Canadian Journal of Arthropod Identification

Identification of Delia pest species (Diptera: Anthomyiidae) in cultivated crucifers and other vegetable crops in Canada

CJAI 29 -- June 29, 2016

Jade Savage, Anne-Marie Fortier, François Fournier, Véronique Bellavance

| Abstract | Introduction | DNA Barcoding | Larval Host Associations | Checklist | Keys to Delia pests | Acknowledgments | References | PDF | Cite |



Feeding damage by root maggot flies (Delia Robineau-Desvoidy spp. (Diptera: Anthomyiidae)) can result in important crop losses in Canada, being especially detrimental to vegetable crops (Griffiths 1986, 1991, 1993; Howard et al. 1994) and, more recently, to the production of a variety of canola cultivars (Brassica rapa Linnaeus and Brassica napus Linnaeus (Brassicaceae)) (Broatch 1993; Soroka et al. 2004; Soroka and Dosdall 2011). As their common names indicate, certain phytophagous Delia species such as the onion maggot, Delia antiqua (Meigen) and the cabbage maggot Delia radicum (Linnaeus), have a relatively narrow range of hosts, in this case feeding exclusively on Allium Linnaeus (Amaryllidaceae) or Brassicaceous species, respectively. However, other primarily saprophagous taxa such as the seedcorn maggot, Delia platura (Meigen), and the bean seed maggot Delia florilega (Zetterstedt) have a much wider range of larval plant food, including decomposing (and sometimes fresh) cultivated crucifers and Allium species as well as many other types such as legumes, cucurbits, a wide variety of garden crops and even certain cereals (Griffiths 1993; Howard et al. 1994).

Because most crops can be infested by more than one species of root-maggot flies, the correct identification of species involved in plant damage is important for the adequate development and assessment of pest-control strategies, as each may respond differently to treatments. This is especially relevant to targeted control methods such as the mass release of sterile insects, a strategy currently used for the control of the onion maggot in southern Québec (Fournier 2014). Much can be learned about the ecology and population dynamics of Delia flies by studying adults  (Broatch 1993; Broatch et al. 2006; Ellis and Scatcherd 2007; Soroka and Dosdall 2011); however, using adults as a proxy for the abundance and species composition of the taxa actually causing the damage can be misleading. Different trap designs will capture different proportions of each species (Finch 1989; Broatch and Vernon 1997); some species can be very abundant as adults but rarely involved in damage, and the relative contribution of different species to plant damage can be over or underestimated (see DNA barcoding and larval host associations section).
Specimen identification of even the most common Delia pest species can be difficult for anyone without extensive training and this challenge is more pronounced when dealing with females or immature specimens which may lack obvious diagnostic features. While excellent, the keys to adult Delia flies published by Griffiths (1993) in his monumental revision of Nearctic Anthomyiidae rely heavily on genitalic characters and include over 150 couplets for each sex, being therefore virtually incomprehensible to non-specialists. The key to third instar larvae by Dahlem and Thompson (1991) remains a good resource for Delia species of economic importance but it contains some inaccuracies (see key to larvae for additional details). The work of Brooks (1951) offers identification keys to the different life stages of Delia pests of cruciferous crops in Canada and remains the most comprehensive taxonomic tool currently available for the group. However, Brooks (1951) includes pests of crucifers only, is poorly illustrated, does not discuss intraspecific variability for certain key structures, and uses obsolete name combinations for most taxa.

The main objective of this work is, therefore, to provide a simple and comprehensive taxonomic resource (identification keys to the different life stages and reference DNA barcodes) for economically important species of Delia flies in cultivated crucifers and other vegetable crops in Canada. New host association data for various crops grown in southern Québec are also presented. 

More than 5000 adults and 3500 immatures from dozens of Nearctic and Palaearctic localities were examined to document intraspecific variability and host associations; however, all specimens photographed in the keys except for those belonging to Delia planipalpis (Stein) were reared from laboratory cultures to ensure the correct association of the various life stages. Live puparia of Delia floralis (Fallén) (cultured from wild Norwegian stocks) were provided by R. Meadows (Norwegian University of Life Sciences, Ås, Norway) and K. Hillier (Acadia University, Wolfville, Nova Scotia, Canada), while laboratory cultures of D. platura, D. florilega, D. antiqua and D. radicum descending from wild Québec populations were maintained by F. Fournier (Collège Montmorency, Laval, Québec, Canada). All wild larvae collected in the province of Québec are housed at the Bishop’s University Insect Collection, Sherbrooke, Québec, Canada (BUIC). Additional adult and/or immature specimens from BUIC, the Canadian National Collection of Insects, Ottawa, Ontario, Canada (CNC), the National Museum of Natural History, Smithsonian Institution, Washington DC, United States (NMNH) and the E.H. Strickland Museum, Edmonton, Alberta, Canada (UASM) were also examined. Provincial acronyms used in the distribution section of each species are based on Canadian provincial and territorial postal abbreviations.

Adults and their external structures were photographed with a Canon DSLR camera equipped with a Canon MP-E65 lens and a Canon MT24EX flash. Male genitalia and immature specimens and structures were photographed using a Luminera 1 digital camera mounted on a Leica M125 dissection microscope. Multiple images taken at different focal points were stacked using Helicon Focus 6.3.0 (Helicon Soft Ltd., Kharkov, Ukraine) and final image editing was completed in GIMP (v.2.8; All larvae were immersed in near-boiling water for a minute before they were measured and photographed.