How to assess leaf area loss for flea beetle scouting

June 3, 2020 – Issue 12

The action threshold for flea beetles in canola crops in Canada is an average leaf area loss of 25 per cent or more. How do you make that assessment?

First, figure out how to estimate 25 per cent loss on one plant. Use the graphics below as guides. This isn’t a perfect science, so do your best and go from there. CCC agronomy director Clint Jurke says, “It seems that our brains are wired well to estimate low percentages (<10 per cent) and high percentages (>90 per cent), and not very good at the numbers in between. I recall when taking my epidemiology course, that it took a lot of training to be able to estimate leaf rusts and leaf area with accuracy and repeatability.” An important note when assessing loss: A plant that has serious (killing) levels of stem feeding can be counted as 100 per cent.

Flea beetle feeding damage scale on canola cotyledons. Source: AAFC (We’re looking for a better version.)
This is what 20-30% defoliation looks like. In this case, flea beetles were the cause but the same scale works for diamondback larvae.
This graphic represents what 25% and 50% leaf area loss looks like. If a stem is clipped, loss is 100%.

Once you have an eye for what 25 per cent looks like, estimate the average leaf area loss for 10 plants in the immediate area. Toss your plant counting hoop or anything (like a wrench) to find a random scout spot. Assess 10 plants closest to where it lands, and compute the average leaf area loss per plant. Note that flea beetles will often enter a field from the edges, so edges will give skewed results. Walk toward the middle of the field to find a start point for your first assessment.

Finally, check a few places throughout the field. The usual scouting technique is to make assessments at five points, in a “W” pattern. Check 10 plants at each point of the “W”. Average the results. Is that average over 25 per cent?

If yes, consider spraying if:

Flea beetles are still present in the field and continuing to feed. If it looks like populations are dwindling, an application may not be necessary. If flea beetles are slow and lethargic, it may mean they have ingested seed treatment insecticide and are no longer feeding. If new tissue and young leaves are free of damage, beetles are no longer feeding, and the seed treatments have done their job.

Note that time of day and weather can influence flea beetle activity. On rainy days, for example, flea beetles will usually take cover, so rain will slow or even stop feeding for the time being. Rain can also help the crop more quickly recover. Cool and windy weather will drive the flea beetles lower to the ground and may result in stem feeding and/or reduced feeding. Insects are completely temperature driven. Warm weather may speed up beetle feeding more than crop growth increases, so scout more frequently in warm weather.

–You can’t afford to lose any plants. With a thin stand of four or fewer plants per square foot, growers can’t afford to lose any plants and may want to take action quickly as damage approaches 25 per cent leaf area loss. But with plant counts in the high end of the recommend range (of five to eight plants per square foot), growers may decide they can afford to lose a couple of plants without sacrificing harvest yield.

–Check the crop stage. After the four-leaf stage (four true leaves), the threat is likely over because the crop usually has enough plant material to feed flea beetles without compromising growth, and the plants can compensate for feeding better by this stage. If the crop is uneven (some plants are at the four-leaf stage and some are earlier), keep scouting until most of the crop has at least three or four true leaves. Again, if new material and leaves are emerging with little or no damage, beetle feeding has ended and no foliar application is warranted.

High risk situation: A thin crop, struggling to grow in hot weather and dry soil conditions may have all three of the risk factors listed above. And since seed treatment insecticide may be wearing out, this crop will require frequent scouting and a potential foliar insecticide.

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