What's the deal with late season flea beetles?

Author: Natalie Hoidal, UMN Extension Educator, Local Foods and Vegetable Production

Typically we think of flea beetles as very tricky early season pests that start to wind down around mid-June. However, I've been receiving consistent reports of flea beetles throughout the summer, and even now in mid-August, growers are seeing substantial flea beetle damage on their most recent successions of cole crops.

Flea beetles, NH

What's going on?

 Unfortunately, we don't have a great answer at this point. There are typically only one to two generations per year, and with the amount of heat we've had, we should have experienced enough degree days / accumulated heat units for them to complete those generations.

If you'd like to help us solve the puzzle, and you're currently seeing flea beetle damage, please reach out to me at hoida016@umn.edu. We're interested in collecting some flea beetles to ID them, since we've seen photos of what look like a couple of different species.

What can you do at this point in the season? 

At this point, most cole crop successions are in the field already. IF you are planting any more successions of kale or quick-growing varieties, it is worth considering a couple of weeks of row cover while the plants establish their root systems, as well as a trap crop. While we typically just recommend a trap crop in early Spring, I am leaning towards recommending that growers start to plant a trap crop of arugula or mustard anytime you are planting a new succession of high-value Brassicas. You can keep it in the ground for a few weeks to allow the flea beetles to accumulate, treat the flea beetles, and then destroy it. All of the farms I saw this summer where there was a trap crop had excellent cabbage, broccoli, and cauliflower with minimal damage, even when the trap crop was full of flea beetles and holes. 

For already established successions, insecticides are your best bet at this point. For organic options, research studies consistently show that Neem and Pyrethrin products are NOT effective; spinosad remains the most effective OMRI approved option. Kaolin clay has very mixed results. For conventional growers, the Midwest Vegetable Production Guide for Commercial growers (page 114) has a long list of product suggestions.

Since flea beetles overwinter in hedge rows, woody areas, and grassy areas, your options for preventing overwintering are limited. However, if you currently have high populations of flea beetles, anticipate higher populations in the year to come and develop a management plan before next season begins. This plan should include a combination of row cover, planting timing, potentially using a high tunnel for the earliest successions, trap crops, possibly a repellent crop, and insecticides if necessary.