Tijdschrift voor Entomologie 155 (2012) 15–28 brill.nl/tve A description of the eggs of seven species of Noctuidae (Lepidoptera) commonly transported by plant trade to the UK, and their separation using stereomicroscopy and scanning electron microscopy Anastasia Korycinska Eggs of seven economically important noctuids commonly transported in plant trade, three of quarantine plant health significance in Europe, are described and illustrated using stereomicroscopy and scanning electron microscopy. Autographa gamma (Linnaeus), Helicoverpa armigera (Hübner), Lacanobia oleracea (Linnaeus), Mamestra brassicae (Linnaeus), Spodoptera exigua (Hübner), S. littoralis (Boisduval) and S. litura (Fabricius) could be separated using external morphological characters on the chorion of the eggs, enabling early identification of quarantine pest species. Keys to distinguish the eggs of the seven species are provided. Anastasia Korycinska, The Food and Environment Research Agency, Sand Hutton, York YO41 1LZ, United Kingdom. [email protected] Introduction The eggs can subsequently be prepared for scanning Immature stages of Lepidoptera, many from the electron microscope (SEM) micrographs, reared to family Noctuidae, are inadvertently, and frequently, confirm the identification, or sent for molecular anal- transported during international plant trade (Ma- ysis, depending on the rapid screening results and the lumphy & Robinson 2002). While the adults of individual requirements of the import sample. most species are well described, there is often little in- Eggs are often less obvious during pre-import in- formation available for other life stages. Lepidoptera spections than the larger and active later life stages, intercepted on plant material by plant health author- and so are often the life stage encountered during ities are almost exclusively immature life stages, i.e., plant health border inspections. There have been a larvae or eggs, and must be reared to late instar lar- number of previous studies on noctuid egg mor- vae or adults for species identification, and this can phology using both light microscopy and SEM mi- take weeks, or even months. However, it is not prac- crographs, and these have established morphological tical to hold plant material for this length of time at characters that are of potential diagnostic signifi- ports of entry until a final identification can be made. cance. A number of previous studies described the To enable a National Plant Protection Organisation eggs of a single species (such as for Hyponeuma tal- (NPPO) to decide upon an appropriate course of ac- tula (Schaus, 1904) (Zenker et al. 2007a); Spodoptera tion within a realistic timescale, accurate and timely cosmioides (Walker, 1858) (Zenker et al. 2007b); and identification of all immature life stages is necessary. S. eridania (Stoll, 1781) (Valverde 2007)). The eggs Morphological screening of eggs can be carried out of multiple noctuid species have been studied in using stereomicroscopy very rapidly (within half a NorthandSouthAmerica;thesestudiesrangefrom day), with little or no specimen preparation required. illustrated descriptions (e.g., Peterson 1964; Salkeld Tijdschrift voor Entomologie 155: 15–28, Tables 1–2. Figs 1–22. [ISSN 0040-7496]. brill.nl/tve © Nederlandse Entomologische Vereniging. Published by Koninklijke Brill NV, Leiden. Published 1 August 2012. DOI 10.1163/221194312X651391 Downloaded from Brill.com10/03/2021 03:48:56PM via free access 16 Tijdschrift voor Entomologie, volume 155, 2012 1984), to more comparative work (such as Hudson within a UK plant quarantine context. The eggs were 1973; Olivares & Angulo 2004; Olivares et al. 2005; described and illustrated from the seven species using Andaur-Arenas & Olivares 2009). Collectively, the both stereomicroscopy and SEM micrographs. Three previous work enables the eggs of selected species and of the species have SEM micrographs provided for genera of Noctuidae from America to be identified, the first time, and comparative characters between many to species. However, eggs from noctuid species all seven species are presented and discussed. Keys are in the Old World have been studied less intensively, provided to distinguish the eggs of all seven species and eggs from these regions cannot be identified studied here, through stereomicroscopy and using using New World sources. SEM micrographs. Seven species of noctuid were selected for the current study: all are polyphagous pest species and they include three species listed in the legislation Materials and methods of the European Union Plant Health Directive Insects (2000/29/EC, as amended), Helicoverpa armigera Supplies of eggs were obtained from the sources (Hübner, 1827), Spodoptera littoralis (Boisduval, listed in Table 1. The