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 |


Larval Host Associations

The abundance of different Delia species in canola and/or vegetable crops is often monitored using passive traps (yellow and/or blue sticky cards, cone traps, water traps, etc.) to capture adults (Finch 1989). While D. radicum and D. antiqua are often assumed to be the main contributors to crop damage in crucifers and Allium, respectively, adults of D. platura and/or D. florilega can be much more abundant in passive traps than primary pests (Broatch and Vernon 1997; Villegas et al. 2009), as they are attracted to organic matter and fresh crop residues (Hammond 1990) and can breed in previously damaged and/or healthy cultivated plants. Little is known, however, of their actual involvement in plant damage. To determine the species composition of Delia species feeding on vegetable crops in southern Québec, more than 3500 third instar larvae were collected from 85 vegetable fields distributed in 21 farms in the Montérégie region (Fig. 2) between 15 May 2015 and 23 September 2015. Each field was sampled at least once and sampling protocol varied according to crop type. In crucifers (Brassicaceae), 150 seedlings each of broccoli (Brassica oleracea Linnaeus var. italica Plenk), cauliflower (B. oleracea var. botrytis Linnaeus) and Napa cabbage (Brassica napus Linnaeus subsp. pekinensis (Lour.) Hanelt) were examined, and 300 seedlings of radishes (Raphanus sativus Linnaeus) were collected. Regarding onions, all yellow onion (Allium cepa Linnaeus) seedlings along 30 lines of 1 meter each were examined, and all green onion (Allium fistulosum Linnaeus) seedlings along 15 lines of 1 meter each were examined. Broccoli and cauliflower were grown in mineral soils while Napa cabbages, radishes and all onions were grown in muck soils. The larvae were identified morphologically and those of D. platura and D. florilega were grouped together since they can’t be distinguished morphologically (see identification key to larvae for details). The abundance of species in samples where at least one larva was collected was compared in each crop using a two-tailed Wilcoxon-signed rank test on paired data. Green and yellow onion samples were divided into two groups according to the presence or absence of mass release (since 2012) of sterile D. antiqua on the farms where they were collected (Fournier 2014). The statistical analysis was computed using the R package “stats” 3.2.4 (R Development Core Team, 2016) and statistical significance was assessed using an alpha-level of P < 0.05 and 0.95 confidence intervals.

As expected, the diversity of collected larvae was low: Delia platura/D. florilega was collected in all crops, D. radicum was found only in crucifers and D. antiqua only in onions (Table 1). In crucifers, most larvae were collected from broccoli (n=2089) and Napa cabbage (n=858). The larval abundance of each species was quite variable but Delia radicum was significantly more abundant than Delia platura/D. florilega only in Napa cabbage (V = 108.0, P = 0.007) and radishes (V = 242.5, P = 0.001) (Table 1), even if D. platura and D. florilega adults were always much more abundant than D. radicum on both sticky cards and emergence traps placed in the fields (A.-M. Fortier and J. Savage, unpublished). Liu and Sparks (1999) reported substantial damage to Napa cabbage by D. florilega in Texas based on rearing data but our results suggest that the pattern may not apply to more northern localities, and that the attribution of feeding damage to Napa cabbage by D. florilega and D. platura in southern Québec based on the high abundance of adults observed laying eggs in the field by Lafontaine et al. (2008) may have been incorrect.

