Hot temperatures and pest management

Hot temperatures can reduce the effectiveness of pest management applications. This article looks at performance of canola herbicides for weed control and insecticides for flea beetle control.


Warm, sunny days when weeds are activity growing are generally the best for herbicide activity, but hot weather reduces efficacy, especially if conditions are also dry.

Glufosinate: BASF notes that Liberty 150, the branded glufosinate product for Liberty Link canola, can perform well in warm, sunny temperatures, but temperature is not the only consideration for weed control efficacy.

On temperature, BASF says: “When temperatures rise above 27°C, plant and weed growth begins to shut down. To maintain product efficacy and crop tolerance, BASF recommends refraining from applying in-crop herbicides and fungicides above 27°C. The one exception is Liberty 150. InVigor hybrid canola is extremely tolerant to Liberty 150 and, as a contact herbicide, Liberty becomes more active as temperatures increase. If applying Liberty above 27°C, note that some additional bronzing may be observed (see the photo) but this will be transient and will not impact yield.”

On other considerations, BASF says: “We can’t consider temperature as the only environmental factor that impacts Liberty’s efficacy. Conditions such as low relative humidity, prolonged drought stress, and extreme conditions that lead to weeds ‘hardening off’ and being unable to take in Liberty can lead to reduced efficacy.”

Liberty can cause “bronzing” on canola leaves when applied on hot days.

Gyphosate: Bayer says: The optimal temperature for applying its branded Roundup WeatherMAX or Roundup Transorb HC is between 16°C and 24°C, and ideally, temperatures should remain within that range for a few hours after application. We assume this would be the same for all glyphosate. Bayer adds that both products can be applied in temperatures up to approximately 34°C without any significant impact on performance. When temperatures exceed 34°C, plants may become heat stressed, and absorption of the product into the plant tissues may be decreased. (While on the topic, when temperatures are lower than 15°C, plant growth begins to slow down, resulting in slower uptake and translocation. This can affect the rainfast period required and can slow the onset of symptoms. Roundup WeatherMAX or Roundup Transorb HC should only be applied if temperatures are forecasted to reach at least 8°C and will not drop below that temperature for a minimum of two hours after the time of application.) Crop effect from hot days: Bayer says they wouldn’t expect to see any major effects on TruFlex canola from hot temperatures.

Imazamox and Imazapyr: These are the actives for Ares (both) and Amity (just imazamox) for the Clearfield system. Corteva says: As with any systemic herbicide, high temperatures (>30°C) can stress the weeds and crop, which may reduce uptake and translocation of “imi” herbicides. Plants adapt to prolonged heat stress by thickening their cuticle and this means the adjuvant becomes especially important for maintaining herbicide performance.  Effects may be more noticeable on larger weeds due to tissue dilution.  High temperatures can also be coupled with other factors (drought, low humidity, etc) and those need to be taken into consideration when deciding when to apply a herbicide. On crop safety, Corteva says: Heat stress can impact crop safety of Ares and Amity, but generally the effects are short-lived and Corteva would not anticipate any major concerns from high temperature applications.  Following label guidelines arounds rates, adjuvant use and crop/weed staging is the best approach to ensure robust product performance and excellent crop safety.

Humidity helps. Western Canadian research suggests that high relative humidity (RH) will improve Liberty performance. The study on wild oats found that exposure to high (>95 per cent) RH as opposed to low (40 per cent) RH increased glufosinate ammonium efficacy, and it was high RH within 12 hours of spraying that was most crucial in affecting the efficacy.

Larger droplets can help. Larger droplet that evaporate more slowly will mean more herbicide enters the plant. Lab work has shown that herbicide that dries on the weed can be reawakened with humidity.

Spraying at night. Herbicide application at night is an option for some products. If you’re not sure (because some products work best in the sun), check with product manufacturers. This Canola Watch article has more, including links to studies.

Flea beetles

Flea beetles might be considered a “temperature driven engine” and warm weather will really get them actively feeding. At least to a point. Flea beetle activity is lower under extreme hot temperatures. 

James Tansey, provincial insect management specialist for the Saskatchewan Ministry of Agriculture, says activity can be tracked along a bell-shaped curve for most insects – low at low temperatures, increasing to a point, then falling off. Tansey cites Chinese research that showed decreasing activity past 26°C for striped flea beetles, and another study (Kinoshita et al. 1979) that found crucifer hatch rates to be greater at 25°C than at 30°C. “This doesn’t mean a hard stop in activity, just that their activity begins to diminish and will continue to decrease with increasing temperature,” Tansey says.

For a Canadian example, Agriculture and Agri-Food Canada (AAFC) research scientist Julie Soroka reported in 2005 that crucifer flea beetles displayed little activity and were not attracted to any traps in very hot weather. In one particular trial, during which maximum temperatures exceeded 35°C for several days, they collected relatively few beetles.

John Gavloski, provincial entomologist for Manitoba Agriculture, was asked whether flea beetles do less damage to canola when the temperatures get above 30°C. “The short answer is that we don’t know, this has never been tested,” he says. “Optimum feeding temperatures have been tested in other insects, and this does vary greatly with the insect. Many insects will feed more in the mid to high 20°C temperatures, but once temperatures get into the 30s feeding often is reduced.” Gavloski says that until a proper study is done, we can only speculate on how hot is too hot for flea beetles.”

Insecticide performance in high temperatures. Perhaps the bigger issue with hot weather is its effect on insecticide activity. Pyrethroids have restrictions for application in higher temperatures. The label for pyrethroid Decis (active ingredient deltamethrin), for example, says: “DO NOT spray under a strong temperature inversion, or when temperature exceeds 25°C as this will result in a reduction in control. Best control will be achieved when deltamethrin is applied during cooler periods of the day.” Check labels and company recommendations for other options as not all have the same specific restrictions.

Tansey says one field study showed that 12 to 72 per cent of deltamethrin volatilized from plant leaves 24 hours after application. (The wide range depends on the plant species.) Volatility is a product of temperature. This same study indicated 100 per cent evaporation after 24 hours in some cases.

This is significant because AAFC research scientist Bob Elliot found, in a study on crucifer species, that insecticides were much more toxic by ingestion (flea beetles eating sprayed leaves) than by topical application (spray hitting flea beetles directly). So we need insecticides to stick around on the plant for them to work effectively. Elliot also found that “mortality in the deltamethrin treatment was higher at 15°C than at 25°C.” This aligns with the label restriction.

A good general rule is to spray in moderate temperatures. Product applied just prior to expected active feeding, such as early morning, may have a higher amount of uptake than product applied in extreme temperatures at mid-day.

Spraying at night. Night spraying is an option if days are too hot for insecticide efficacy. Because nighttime temperatures are cooler, this can be an advantage for some insecticides, such as pyrethroids that have high-temperature restrictions. John Gavloski says night spraying does present some uncertainty with regard to the flea beetle target. “Little is known about flea beetle behaviour at night, such as how much time are they spending up on the plants, where contact with them would be easier. The insecticides should have enough residual though that I would expect good control the following day should feeding have declined over the evening.”