Insect Degree Days

Weather Station Location Onion Maggot Seedcorn Maggot Cabbage Maggot Carrot Weevil Carrot Rust Fly Aster Leafhopper
(base 4°C) (base 4°C) (base 6°C) (base 7°C) (base 3°C) (base 9°C)
Harrow 1265 1265 1044 938 1380 746
Ridgetown 1153 1153 937 838 1268 660
Delhi 1125 1125 910 813 1237 636
Goderich 973 973 775 684 1076 533
Guelph 928 928 735 646 1034 492
Bradford* 987 987 795 708 1086 547
Kemptville 969 969 786 700 1069 544
Sudbury 754 754 597 525 845 394

*Bradford weather, degree day data and information courtesy of the University of Guelph – Muck Crops Research Station


Cabbage, onion and seedcorn maggot fly activity should be beginning to increase from Norfolk across to Lambton and south. Most of the province is in-between generations currently but should start seeing the next generation emerge in the next few weeks depending on weather.

For carrot weevil we are now well past the degree day threshold for 90% oviposition across the province. This year we will be monitoring for the presence of a possible 2nd generation of carrot weevils. If you think you are having late season carrot weevil issues, please contact Dennis (

We have reached the DD threshold for carrot rust fly in Harrow only. The rest of the province should be between generations currently.

Aster leafhoppers adults are now present across the province.

Disease Forecasting

Onion Downy Mildew (DOWNCAST disease forecasting model):
In Bradford, conditions have not been conducive to sporulation so risk of downy mildew is low to moderate.
In Lambton county, sporulation infection periods (SIP) have been accumulated from July 2-5 and then again on July 12 meaning conditions and risk is high in transplant onions and moderate to high in seeded onions.

Late Blight:
Late blight DSVs are accumulating quickly in most areas of the province.
Late blight on potatoes has been confirmed in the Simcoe/Durham county region but the risk remains high across the province. Growers should be applying late blight specific fungicides along with a broad-spectrum fungicide like chlorothalonil (Bravo, Echo), metiram (Polyram) or mancozeb (Manzate, Penncozeb, Dithane).

Check OnPotatoes for up to date late blight risk information and USAblight for currently reported late blight incidence in the Great Lakes region.

Crop Updates

Onions: Earliest direct seeded onions have six true leaves while the majority are at the four to five leaf stage. Stemphylium leaf blight, purple blotch, onion smut and Botrytis leaf blight has been detected in Ontario. See the Stemphylium leaf blight article in a previous OnVegetables post for more information.

Garlic: Leaf dieback has been seen in multiple areas with Fusarium basal rot and bulb and stem nematode. Carefully inspect the basal plate and look for fungal growth or rot on the stem. For confirmation, send suspect cloves to a certified pest diagnostic clinic for bulb and stem nematode extraction. Leek moth has been identified in pockets of Southwestern Ontario.

Brassica: Cabbage maggot is active and severely infested plants may wilt in hot weather. Wilt can also be caused by clubroot, a soil-borne pathogen infecting the roots and causing club-like symptoms. A clubroot survey is underway; please contact for more information if you find clubroot in your field. Swede midge has also been reported throughout Southwestern Ontario and is affecting the meristems later seeding dates of multiple brassica crops. Continue to scout for flea beetles, diamondback moth, cabbageworm, aphids, thrips and tarnished plant bugs.

Celery: Tarnished plant bugs have been detected in Thedford and the Holland Marsh. Scout for damage from carrot weevils laying eggs.

Below are the various insect DD thresholds for reference.


Use these thresholds as a guide, always confirm insect activity with actual field scouting and trap counts.

Degree Days
1st generation 2nd generation 3rd generation
Onion Maggot 210 1025 1772
Seedcorn Maggot 200 600 1000
Cabbage Maggot 314-398 847-960 1446-1604
Carrot Rust Fly 329 – 395 1399-1711 n/a
Carrot Weevil 138 – 156 Egg laying (oviposition) begins
455 90% of the egg-laying (oviposition) is complete
Aster Leafhopper 128 Overwintering eggs hatch
390 Local adult emergence

Ontario DD Map July 13

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