Grower Grind

The heat was on this week, with much of the province experiencing several days of hot, humid conditions. Fruit are sizing rapidly – reaching 30 mm in the early regions – and degree days are accumulating quickly, accelerating orchard development and pest activity alike. While the weather has helped move the crop forward, it has also compressed management timelines, making regular scouting and attention to model predictions especially important as we move towards summer solstice.

How Hot is Too Hot?

With temperatures reaching 27-30^oC this week, I often get asked whether heat can slow insect or disease activity. For most apple pests, the short answer is no. Hot days and warm overnight temperatures remained well within the favourable range for development. In fact, these conditions accelerated degree day accumulation and likely advanced pest development for many regions.

Too Hot for Bugs?

For common insect pests this time of year, including aphids (though numbers overall have been low for rosy and green apple aphid this year), codling moth, obliquebanded leafroller, potato leafhopper and San Jose scale, this week’s temperatures were more likely to increase development than suppress it. Warm nights were particularly important, allowing insects to continue accumulating degree days around the clock. While some insects may be less active during the hottest parts of the day – such as plum curculio, temperatures generally need to exceed 32-35^oC for extended periods before significant reductions in activity or survival occur. If running developmental models, consider including the maximum temperature thresholds days are above 30^oC (eg., codling moth maximum temperature threshold is 31^oC).

Too Hot for Infection?

Disease responses are more nuanced…

Apple scab still requires leaf wetness and rainfall for infection. That said, while scab can infect over a broad temperature range, infection is generally most efficient under moderate temperatures. Sustained temperatures over 28^oC become less favourable for ascospore survival, germination, and infection. However, there does not seem to be a specific temperature threshold above which infection ceases. If wetting events occur, infection remains possible, particularly during cooler overnight periods.

Bitter rot is a different story. Warm temperatures, high humidity, and mild nights create ideal conditions for infection. Temperatures between 25-30^oC are near optimal for disease development. Orchards with a history of bitter rot should consider fungicides with efficacy against this disease, especially if thunderstorms are in the forecast. Susceptibility studies led by University of Guelph in recent years have shown fruitlets can be infected with bitter rot shortly after petal fall; however, symptoms often do not become apparent for several weeks.

For more information on products with efficacy against bitter rot, see the Bitter Rot Efficacy Table on the Ontario Crop Protection Hub.

Too Hot to Spray?

The greater challenge during hot weather is often spray timing and crop safety. High temperatures can increase the risk of phytotoxicity from certain products, particularly on rapidly expanding leaf tissue.

Heat stress can also alter leaf physiology and spray performance. Under hot, dry conditions, leaves may develop a thicker cuticle layer, while rapid evaporation can concentrate spray residues on the leaf surface. Together, these factors can influence uptake and increase potential for crop injury.

When possible, target applications during cooler parts of the day and pay close attention to label precautions, especially for products such as captan, oils, sulphur and certain foliar nutrient formulations.

Bugs Behaving Badly

Things have been pretty quiet on the insect front this season. Don’t be surprised to see insect activity take off now that we are seeing warmer temperatures.

Pay close attention to scouting reports and be prepared to pivot or adjust control timings, where needed, based on pest activity.

Codling moth

Egg hatch for 1^st generation codling moth has started and spray timings are being triggered over the next week or two across the province.

Following egg hatch, codling moth larvae search out the fruit where they begin to tunnel in. This can happen as quickly as 24 hours after egg hatch which is why timing of control strategies are so important. After entering fruit, the larvae will feed for approximately 3 weeks before leaving the fruit to pupate (usually on tree trunk or larger branch of the tree). Pupation generally lasts 14-21 days. Some larvae do not pupate at this time, but remain as larvae until the next year. Second generation adults begin to appear in orchards as early as July and lay eggs over two months.

For more information on products with efficacy against codling moth, see the Codling Moth Efficacy Table on the Ontario Crop Protection Hub.

