Investigate factors that determine the occurrence of and level of damage from seed corn maggot in key onion production regions of Ontario,Quebec and the Maritimes

Project Code PRR14-040

Project Lead

Suzanne Blatt  Agriculture and Agri-Food Canada


Monitor population levels and compare emergence timing of the Delia spp. complex in onion crops and investigate factors, including entomopathogenic fungi, influencing the level of damage caused by these pests

Results Summary


A complex of root maggots (Delia species (spp.)) can cause important damage to onion crops in Canada. While the onion maggot (Delia antiqua) is often considered the primary pest, recent studies have demonstrated that the seed corn maggot (Delia platura) and the bean seed maggot (Delia florilega) may be causing a larger proportion of root maggot damage than previously thought. In this project, field and lab studies were conducted in Ontario, Québec and Nova Scotia to gain an understanding of population dynamics of Delia spp. in the field, as well as temporal and spatial locations of these species in the field. Correlations between plant damage and trap captures were also assessed, as well as intrinsic and extrinsic factors affecting Delia populations. In addition, surveys of growers’ knowledge of the seed corn maggot and pest management practices were carried out. This work was identified as a priority for action under the Pesticide Risk Reduction Strategy for Root Insect Pests of Carrot, Parsnip and Onion of Agriculture and Agri-Food Canada (AAFC).


A written survey to determine how well-acquainted onion producers in Nova Scotia (NS) and Ontario (ON) were with the seed corn maggot was developed. The survey was administered to seven onion producers in NS in 2014 and to nineteen onion producers in ON in 2016.

Field studies were conducted in onion fields located in NS, ON and Quebec (QC) to determine the presence and relative abundance of Delia flies. Blue sticky traps were placed just inside the perimeter of fields in ON and QC and both just inside the perimeter and in the center of the fields in NS. Traps were collected twice each week and brought to the lab for counting and identification of the flies. Adult population levels of each species were monitored and their emergence was plotted against the accumulated degree-day (DD) to illustrate when each species was active in the field. Weather data was collected near each site.

Field larval collections were conducted in onion fields located in QC throughout the 2015 growing season to determine the presence and relative abundance of Delia maggots in the soil. Correlations between larval collections and trap captures, as well as between the plant damage caused by larvae and accumulated DD were conducted.

The type of vegetation surrounding each field site was surveyed and categorized as either woody, shrubby, crop or urban. The percentage of the field perimeter harboring these categories was determined by measuring the field perimeter using Google Earth. Other parameters (proximity to water, previous crop, onion variety, total field size, perimeter length and whether there was a Lorsban application) were also recorded for each field. Correlations between all variables, singly or in combination and trap captures were conducted.

The infection of Delia spp. by entomopathogenic (EN) fungi was investigated. In 2014 and 2015, adult Delia flies were collected from the field using sweep nets. These were maintained in the lab and observed for the development of spores.


Grower survey

The written survey administered to onion growers in NS and ON showed that scale of onion production, pest management practices, and knowledge of the seed corn maggot within these two provinces varied greatly. Most growers in NS knew that the seed corn maggot was a potential pest, however, monitoring for the pest was not carried out in either province. More than half of the growers reported using cover crops and a high percentage of NS growers reported cultivating organic onions compared to the ON growers surveyed. Crop rotations employed in NS were generally longer than those used in ON (5 years and 2 years, respectively), and more diverse. The number of years in crop rotation to non-host crops is likely to have an impact on pressure from Delia pests; however despite this, high populations of Delia species can be found in monitoring traps in NS.

Field survey of adults

Emergence data was predicted with the use of a 2006 bioclimatic model developed with Western Canadian data in which the first and second generation for D. platura is predicted at 255 DD and 639 DD, respectively1.

