Are those flea beetles worth spraying?

May 31, 2017 - Issue 10

A few flea beetles in a field are not worth the time and investment to spray. They will not cause economic losses. Only when defoliation reaches 25% across the field and feeding pressure continues does it make sense to spray for flea beetles.

Seeing a few flea beetles is not a call to action, but it does suggest that regular scouting is necessary.

Most flea beetles found in canola in Western Canada are either crucifer or striped species. Each is 2-3 mm long, but crucifer are all black and striped have two cream-coloured stripes down their back. Striped flea beetle populations are rising in some regions, so look to see which species are present.

The two species have a couple of important differences when it comes to management:
—Striped adults emerge first each spring, one to 4 weeks earlier than crucifer adults. Peak emergence of crucifer flea beetles occurs when ground temperatures reach 15°C.
—Striped flea beetles can be more tolerant of insecticide seed treatments. Flea beetles need to ingest treated plants in order to take in the pesticide, so some feeding will occur with treated seed. Don’t panic at a few bites, but if feeding is overtaking the plant, a foliar insecticide application may be required. High-rate seed treatments will be active for about 4 to 5 weeks after seeding.

The decision to spray for flea beetles rests on 5 field observations:

1. Assess the level of damage

The economic threshold for flea beetles is when canola plants over a wide area of the field have 50% leaf defoliation. However, entomologists developed a 25% “action threshold” because under intense flea beetle feeding, a crop can go from 25% to 50% leaf area loss in a short time.

To assess damage, check 20 plants at 10 sites throughout the field. Flea beetles tend to move into a field from field edges, so check the middle of the field to see how widespread they are. What does 25% and 50% defoliation look like?

2. Look under leaves and on stems

Leaves are the feeding site of choice, but if cool conditions force flea beetles to move down and feed on stems, a little feeding can be fatal to the seedling. If flea beetles are actively feeding on stems, action may be required — even if leaf defoliation is below 25%.

3. Assess the plant stand

The economic threshold is based on a stand of 7 to 14 plants per square foot. In thinner stands, growers may want to reduce the action threshold somewhat. At 4 to 5 plants per square foot, growers can’t afford to lose very many.

4. Look at the newest leaves

If newest leaves are growing fast and virtually untouched, spraying is probably not necessary — even if cotyledons sustain heavy damage. The flea beetle threat has likely subsided. If newest leaves are 25% defoliated and flea beetles continue to feed, then spraying may be warranted.

5. Check the crop stage

After the 4-leaf stage, canola plants are established and can outgrow flea beetle feeding without economic loss. If the crop is uneven, keep scouting until most of the crop has passed the 4-leaf stage. Note that heavy feeding may be concentrated on volunteers. If some plants are heavily damaged and others are not, check to see if those plants are growing between the rows. They may be volunteers and won’t have seed treatment.

Canola Watch