Survey of Delia species of maggots affecting onion in Quebec, Ontario and Eastern Provinces

Project Code: PRR17-010

Project Lead

Julia Mlynarek - Agriculture and Agri-Food Canada

Objective

Evaluate damage to onion crops caused by Delia root maggot species and determine which species are the major contributors of damage

Summary of Results

Background

Root crops grown in Canada are susceptible to root maggot pests (Delia platura, Delia florilega, and Delia antiqua). Although D. antiqua, the onion maggot, is often considered the primary pest, of onion recent studies have demonstrated that the seed corn maggot (D. platura) and the bean seed maggot (D. florilega) may be causing a larger proportion of root maggot damage than previously thought. These pests cause damage by feeding on the seedlings and bulbs. Current management strategies for Delia species include pesticide treatments, however it has been shown that D. antiqua has developed resistance to chlorpyrifos, the most commonly used pesticide for this pest. Knowledge on which Delia species cause the most damage to onion crops is also limited due to challenges associated with Delia species identification. A better understanding of which root maggot species are affecting onion crops will lead to the development of management strategies to effectively address each species, which would benefit the industry by targeting controls at the species level, and by reducing ineffective pesticides use.

This work was identified as a priority for action under Pesticide Risk Reduction’s Strategy for Root Insect Pests of Carrot, Parsnip and Onion and builds on the work started through a previous Pest Management Centre project PRR14-040 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. This project aimed to determine which species has the most potential to cause damage specifically to onions, and investigate methods to quickly and accurately identify which species are present in crops.

Approaches

The project included field assessments to determine the presence and damage caused by Delia species in onion crops grown in Southwestern Ontario (Harrow), Quebec and Nova Scotia. In the first year of study (2017) sites in Nova Scotia included five commercial onion fields and a site located at Kentville Research and Development Centre. In Quebec five commercial onion fields were monitored. In Ontario-Guelph five commercial sites and a site located at the University of Guelph Muck Research Station were monitored. In 2018 sites at Harrow and Nova Scotia, four onion fields were selected for follow-up monitoring from the previous year. At all site blue sticky traps were placed around the field to collect the adult Delia species. The traps were collected weekly from seeding to harvest. If onion plants looked damaged (wilting or stunted) they were removed and verified for larvae feeding on them. If larvae were present, they were collected into 70% ethanol for identification in the lab.

Greenhouse studies with a growth chamber experiment were conducted to monitor the extent of damage (percent of root affected and severity) done by D. antigua and D. platura with increasing numbers of larvae. Experiments evaluated the number of larvae required to effect damage to a single onion seedling, the growth stage of onion most impacted by Delia larvae and preferred by female flies for oviposition.

The adults and larvae were identified using the Delia key developed by the Savage Lab at Bishops University. A molecular method to identify species of Delia was also developed for this project.

Results

For adult captures on the sticky traps, Delia platura dominated in all years, at all sites. In Harrow, Ontario, D. platura represented more than half of all the adults collected. In Nova Scotia adults captured on blue sticky traps were predominantly D. platura, and very few D. antiqua were captured. In Quebec Delia platura was the species that was mostly caught as adults in onion fields followed by Delia florilega. The Muck Research Station had the highest populations of D. antiqua but still in lower numbers than D. platura and D. florilega.

Larva collected from damaged plants were lower than expected. For larva from commercial fields in 2017 in Nova Scotia three from the commercial fields were identified as D.platura/D.florilega and one was identified as D. antiqua. No larvae were recovered from onion in Nova Scotia in 2018. The largest numbers were collected from damaged onions in the Muck Research Station with 271 larvae identified as D. antiqua. No Delia larvae were recovered from sites in and around Harrow.

In the greenhouse experiments data for D. antiqua has been analysed and strongly suggests the potential of this species to cause significant damage to onion. Larvae feeding on the younger growth stages will decimate the plant, while older plants were able to recover and continue growing. While D. antiqua was able to complete its development on onion, D. platura did not. D. antiqua preferred to oviposit on onion post-germination A single female would lay, on average, 12 eggs and 16 eggs, D. platura only oviposited on the 5-7th true leaf stage, but in very low numbers, less than 1 egg/plant. These results demonstrate, in combination with the low number of D. platura larva collected in the field that onion is not a suitable host for D. platura larvae, nor a preferred oviposition site for the adult flies. Results also suggest that the inability to distinguish D. platura from D. antiqua on the monitoring traps could grossly over estimate the population effecting damage in the onion field.

A molecular method (PCR-RFLP) to identify Delia species that are present in onion crops was developed and tested. Validation demonstrated 100% efficacy. This method is faster, more efficient and cheaper than standard molecular barcoding and takes less time and less taxonomic expertise than morphological identification.

Conclusions

The results from this project suggest that while D. platura, the seed corn maggot has the largest adult population flying in onion fields and caught in blue sticky traps it does not cause economic damage to onions. Even in areas with high counts of D. platura the larvae collected were mostly onion maggot. The greenhouse studies confirmed onion is not a crop that is suitable for D. platura to complete its development.

While the number of maggot damaged onions was not abnormal, results from this project indicate the larvae are not causing as much damage as previously believed. It may be worthwhile to revisit the adult sampling method to ensure it is quantifying accurately the larvae present in the field. Further research in the development of other traps such as adding potential pheromones/attractants to attract the Delia that are affecting damage in onion may improve this method as decision making tool. Future projects might also pilot an identification network using the new PCR-RFLP identification method. In addition further research is required to determine whether there are population fluctuations between years and how changes in climate will affect these species to provide appropriate changes to integrated pest management approaches.