porina is a complex of seven closely related species (Dugdale, 1994; Richards et al. 2017) which are major insect pests of pastures, feeding on ryegrass and clover (Harris 1969; Jensen & Popay 2004). Porina have one generation per year and adults fly from spring to autumn with flight times that differ among species, locality and year (French 1973; Carpenter & Wyett 1980; Barratt et al. 1990). The larva is the damaging stage of porina and has the characteristic ability to form a subterranean burrow lined with silk (Miller 1971; Zydenbos et al. 2011). Burrows are 30-100 cm deep depending on soil type, with the burrow entrance also intertwined with ‘silk threads’ that hold soil particles together (Barlow et al. 1986). Adult female moths are very fecund (French 1973), with a single female producing between 450-3000 eggs (Stewart 2001; Jackson et al. 2012). Depending on temperature and species involved, eggs will take about 9-84 days to hatch (French & Pearson 1976; Ferguson & Cook 2004; Atijegbe et al. 2017) and the small larvae remain in litter on the soil surface for 6-12 weeks (French & Pearson 1981; Barlow 1985; Fleming et al. 1986). The young larvae initially live protected by their silken webbing in small crevices in the soil surface (Stewart 2001). The larvae then burrow underground and remain in the soil for about 6-9 months feeding and grow to between 0.75-2g in weight and 6-7 cm in length before pupating (Barlow et al. 1986). As with all insect pests, effective management requires understanding not only the species’ basic biology and overall life cycle, but also a detailed study of all its life stages. The pupa is an often-overlooked life stage in many holometabolous insect studies - this is also true of porina. This might be because it is assumed that the pupae of most insects are essentially an immobile stage, at most able to wriggle the abdominal segments (Resh & Cardé 2009) and incapable of evading or defending themselves against SR Atijegbe et al. 21 predators. Hinton (1946) was first to suggest that pupae of ten coleopteran families and three lepidopteran families have a defensive mechanism called “gin traps” that deter predaceous mites. It was Eisner & Eisner (1992) that showed these structures serve as a defensive mechanism to protect the coccinellid pupa (Cycloneda sanguinea) against predatory ants. Gin traps are essentially mouth-like elaborations of the intersegmental regions of the pupal abdomen where stimulation by a potential predator triggers a flipping response (Eisner & Eisner 1992). More recent research has demonstrated a wide variety of defence mechanisms used by insect pupae. Mechanosensory mechanisms have been observed in the pupae of the tenebrionid beetle, Zophobas atratus (Kurauchi et al. 2011; Ichikawa et al. 2012; Ichikawa & Sakamoto 2013). The abdominal rotation response by pupae of the tenebrionid beetles, Tenebrio molitor and Z. atratus functions as an effective defence against larval cannibalism (Ichikawa & Kurauchi 2009). Smedley et al. (2002) also showed that glandular hairs on a coccinellid pupae (Subcoccinella vigintiquatuorpunctata) secrete three polyazamacrolide alkaloids that are potent defences against the predatory ant Crematogaster lineolata. Additionally, Alabi et al. (2011) showed that cuticular hydrocarbons expressed by Tribolium brevicornis pupae also have a deterrent feeding effect on both conspecific and congeneric adults, serving as a defence against both cannibalism and predation. Kojima et al. (2011) in a novel study of the Japanese rhinoceros beetle (Trypoxylus dichotoma), established vibratory communication between larvae and pupae co- inhabiting the same soil where pupae produced vibrations in response to larvae approaching their pupal cells, thus showing anti-predatory and sib- killing-avoidance behaviour. Significant movement of porina pupae within their burrows had been noted by Dick (1945) and Barratt et al. (1990), but details of this behaviour have never been fully described. Porina pre-pupae and pupae are vulnerable in their burrows not only to predators but also to drowning during heavy rainfall over the pupation period of about 30 days. These significant risks for pre-pupae/pupae may have prompted the evolution of specific 22 The Wētā 54:19-31 behaviours to avoid predation, cannibalism and drowning. Porina larvae do cannibalise one another but are not known to cannibalise pupae (S. R. Atijegbe, personal observation). Before the introduction by humans of small mammals and exotic birds to New Zealand, porina larvae and pupae were probably preyed on by native birds such as kiwi, pukeko, weka, rock wren, and possibly ground-foraging lizards or carabid beetles. While porina larvae have some ability to defend themselves against predators by rapidly crawling away, ‘playing dead’or regurgitating an offensive brown liquid from their mouth (Barratt et al. 1990), very little is known about any defence strategy/mechanism employed by the porina pupae. In this study, we observed the locomotory ability of porina pupae to move up and down their burrows in response to vibrational stimuli and described key morphological features of the pupae that support this movement. Materials and Methods Insects Adult female porina moths were hand collected from a light trap located at the AgResearch farm on Springs Road, Lincoln, New Zealand, between 21:00 h and 01:30 h during the flight season (October - March). Individual females were placed into numbered 120 mL PP screw cap plastic bottles (Labserv®) so that offspring could be traced back to their mothers after each female had been identified to species. Moths were kept at room temperature close to a 40-watt light source for about 4 hours and were excited by shaking the bottle once or twice. This procedure encouraged oviposition within 1-2 days. After egg laying, moths were stored in tubes of 96% ethanol at 4°C for identification. The preserved adult females were identified using a restriction fragment length polymorphism method based on the mitochondrial cytochrome oxidase I gene (Richards et al. 2017). Eggs of each female were hatched on filter paper (90 mm diameter) that had been immersed in a solution of 1 mg/L of copper sulphate for 2 SR Atijegbe et al. 23 minutes and drained on paper towels to remove excess solution before use. The eggs were sealed in a Petri dish with Parafilm® and stored in a constant temperature (CT) cabinet at 15°C. On hatching, larvae in each Petri dish were transferred into a 3 L KLIP IT™ container (23.5 cm × 17 cm × 12 cm, Sistema Plastics, New Zealand) half full with Grade-2 moist horticultural bark (HB) sourced from Intelligro, New Zealand. The larvae were reared for eight weeks according to the method described by Atijegbe et al. (2017). After eight weeks, larvae of W. cervinata and W copularis were transferred individually into 120 mL plastic bottles half filled with moist HB and fed twice weekly with chopped carrot (Daucus carota) roots until pupation. Glass-sided container and artificial burrow for observation: A two- sided glass container, length 27 cm × 5.5 cm × 22.5 cm (Figure 1a) was filled with moist HB and compacted. An artificial vertical burrow 20 cm deep was made by pushing a 4 mm diameter stick down the glass/HB interface to the bottom of the container so that the burrow was visible from the side and pupal behaviour could be observed easily (Figure 1b). The pupa was placed at the mouth of the burrow at the beginning of each observation. Vibrational stimulation: Pupae of W. copularis (n =5) and W. cervinata (n =5) were used for this experiment; sample size was limited due to larval mortality before pupation. A small metal spatula was used to stimulate the larvae by gently tapping the glass and the surface of the moist HB for about 20 s to create vibrational stimuli. Each pupa was observed for about 30 min after stimulation. Images and videos: Behavioural responses of all pupae were captured on video using a camcorder (Sony HDRPJ710V High Definition Handycam 24.1 MP, 10x Optical Zoom, 32 GB Embedded Memory and Built-in Projector). Videos of porina burrow movement were analysed using Behavioural Observation Research Interactive Software (BORIS) (Friard & Gamba, 2016). Digital images of pupal specimens were taken using a 24 The Wētā 54:19-31 Nikon SMZ25 stereomicroscope with a Nikon NIS-Element Imaging Software. Figure 1. (a) Glass-sided container and (b) pupa in the burrow.