By: Dennis Van Dyk, Vegetable Crop Specialist, OMAFRA, Guelph
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)|
*Bradford weather, degree day data and information courtesy of the University of Guelph – Muck Crops Research Station
Onion maggot flies are active in most areas of the province.
Cabbage maggot has reached the DD threshold for emergence in all areas of the province except up North.
The 2nd generation of seedcorn maggot flies have reached the DD threshold for emergence in Harrow. In most other areas of the province, we are just getting over the 1st generation or are currently between generations.
Carrot weevil adults are currently laying eggs, if they are present. In Harrow we have past the degree day threshold for 90% oviposition. This means if carrot weevils are present then almost all of the egg-laying has been completed.
The 1st generation of carrot rust fly is emerging or has emerged in the province.
All areas of the province have reached the DD threshold for overwintering Aster Leafhopper eggs to hatch. Harrow is nearing adult emergence of the local population of Aster Leafhopper adults.
Onion: Earliest direct seeded onions are at the fifth leaf stage while the majority are the third leaf stage. There is a low risk for Botrytis leaf blight. When scouting, check the underside of the outer leaves and look for small, light green to yellow lesions surrounded by white halos. Herbicide injury will be often more white than green but can be mistaken for Botrytis leaf blight. Thrips have been detected in most onion growing areas; pull the leaves apart and look down into the leaf base for 2mm-long grey adults or very small yellow nymphs ~1mm in length. Onion fields next to hay or overwintering rye are at a greater risk as thrips move once the hay or rye is cut.
Garlic: Continue to scout for symptoms of rot in the basal plate which may be due to bulb and stem nematode, Fusarium basal rot or white rot. Look for plants that have yellowing of foliage or tip dieback and dig carefully around the plant and look for fungal growth or rot on the stem. Onion thrips and onion maggot flies have been detected.
Brassica: First generation of cabbage maggots are active. Continue to scout for diamondback moth, cabbageworm, aphids, thrips and tarnished plant bugs.
Celery: Scout for tarnished plant bugs as they are emerging and are typical found first around field borders.
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.
|1st generation||2nd generation||3rd generation|
|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|
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