9 steps to make the right flea beetle spray decision

May 17, 2017 - Issue 8

While only a small percentage of canola fields tend to require flea beetle management in addition to seed treatment, all fields should be monitored to assess the potential threat. Begin monitoring right after emergence and through until at least the four-leaf stage. Seed treatment can be effective through most of this period, but slow starting or slow growing crops under intense flea beetle pressure may require a foliar spray.

This graphic represents what 25% and 50% leaf area loss looks like. If a stem is clipped, loss is 100%.

Canola seedlings are withered from flea beetle damage and dry conditions.

9 steps to make the right flea beetle spray decision:

Step 1. Assess damage. The action threshold for flea beetles in canola crops in Canada is an average leaf area loss of 25% or more. The point at which foliar insecticide provides an economic benefit could be when damage is closer to 50%, but leaf area loss can reach this stage fairly quickly when flea beetle numbers are high, they are actively feeding and damage is already at 25%.

Flea beetles have attacked the leaves and stem of early canola seedlings. Credit: Keith Gabert

Flea beetles have attacked the leaves and stem of early canola seedlings. Credit: Keith Gabert

Step 2. Look under leaves and on stems. This is especially important in foul weather. With high winds or cooler conditions, flea beetles will move off leaf tops and down to leaf undersides and leaf stems. This could actually make the situation worse, since it takes just a few bites on a stem to nip off a whole cotyledon or sever the stem. A plant with stem feeding that will kill the plant would be considered 100% lost, so that would factor into the average 25% leaf area loss threshold. As a general observational guide, if the field seems thinner than it was 2-3 days earlier and if flea beetles are the cause, action is likely required. Note that flea beetles will usually take cover in the rain, so rain will slow or even stop feeding for the time being. Rain can also help the crop more quickly recover. 5 things you need to know about flea beetles

Step 3. Assess flea beetle numbers. Are the insects still present in the field and continuing to feed? If it looks like populations are dwindling, a spray may not be necessary. If flea beetles are slow and dopey, it may mean they have ingested seed treatment insecticide and are no longer feeding. But check again to make sure. Time of day and weather can influence flea beetle activity. On rainy days, for example, flea beetles will take cover in the soil. Weather influences activity

Step 4. Scout the newest leaves. If flea beetle feeding continues and the newest leaves are 25% defoliated, then spraying is probably warranted. Look closely. Under intense pressure, flea beetles may take out first true leaves before they amount to anything. If newest leaves are growing fast and virtually untouched, spraying is probably not necessary — even if the cotyledons sustained heavy damage. This crop will likely outgrow the threat.

Step 5. Consider the plant stand. This may influence your approach to the thresholds. With a thin stand of 4 plants per square foot, for example, growers can’t afford to lose any plants and may want to take action when damage is only 20-25%. But with 7 to 10 plants per square foot, growers can afford to lose a few plants without sacrificing harvest yield.

Step 6. Check a number of areas within the field. A sampling protocol could include inspecting 20 randomly selected seedlings at each of 10 sites in a field. Mark specific locations in the field to revisit the same affected plants. This may increase your confidence in the flea beetle damage assessment. Sites should include field edges, within-field locations, and topographic features such as tree lines, hedgerows, and bluffs. Note that checking field edges is not enough. Flea beetles, especially earlier in the season, tend to be more plentiful at field edges, particularly edges beside fields that were in canola the year before. Spot spraying field margins may be an option if damage is primarily in that part of the field, but one nice day may be enough to spread them across the field.

Step 7. Check the crop stage. After the 4-leaf stage, the threat is likely over because the crop usually has enough plant material to feed flea beetles without compromising growth. If the crop is uneven, and some is at the 4-leaf stage and the rest is earlier, keep scouting until most of the crop has passed that stage.

Step 8. Check canola fields frequently. Seed treatment insecticide starts working right after seeding. If you have enough moisture for seed to imbibe and germinate, you have enough to solubilize the active ingredient and allow it to move into the plant. Once that happens, high rate seed treatments should be active up to 4 weeks. However, high levels of flea beetles can overwhelm seed treatments. And in a slow developing crop, seed treatments can lose their protective capacities before the crop reaches the 4-leaf stage. Scouting often during these first three weeks, and if levels are building and are close to thresholds, daily checking for a few days may be required.

Step 9. If necessary to spray, use only pesticides registered for control of the pest in canola. Refer to product labels for proper use instructions. No pre-harvest interval concerns exist at this stage, but this could be a deciding factor on which product to use later in the season to avoid repeated use.

Further reading:

Five things you need to know about flea beetles
Canola Watch library of flea beetle articles
Canola Encyclopedia chapter on flea beetles

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