8 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.

8 steps to make the right flea beetle spray decision:

Step 1. Know the spray threshold. The action threshold for flea beetles in canola crops in Canada is an average leaf area loss of 25% or more. Research to re-evaluate these thresholds found that canola with up to 25% leaf area loss will yield the same as canola with no leaf area loss at al. Yield loss starts to show after 25% leaf area loss and becomes economic (yield loss is more than the total cost of the foliar spray operation) at around 50% leaf area loss. The reason for the ‘action threshold’ for flea beetles is that leaf area loss can escalate quickly from 25% to 50% and beyond in a severe infestation.
Step 2. Assess leaf area loss.
With thresholds in mind, check plants in a number of areas of the field to get an average damage level. Scout the newest leaves. Flea beetles tend to prefer fresh new leaves. If cotyledons are chewed up but newest leaves show very little feeding, then plants may be outgrowing the threat or seed treatments may be having an effect (or both). On the other hand, if cotyledons are under 50% but first true leaves are not present, flea beetles may be nipping off fresh apical meristem – the growing point for first leaves. If the meristem is lost, these leaves could be considered 100% lost. Mark those plants and return in a day or two to see if true leaves are showing.
Step 3. Assess stem damage.
This is especially important in foul weather (wind, cool) that drives flea beetles down to leaf undersides and leaf stems. This relocation could actually make the flea beetle 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 plants don’t seem to be growing and have stem feeding damage from flea beetles, action may be required. Scouting note: Daily scouting needs to happen in areas approaching action threshold. Within two or three days, flea beetle feeding can escalate way beyond threshold, and actually wipe out whole areas of a field, especially in hot, dry and windy conditions. accompanied by hot/dry and windy conditions.
Step 4. Assess flea beetle feeding activity.
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.
Step 5. Consider the plant stand.
This should 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 counts in the high end of the recommend range (of 5 to 8 plants per square foot), growers can afford to lose a couple of plants without sacrificing harvest yield.
Step 6. Check the crop stage.
After the 4-leaf stage (4 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 4-leaf stage and some are earlier), keep scouting until most of the crop has at least 3 or 4 true leaves.
Step 7. Check canola fields frequently.
Seed treatment insecticide starts working when the seed imbibes water and it can remain active for about 3-4 weeks after that point. 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. High levels of flea beetles can overwhelm seed treatments. Flea beetles need to consume some plant tissue to get a dose of the protectant. 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 8. If spraying is required, only use pesticides registered for flea beetles 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. Specific products are limited to one application per season and a grower may want to keep these in reserve.

Further reading:

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

Subscribe to our podcast

Canola Watch