How to make the flea beetle spray decision

June 6, 2012 - Issue 15

20% flea beetle defoliation is on the left. 30% is on the right. Source: AAFC

50% defoliation

Striped flea beetles. Credit: Denis Pageau, AAFC

Striped flea beetles. Credit: Denis Pageau, AAFC

Cotyledons may show significant damage, but if the first leaves are in good shape, the flea beetle threat has subsided and a spray isn't needed.

Seed treatments will provide 3 to 4 weeks of protection for canola that emerges within a week of seeding. Scouting is still necessary early in the season to make sure the seed treatment provides enough protection. Here’s why: (1) Flea beetles need to take a bite of the plant in order to take up the insecticide, so there will be some feeding even while the seed treatment is working. In some cases, intense nibbling by a very large flea beetle population could overwhelm the seed treatment. (2) Seed treatments may wear out in slow germinating or growing crops before the plants are large enough to tolerate much feeding. And (3), the two most common seed treatment insecticides may be a bit less effective on striped flea beetles, so check which one is most common in your field. Most areas of the Prairies have both striped and crucifer flea beetle species.

Here is how to make the 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 reaches 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%.

This is what flea beetle feeding on the stem looks like.

This is what flea beetle feeding on the stem looks like.

Step 2. Look under leaves and on stems. High winds may force flea beetles 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. Stem feeding, if it’s happening on a lot of plants, has a lower control threshold than the 25% damage recommended for leaf feeding.

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.

Step 4. Scout the newest leaves. If newest leaves are 25% defoliated and flea beetles continue to feed, 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. 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 10-14 plants per square foot, growers can afford to lose a few plants without sacrificing harvest yield. But scout closely. Heavy feeding pressure can wipe out even a thick stand.

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. 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. 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. Check canola fields daily for flea beetle damage, especially if the weather is warm and if the fields are seeded near canola stubble. Keep in mind that for canola seeded in early May, seed treatment protection will be breaking down. 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 will be active up to 4 weeks.

Step 8. If necessary to spray, use only registered pesticides. Refer to product labels for proper use instructions. (See the table below.)

Click here for more information on flea beetle management.

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