Agronomy Update
Agronomy update: Revising lygus bug economic injury levels

The historical lygus bug economic threshold of 15 lygus bugs per 10 sweeps was established in the early 1990s on conventional canola cultivars and canola prices in the $6 to $7 range. With the development of higher-yielding and more vigorous hybrid canola, there was a need to update the economic threshold for lygus bugs in canola.

Research led by Hector Cárcamo with Agriculture and Agri-Food Canada at Lethbridge was conducted in Alberta to validate a Lygus threshold for canola. It used data collected over 23 years from 1999 to 2021 from small plots and commercial fields. The 97 study sites were in southern and south-central Alberta.

 Canola yield and lygus abundance of both adults and juveniles were collected for all trials. Canola yield in commercial field trials was obtained from combine yield monitors or weigh wagons, and in some trials, were harvested manually at two locations in each plot to provide an additional estimate of yield. Lygus sampling was conducted at late flower or early pod stage before insecticide application and within one week after application. 

Commercial field trials from 2010 to 2019 (farm study one 2010-2013; farm studies two and three 2016 – 2019) were used to help validate the lygus threshold. These trials had four blocks per farm, and each block contained large plots of sprayed and unsprayed treatments. In the sprayed plots, a pyrethroid insecticide of either lambda-cyhalothrin or deltamethrin was applied. Lygus bug abundance in these trials averaged from 15.4 to 25.5 lygus per 10 sweeps.

 In 2021 during a severe lygus outbreak, three commercial fields (farm study four) were studied where farmers left unsprayed strips to assess feeding damage. The fields were near Airdrie, Nanton and Stavely. Lygus bug abundance was approximately 220 Lygus per 10 sweeps but reached up to 1,400/10 sweeps at the Airdrie field.

 Plot trials were also conducted intermittently between 1999 and 2019. These trials typically had randomized sprayed and unsprayed treatments within a matrix of plots. Lygus bug abundance was around 58 lygus per 10 sweeps. 

In farm study one from 2010 to 2013, sprayed plots yielded 10.6 per cent higher than unsprayed plots.

 For farm study two from 2016 to 2019, data analysis identified three lygus population groupings that influenced yield. In 141 observations with less than 17 lygus per 10 sweeps, canola yield was slightly higher in unsprayed plots than in sprayed plots. For populations between 17 and 30 lygus per 10 sweeps, yield was similar between sprayed and unsprayed treatments. This suggested that canola plants compensated for lygus feeding damage. When lygus populations were greater than 30 lygus per 10 sweeps, yield declined significantly in the 45 treatments analyzed. 

In farm study three from 2017-2019, conducted in south-central Alberta (Lacombe area), there were no treatment effects as far as yield responses, suggesting that lygus are less damaging to canola in this fertile and moist black soil zone. This agrees with the observation from local canola growers that even 50 lygus per 10 sweeps did not appear to cause damage. It also confirms an earlier study from the Vegreville area in the same eco-region, which noted that yield losses were highest when lygus surpassed 50 per 10 sweeps. 

In farm study four during the 2021 severe outbreak, crop yields were significantly higher at the three sites when sprayed with an insecticide. At the field near Nanton, yield was 20 per cent higher in the sprayed treatments, while the Stavely site had 15 per cent higher yield and the field near Airdrie had 2.8 per cent higher yield. 

To determine the economic injury level (EIL), data from all plots and commercial fields (1,229 observations) were merged. Yield was modelled as a function of lygus abundance. The 2023 canola commodity price used in the calculations was $15.47/bu. ($682/tonne) and an aerial application cost of $18/ac. ($44.48/ha) was used.

 The researchers considered that the EIL and the economic threshold would be the same since spraying often takes place quickly after scouting. Linear and non-linear models suggested the EILs ranged from 8.9 lygus per 10 sweeps up to 23.2 lygus per 10 sweeps at the pod stage, which were similar to the historical threshold. But a detailed analysis of precision yield data from commercial fields found yield was protected from lygus feeding by spraying a foliar insecticide at the early pod stage only when populations exceeded 30 lygus per 10 sweeps.

 Modelling also found the threshold can rapidly rise when canola prices are in the $11.34 to $13.60/bu. ($500 to $600/tonne) range. Conversely, the threshold population would be lower when canola prices move higher than the 2023 commodity price used in these calculations.

 The study highlighted the value of on-farm yield and long-term plot data in validating decision-making tools and is now leading to the era of big data in agricultural research.  

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.