In spite of intensive sampling efforts in onions, infestation rates were low in 2015, with damages averaging 0.7% and 1.6% in yellow and green onions respectively, independently of the control method used (F. Fournier, unpublished) and relatively few Delia larvae were collected, both in fields where sterile D. antiqua had been released and those where they had not (Table 1). Overall, we collected more larvae of Delia platura/D. florilega (n=360) than of D. antiqua (n=213), but found no significant differences in species abundance (Table 1). In onions, D. platura is often reported in joint infestations with D. antiqua (Merrill 1951; Merrill and Hutson 1953; Finlayson 1956). However, in the present work, D. platura/D. florilega larvae were collected alone in 38 of 72 yellow onion samples (53%) and in 17 of 37 green onion samples (46%), suggesting primary infestations.  
Since the larvae of D. florilega are identical to those of D. platura and because of the presence of two distinct BINs for D. platura (see DNA Barcoding section), we used DNA barcodes to determine larval host associations of 288 randomly selected D. platura/D. florilega larvae collected in various vegetable crops in southern Québec (Table 2). Both BINs of D. platura were recorded from all hosts – broccoli, cauliflower, Napa cabbage, radish, green and yellow onions. In crucifers, the larvae from BIN AAG2511 (n=139) were more than 2.5x as abundant as those from BIN AAA3453 (n=52) but the trend was reversed in onions, where 49 out of 64 sequenced individuals belonged to BIN AAA3453 (Table 2). Delia florilega was always less abundant than D. platura (BINs combined) but recorded from all sampled hosts with the exception of cauliflower (Table 2).

The host association data presented here is based on a large number of sampled plants and localities throughout the season, but it was all collected over a single year (2015) in southern Québec. It should therefore be used as baseline data to further document host preference (and its variability) in Delia flies found in vegetable crops in Canada.

Fig. 2. Location of farms from Montérégie, southern Québec, Canada, where Delia larvae were collected in 2015.


Table 1. Mean, standard deviation (SD) and range of Delia species abundance per sample (n) of larvae collected in crucifers and onions in 2015 in Montérégie, southern Québec, Canada, with a comparison of mean species abundance for each crop type. S = samples collected in fields where sterile males of D. antiqua were released, NS = samples collected in fields where no sterile males of D. antiqua were released. 

Crucifers D. radicum   D. platura/D. florilega* Vα p-value
Mean ± SD range   Mean ± SD range
 Broccoli (n=37) 23.8 ± 34.2 (0-135)   32.7 ± 94.5 (0-417) 409.0 0.39
 Napa cabbage (n=15) 48.9 ± 75.5 (0-281)     8.3 ± 17.0 (0-64) 108.0 0.007
 Radish (n=24) 5.0 ± 5.0 (0-17)   2.2 ± 5.3 (0-20) 242.5 0.001
 Cauliflower (n=10) 5.8 ± 9.5 (0-29)   2.6 ± 4.5 (0-12) 34.0 0.54
Onions D. antiqua   D. platura/D. florilega* Vα p-value
Mean ± SD range   Mean ± SD range
 Yellow onion S (n=35) 2.3 ± 5.6 (0-31)   3.3 ± 4.5 (0-17) 205.5 0.18
 Green onion S (n=30) 1.8 ± 2.9 (0-10)   2.9 ± 4.1 (0-17) 165.5 0.26
 Yellow onion NS (n=30) 2.4 ± 5.0 (0-22)   4.7 ± 8.0 (0-42) 139.5 0.09
 Green onion NS (n=6) 1.2 ± 1.5 (0-4)   3.0 ± 3.6 (0-8) 6.0 0.40

* The larvae of these two species cannot be distinguished morphologically.
αV value from two tailed Wilcoxon-signed rank test on paired data with confidence interval of 0.95.


Table 2. Number of Delia platura and D. florilega larvae per species (based on DNA barcodes) and Barcode Index Number (BIN) for a selected subset of the larvae collected in 2015 in Montérégie, southern Québec, Canada (n = number of farms from which at least one larva was sequenced).

Crop type
D. platura
(BIN AAG2511)
D. platura
(BIN AAA3453)
D. florilega
(BIN ACR4394)
Broccoli (n=4) 98 34 9
Cauliflower (n=2) 21 3 0
Napa cabbage (n=3) 12 11 11
Radish (n=2) 8 4 1
Green onions (n=2) 7 28 9
Yellow onions (n=8) 8 21 3