One bertha armyworm per square metre can reduce canola yield by 0.058 bu./ac. So, 17 of them per square metre will drop yields by around 1 bu./ac. This information helps growers assess the threat and spray only when numbers are sufficient to provide a return on investment. How do we know these thresholds? Research.

Insect thresholds drive profitable pest decisions

By Taryn Dickson

Published on 20 November, 2017

They vary from insect to insect, and they help canola growers decide if an insecticide application is necessary or not. What are they? Insect thresholds.

These economic thresholds are an important consideration every growing season. For farmers and agronomists, it’s worth knowing where they came from and how they might change over time.

The “Holy Grail” of insect thresholds for canola and other oilseeds is the Western Committee on Crop Pests Guide to Integrated Control of Insect Pests of Crops. You can find it online at westernforum.org under “Control Guidelines”. The guide is reviewed and updated annually. Manitoba Agriculture entomologist John Gavloski provided the 2017 update.

Find all previous Science Editions online at canoladigest.ca
and more on research discoveries at canolaresearch.ca

“The basic research behind a number of insects and their impact on the plant has stayed the same over time,” Gavloski says. Hybrid varieties yield much higher now than varieties did decades ago, seeding rates have changed over time and the number of canola fields across the Prairies
has grown dramatically, but the insects are still feeding
the same way.

Economic or nominal

Whenever discussing insect thresholds, Gavloski cautions that the type of threshold must be clearly distinguished: nominal or economic. Many of the thresholds used are nominal thresholds, which are based on experience rather than research to quantify the impact of the insects on crops.

Examples of nominal thresholds are those for diamondback moth (DBM) and flea beetles. The current nominal threshold for DBM is around 20-30 larvae per square foot in plants with flowers and buds, but Gavloski suggests that research could make this threshold more precise and should be more crop-stage specific in order to make it more robust.

Lygus has a research-derived economic threshold: when flowering is complete, canola fields can experience yield loss of 0.1235 bu./ac. for each lygus bug per 10 sweeps. New research is seeing if this yield loss per lygus should be lowered for good growing conditions, which means growers and agronomists may want to check for threshold updates
each year.

Examples of economic research-based thresholds are those for bertha armyworm (BAW) and lygus. True economic thresholds will vary with costs, so simply revising production costs will update the values.

An economic threshold doesn’t mean all spray decisions will be straight-forward, though. For example, BAW has an economic threshold, but since their cycles are driven by weather (cold temperatures combined with little snow-cover can kill overwintering pupae) and natural enemies (predators, parasitoids and pathogens), their populations are influenced by more than just access to food and crop stage.

Another complication to consider is the multiple species of some pests, such as flea beetles which now have populations of crucifer, striped and hop species.

Farmer perspective

Katelyn Duncan, who farms with her sister, brother and dad near Regina, Saskatchewan, says they consider crop potential when making insect spray decisions.

“In addition to the damage that can be caused, we factor in yield potential,” Duncan says. “If the field has higher yield potential, then we want to protect that and we’ll be more likely to spray. We always look at the economics.”

Fortunately, the industry is becoming much more familiar with thresholds and how to use them to make better decisions. Insect identification and threshold materials are available in print form and online from the provincial agriculture departments, life science companies, local retails and various organizations, including the CCC.

“Working with a qualified and trusted agronomist is important but isn’t a complete replacement for scouting on your own farm,” says Duncan, “especially when we see spray planes out on neighbouring canola fields. Then we definitely want to double-check the sample numbers and threshold values to make sure we’re making the right decision.”

This schedule shows when to scout for each insect in canola. Symbols beside each insect name indicate how to scout. For more information on this table, download the Canola Insect Scouting Guide at canolacouncil.org

Insect monitoring

Provincial governments, with help from CCC agronomy specialists, conduct annual insect surveys to provide a fairly comprehensive picture of insect populations in each region. The Prairie Pest Monitoring Network shares weekly updates on population predictions and relevant information (such as the wind trajectories that could be bringing up DBM populations) and the Alberta Insect Pest Monitoring Network has survey results, forecasts and real-time maps for specific pest insects.

Growers and agronomists can also get timely advice at canolawatch.org, and meet the experts at workshops and meetings such as the provincial crop diagnostic schools and the canoLAB and canolaPALOOZA events that the CCC puts on with Alberta Canola, SaskCanola and Manitoba Canola Growers.