It’s important to note for resistance management, if re-application is required for control of a pest with discrete generations, use the SAME product within a generation. For instance, codling moth is still in the first generation window. Second generation larvae activity will not begin for another few weeks – typically early August. Rotate to another insecticide class at that time to ensure each generation is only exposed to one active ingredient.

Mullein bug

Mullein bug stings on developing fruitlets have been reported in orchards. However, overall damage from this pest has been fairly low.

At this point in the season, damage is not a concern as most mullein bugs are adults and fruit is larger than dime- to quarter-sized. In fact, mullein bugs are very effective predators of many orchard pests so supporting these populations from this point on in the season is encouraged. A good indication mullein bugs are beginning to become predaceous is their bellies become orange or red as they feed on pests like leafcurling midge or mites (as shown in the picture below).

Plum curculio

Low levels of plum curculio damage has been found on developing fruitlets. Often movement into the orchard follows a period of sustained warm weather and rain so monitor closely for signs of damage. Management is warranted at first signs of fresh injury to the fruit (as shown below).

For more information on products with efficacy against plum curculio, see the Plum Curculio Efficacy Table on the Ontario Crop Protection Hub.

Degree (Days) of Separation

Scale

Based on current degree day accumulations, most regions of the province are reaching 1st generation San Jose scale crawler emergence (predicted emergence of 278 DDC). We are continuing to monitor several high pressure orchards and will report when crawlers have been observed.

For a full list of registered products, see San Jose Scale on the Ontario Crop Protection Hub.

Predicted emergence date of first generation San Jose scale, by region (biofix March 1st, base 10C)

Region	Current Degree Day Celsius (DDC)	Predicted Crawler Emergence (278 DDC)
Essex	391 DDC	Passed
Lambton, Middlesex, Elgin	279 DDC	Passed
Norfolk	302 DDC	Passed
Wellington	216 DDC	June 20
Niagara	292 DDC	Passed
Grey	240 DDC	June 17
Durham, Northumberland	221 DDC	June 20
Ottawa	236 DDC	June 16

Apple leafcurling midge

Apple leafcurling midge infested terminals have become more apparent in orchards this week. Larva inside are beginning to turn orange, indicating later stages of development and will soon pupate.

Adult flight for the first generation has now come to an end for all areas. Earlier regions of the province will start seeing adult flight for the second generation kicking off now. Emergence predictions for the 1st and 2nd generation are summarized below.

For a full list of registered products, see Leafcurling Midge on the Ontario Crop Protection Hub.

Predicted emergence date of first generation adult apple leafcurling midge, by region (biofix March 1st, base 9C)

Region	Current Degree Day (DDC)	5% Gen 1 (76 DDC)	50% Gen 1 (132 DDC)	95% Gen 1 (235 DDC)	5% Gen 2 (430 DDC)	50% Gen 2 (556 DDC)	95% Gen 2 (701 DDC)
Essex	455	Passed	Passed	Passed	Passed	June 20	>14 days
Lambton, Middlesex, Elgin	330	Passed	Passed	Passed	June 22	>14 days	>14 days
Norfolk	355	Passed	Passed	Passed	June 19	>14 days	>14 days
Wellington	262	Passed	Passed	Passed	>14 days	>14 days	>14 days
Niagara	343	Passed	Passed	Passed	June 20	>14 days	>14 days
Grey	283	Passed	Passed	Passed	>14 days	>14 days	>14 days
Durham, Northumberland	260	Passed	Passed	Passed	>14 days	>14 days	>14 days
Ottawa	280	Passed	Passed	Passed	>14 days	>14 days	>14 days

Want to Talk Heat?

Join us for another North American Orchard Meetup series to learn practical strategies and grower insights to tackle heat stress and maintain fruit quality – next webinar on Thursday, June 18th!

For more information and to register, click on the post below:

Heat Is On: 2026 Virtual Orchard Meetup Series