Unfortunately, the field surveys conducted in 2014 did not yield useful data due in part to a late start to the project. In 2015, traps to monitor adult Delia populations were installed in three fields in ON, fifteen fields in QC and six fields in NS. In ON, traps were installed late and missed the first generation of Delia emergence. In NS, hardly any D. antiqua was captured in three fields probably due to the unforseen application of Lorsban around the field perimeter. As a result, only D. platura and D. florilega species were monitored. When trap captures were plotted against DD (base temperature 3.9 degrees Celsius (°C)), first emergence in NS occurred earlier than the model, (roughly at 230 DD and before 180 DD, at two sites) and later in QC (around 320 DD). For the second generation, emergence was observed earlier and at the predicted DD in ON (591 DD and 631 DD, at two sites), later in NS (725 DD and 764 DD, at two sites) and in QC (841 DD). In ON and QC, D. platura and D. florilega were by far the most abundant species and hardly any D. antiqua was captured throughout the growing season. In all provinces surveyed, D. platura and D. florilega males tended to be more abundant than females and males emerged prior to females in QC. Further, in ON, it appeared that a potential fourth generation was also possible towards the end of the growing season as Delia spp. captures started to increase past 1650 DD.

Collectively, these data showed slight differentiation of Delia spp. phenology in the field, in which first and second generation emergence in NS, ON, and QC did not correlate well with the prediction model. Because of the inability to distinguish between D. platura and D. florilega females, the grouping of these two species in the analysis hindered their evaluation in relation to each other and to their respective male counterparts. The ability to distinguish between the females of these two species would provide better knowledge on population dynamics which could eventually lead to better management options for these pests in onions. Morphological keys and deoxyribonucleic acid (DNA) barcoding have recently been developed by Savage et al. (2016)2 and may help provide a more complete picture in future studies.

Field Survey for Larvae

The majority of larvae found in the QC fields were D. antiqua and there was no significant correlation between observed number of D. antiqua flies on the traps and D. antiqua larvae in the field. When the percentage of plants damaged by larvae was evaluated in relation to DD, it showed that most damage occurred between 400 DD and 750 DD, peaking around 550 DD. However, preliminary analysis using Delia adult females did not indicate a strong correlation between trap captures and percentage of damaged plants. Damage caused by D. platura and D. florilega in onion remain largely unknown. Better soil sampling methods for Delia larvae need to be developed in order to capture all Delia species that may be involved in onion damage.

Impact of field/environmental parameters

Many of the field variables examined in this study did not significantly correlate to any of the Delia species present in the fields. The ability of Delia spp. to fly great distances (sometimes kilometers) may be affecting populations on a scale larger than examined in this study. Preliminary data seem to suggest a connection between field size and D. florilega male population, as well as previous year’s crop and D. florilega male population, however, more investigation is needed to confirm any such correlation. Among the variables studied, the variety of onion grown correlated most significantly with Delia populations. For example, white onion correlated with the absence of adult D. antiqua and fields with mixed onion varieties had the highest populations of D. platura and D. florilega males. Since the onion variety grown in each province tends to vary, a more detailed examination of the cultivar effect within each province would be necessary in order to validate these correlations and to provide insight into the impact of these species.

Entomopathogenic fungi

None of the flies captured in the fields and maintained in the laboratory produced spores, even after death. Therefore no conclusions can be drawn regarding the impact of these organisms on naturally controlling Delia species in onion fields.


The data presented here demonstrate that improved bioclimatic models are required in NS, ON and QC for the accurate prediction of Delia spp. emergence in onion fields. Further analysis is warranted to establish plant damage caused by Delia spp. in relation to adult trap captures and/or in relation to larvae in the soil. Better understanding of the damage caused by D. florilega and D. platura is required in order to be able to provide recommendations concerning their management. A new project PRR17-010 - Survey of Delia species of maggots affecting onion in Quebec, Ontario and Eastern Provinces, initiated in the spring of 2017 will examine damage potential from D. florilega and D. platura under laboratory conditions, and investigate when best to sample for Delia larvae in the field. A more thorough examination of the effect of onion cultivar on damage caused by Delia maggots may also provide insight on resistant/susceptible varieties. Development of cultural control methods effective against onion root maggots, regardless of species, would be beneficial for the reduction of damage to onion crops and would ensure that onion production in Eastern Canada is sustainable and ecologically sound through the use of non-pesticide control techniques.

1Broatch et al. (2006) Environmental Entomology, 35:1166-1177


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