Social media also makes the sharing of information and diagnosis of insect issues quicker and easier than ever, although users should always consider the source.

“A farmer can ask a question on Twitter and very quickly get an answer or be referred to a credible resource with the answer,” Duncan explained. “It’s pretty handy and incredible valuable for farmers to have a network of trusted sources at their finger tips.”

Insect research continues

Research into insect monitoring and to update or create economic thresholds remains necessary. Hector Carcamo, research scientist at AAFC Lethbridge, is part of a team currently working to validate or redefine the 25 per cent damage threshold for flea beetles and to update those for lygus bugs in canola. This will be a great asset to canola farmers (especially with suspicions that current lygus economic thresholds are too low).

The pest-reducing impact from beneficial insects is also being researched. Although quantifying the effect that beneficial insects have on insect pest populations is very challenging, it would add value to thresholds once completed.

The bottom line with all of these thresholds is to help farmers realize that some insect feeding is acceptable, and that only when numbers or levels of damage reach a certain point will the time and cost to spray provide a payback.

Research Pays Dividends

Danny Dion | Donnelly, Alberta

On our farm, Dion Brothers Ltd, which I run with my brother, we have made good use of a few Smoky Applied Research and Demonstration Association (SARDA) projects that have received grower-funding through Alberta Canola. Through these local fertility trials, we learned how far we can push modern hybrids for our climate. Knowledge gained through this research has helped us make better fertilizer-rate decisions, which have made a big difference to the bottom line for our farm. We also help SARDA with variety trials. We get to know how certain varieties will help with our harvest time management. By choosing a combination of early- and later-maturing varieties that are suited to our area, we can spread out our harvest and make better use of labour and machinery.

Tracy Popp | Selkirk, Manitoba

Flea beetles have become a particular bad pest in our area. We had to reseed 300 acres in 2015 because flea beetles wiped them out. Everything looked fine in the field, but when we came back three or four days later, there was nothing left. Seed treatments are important, but we need more management tools. Grower-funded research led by Alejandro Costamagna at the University of Manitoba is looking at integrated approaches that include prediction models, true economic thresholds and landscape effects – all of which should improve our flea beetle management and improve canola profitability in the face of this major insect pest. I will also add that in my area, canola acres in general are under pressure from soybeans and corn. Going forward, canola growers will have to invest in whatever research we need to keep canola in the rotation.

Thresholds for Common canola pests

Alfalfa looper: No thresholds. Use bertha armyworm levels for a guideline.

Aphids: Nominal. Control if densities exceed an average of 25 aphids per 10 cm of shoot tip after flowering. Find this average from a sample of 20 or more shoots.

Bertha armyworm: Economic. A loss in canola of 0.058 bu/acre for each larvae per square metre. Use the thresholds table from the bertha armyworm chapter at canolaencyclopedia.ca.

Cabbage seedpod weevil: Nominal. Use a sweep net. The threshold is 3-4 adult weevils per 180° sweep.

Diamondback moth: Nominal. 100-150 larvae per square metre in immature to flowering plants, and 200-300 per square metre in plants with flowers and pods. If larvae are feeding on seedlings, follow the flea beetle thresholds as a guide.

Flea beetles: Nominal. If seed treatment is not providing enough protection, consider a foliar spray if average leaf area loss exceeds 25 per cent and flea beetles are still present and feeding.

Flea beetles can return later in the season to feed on maturing canola plants but are rarely an economic concern.

Grasshoppers: Nominal. 8-12 per square metre in canola.

Lygus: Economic. At pod ripening, a loss of 0.0882 bu./ac. for each lygus bug per 10 sweeps (with a 38 cm diameter sweep net and a minimum of 15 samples per field). When flowering is complete, a loss of 0.1235 bu./ac. for each lygus bug per 10 sweeps. Use the thresholds table from the lygus chapter at canolaencyclopedia.ca.

Cutworm: Nominal. Thresholds vary by species and by crop. Redbacked and pale western threshold for canola is 4-5 larvae per square metre.

Red turnip beetle: No thresholds.

Root maggot: No thresholds and no insecticides are registered for control of root maggots in canola.

Swede midge: No thresholds for Western Canada, but research is underway.

Wireworm: No thresholds for canola in Western Canada.

To assist you on your annual scouting schedule and method of scouting for each insect, download the CCC Insect Scouting Guide at canolacouncil.org and read the insect articles at canolawatch.org.