identity of all the mate- 1833) and S. litura (Fabricius, 1775). The other four rial used in the study was confirmed by identifica- species (S. exigua (Hübner, [1808]), Mamestra brassi- tion of adults reared from the source material, by cae (Linnaeus, 1758), Lacanobia oleracea (Linnaeus, comparison with validated reference specimens and 1758) and Autographa gamma (Linnaeus, 1758)) are with published morphological descriptions (Hard- also frequently found on plants moving in trade by wick 1965; Mochida 1973; Skinner 1984; Waring the plant health authorities of England and Wales & Townsend 2003). All eggs were preserved by ini- (Food and Environment Research Agency (Fera), un- tially placing them in 30% ethanol in labelled 1.5 ml published data). There is thus a quarantine need to microtubes for a minimum of 48 hours and then distinguish these species, listed and unlisted, from transferring them to 70% ethanol and storing them each other as quickly and accurately as possible. at 4°C. Two of the populations were sourced from Stand alone descriptions of the eggs based on SEM quarantine interceptions with a limited number of micrographs are available for S. littoralis (Fehrenbach adult females available: S. litura from India (one fe- et al. 1987), S. exigua (as S. exiqua; Skudlik et al. male); and H. armigera from Egypt (two females). 2005) and M. brassicae (Sannino & Espinosa 1999), For all the other populations, a number of moths but not for the other four species studied here, al- laid eggs in culture on the leaves of Brassica rapa though Matthews (1999) did provide an overall SEM L. (Pekinensis group) plants, and eggs were pre- micrograph of an egg of H. armigera. Döring (1955) served from a minimum of five different batches, or provided a key, general descriptions and diagrams of the eggs of a number of noctuid species, including from different areas of the cage if the eggs were laid A. gamma, L. oleracea and M. brassicae, but was re- singly. stricted to light microscopy. The aim of this study therefore was to examine Terminology and describe the eggs of all seven species in order Egg terminology follows Peterson (1964) and Döring to produce a practical means of distinguishing them (1955). The major terminology used here is defined Table 1. Sources of eggs and the number from each species viewed (+: some eggs partially collapsed under vacuum, although features remained useable). Species Source Geographical origin Number of eggs viewed Stereomicroscope SEM Autographa gamma CEH, Oxford, UK Not known 20 12+ Helicoverpa armigera CEH, Oxford, UK Not known 10 – H. armigera PHSI quarantine interception on Pisum sativum Egypt 20 10 Lacanobia oleracea Fera, UK Jersey 20 13+ Mamestra brassicae Fera, UK UK 20 10+ Spodoptera exigua CEH, Oxford, UK Not known 20 15+ Spodoptera littoralis Fera, UK (culture originally from the University of Durham) Not known 20 20+ Spodoptera litura PHSI quarantine interception on Ocimum americanum Thailand 20 10 S. litura PHSI quarantine interception on Momordica sp. India 15 12+ S. litura PHSI quarantine sample on Begonia hybrids Not known 20 7+ Abbreviations: CEH, Centre of Ecology and Hydrology; PHSI, Plant Health and Seeds Inspectorate (England and Wales); Fera, The Food and Environment Research Agency. Downloaded from Brill.com10/03/2021 03:48:56PM via free access Korycinska: Noctuidae egg morphology 17 Fig. 1. Generalised diagram of the surface features of an egg from the family Noctuidae in dorsal view. Terminology follows Peterson (1964) and Döring (1955). as follows. The micropylar plate is a small flattened the overall shape of the egg; the form of the cells area on the dorsal pole of the egg. The micropylar in the micropylar zone; the comparative heights of rosette is the region of cells immediately surround- the ribs and cross striae; the comparative heights of ing the micropylar plate. The micropylar zone is a the micropylar rosette cells and other cells on the egg region of cells, usually ill defined or very irregular, surface; and the colouration of the egg. immediately surrounding the micropylar rosette (the division between this region and the ribs and cross Morphological examination: scanning electron striae is not always clear). Ribs are radial structures: microscopy first-order ribs reach the micropylar rosette while Preserved eggs were prepared for scanning electron second-order ribs terminate before this region. Cross microscopy by air drying them briefly and removing striae (singular stria) encircle the egg, usually more surface debris, then mounting on an SEM stub us- or less perpendicular to the ribs. A cell is any re- ing double-sided carbon tape. No further treatment gion bounded by raised sculpture: if no region is was performed before sputter coating with gold. The