Things are Heating Up for Insects

Insect development relies on the accumulation of heat throughout the season to progress through their life stages, allowing their development to be tracked with the use of degree day models specific to the insect. By tracking the degree days, the timing of key activities such as adult flight and egg hatch can be predicted, facilitating scouting and pest management activities. To calculate degree days for a particular insect, there are two pieces of information that need to be known: the base temperature (the minimum temperature required for the development of that insect), and the biofix date (the date on which degree day calculations for that insect will begin). The equation used to calculate degree days for insect development is:

GDD = ((Tmax + Tmin)/2) – Tbase

In the equation, Tmax and Tmin refer to the maximum and minimum temperatures on a single day. Using this equation, the growing degree days can be calculated anywhere that has daily temperature information available, making it possible to tailor the model to a particular geographic area. Being able to customize data is important in Nova Scotia where we have a number of microclimates with sometimes significantly different weather often caused by the geography and proximity to the Atlantic Ocean or the Bay of Fundy. Environment Canada lists all of the weather stations available in Nova Scotia both past and present, and can provide the temperature data needed to calculate degree days. Choose the closest weather station logging hourly data for the current year. It is important to note that while degree day modeling can be a useful tool, development models from other geographic areas are not necessarily validated for use in Nova Scotia. Degree day modelling should be used in conjunction with, not as a replacement for, regular scouting.

European corn borer (ECB) is a pest of a number of crops in Nova Scotia. There are two types of ECB, with significantly different development schedules. The univoltine type completes one life cycle in a growing season while the bivoltine type can have two lifecycles in a growing season, though in cool summers the second generation of the bivoltine corn borer may not be able to complete its development.

European corn borer larva in a corn stalk. Photo: https://extension.entm.purdue.edu/fieldcropsipm/insects/euro-cornborer.php  

Both univoltine and bivoltine types of European corn borer use a base of 10°C and a start date of April 1st for degree day development models. As of June 21^st, 239* degree days had been accumulated in Kentville and 190 in Debert. According to the model, at 231 degree days, about 5% of pupae are emerged for univoltine types, with egg laying starting to occur around 425 degree days. For bivoltine types, about 50% of 1^st generation adults are emerged by  281 degree days, with a second generation starting their flight around 792 degree days.

For more information on European corn borer in Nova Scotia, check out Perennia’s new fact sheet!

*These numbers were generated with CIPRA software, AAFC, Saint-Jean-sur-Richelieu, Quebec.

Growing degree days and crop and pest development

I was talking to Michelle Cortens, Perennia's Tree Fruit Specialist, this morning and she mentioned something interesting...  By June 5^th, we had achieved the status of ‘coldest spring in the last 20 years on record’. I'm sure none of you are surprised to hear that.  Many crops use a 5℃ Growing Degree Day model, while many insect pests use a 10℃ Growing Degree Day model.  

Graph courtesy of Orchard Outlook.  Click here to subscribe or read past issues.

We were pretty far behind in insect degree days by about 44-49% less than average.  By comparison, the plant development heat units were at about 21-28% of average.  Some growers may be noticing this in their fields - pests that you normally expect at, say the end of May/early June showed up later, and are continuing to cause problems longer than they normally would (i.e. flea beetle).

The good news is, now that we're starting to get some heat, Growing Degree Days are starting to look more like what you'd expect at this time of year.  We're still about 13% less plant development heat units (base 5℃) compared to the 5 year-average, and 17% less compared to the 10-year average as of July 2.  We have 25% less insect development heat units compared to the 5-year average (10℃ base), and 27% less compared to the 10-year average.

Graph courtesy of Orchard Outlook.  Click here to subscribe or read past issues.