Author: Natalie Hoidal
In most of the main vegetable crop groups, there are a few insects that are exceedingly difficult to manage. As you prepare for the growing season, it's a good idea to have scouting and management plans in place for your most difficult to manage insects so that you can practice preventative tactics. I've come up with a list of the insects that I consider the most difficult to manage in each major crop group, and I hope you can use it to prepare for the 2020 season. This week we'll cover flea beetles, an early season pest of cole crops.
Tricky vegetable insects to keep an eye on in 2020: Cruciferous Flea Beetles
Flea beetles (mostly Phyllotreta cruciferae in MN cole crops) overwinter in leaf litter or hedgerows at field margins. Females emerge early in the spring and lay their eggs in the soil; eggs hatch into larvae, which feed on plant roots and pupate in the soil. We typically start to notice flea beetles when the new adult generations emerge in the spring and create pinhole like damage on leaves. While the holes are quite small, enough of them can add up to major plant damage and even plant death. Flea beetles have 1-2 generations in Minnesota, and are no longer much of a problem after mid-June.
Strategies include:
Waiting to plant: planting in June or later will allow you to avoid flea beetle damage altogether. Starting your seeds a bit early so that your transplants are larger will also help - larger plants are better able to withstand some feeding damage.
Using row cover: make sure to get your row cover out early enough, ideally right away when you plant. Row cover can be annoying to manage if you're trying to weed your fields every 5 days or so, but it very effectively keeps flea beetles off of your crops when they're small and most susceptible. Usually after your plants have been in the field for around 3 weeks, you'll no longer need to use row cover.
Trap crops have been somewhat successful on farms in Minnesota. Flea beetles tend to be most attracted to the spiciest cole crops like mustard and arugula. They'll be attracted to the tallest and earliest crops, so consider planting a trap crop a week or so before your main crop, or starting it in the greenhouse ahead of time so it's more mature at transplanting. Many Extension publications promote marigolds and green onions as repellent crops, but more recent research has shown these strategies to be ineffective. Keep in mind that if you use a trap crop, you may need to manage the flea beetles in it, usually with an insecticide. Flaming them may also be an option. To read more about cover crop trials in cruciferous flea beetles, see the following:
Physical control: Many Minnesota growers use Kaolin clay to create a physical barrier between plant leaves and flea beetles. While in some cases it can be effective, research results are very mixed and location dependent.
High tunnels: Recent work by Dr. Mary Rogers at the U of M (2019 data only - she'll have more complete results in the near future) showed that flea beetle populations were significantly lower in high tunnel grown kale, collards, and broccoli compared to field grown crops. However, cabbage worms (imported, diamondback moth, and cabbage looper) were more abundant in tunnels. Growing your brassicas in tunnels early in the year may be a good strategy to avoid flea beetles and get an early season brassica crop. This would allow you to move to field production of brassicas a little later in the year to avoid the major period of flea beetle damage and free up your high tunnels for other later season crops.
Chemical control: Insecticides are the primary control option for flea beetles. The action threshold for spraying is when 10-20% of the leaf area of your plants is damaged. While the Midwest Vegetable Production Guide For Commercial Growers is a good resource for conventional insecticides, they don't list many organic options. Spinosad has been the go-to organic product for flea beetles, but any time we use the same chemical too often we should start to worry about resistance issues. Pyrethrin products also help to reduce flea beetle populations, though not quite as well. There's been some recent work with biopesticides for cruciferous flea beetles in canola (Briar et al., 2018)* that suggests beneficial nematodes (Steinernema feltiae) may help to reduce flea beetle populations. Typically with biocontrol agents, success is very location dependent, and is also influenced by soil moisture and temperature. We have yet to learn how this system might work in Minnesota, but we're encouraged by the development of new products for organic growers!
What if you see an insect you don't recognize?
Unless you know exactly which insects you’re dealing with, it’s impossible to develop an informed management plans. A few key tools that I like to use for insect ID are:- What Insect is this? This tool is user-friendly and picture-based. It’s a good place to start if you’ve got the insect in front of you, but doesn’t always include photos of the entire lifecycle.
- What’s Wrong with my Plant? This tool is also quite user-friendly. If you’re seeing symptoms but don’t know what’s causing them, this is a great place to start.
- Veg Edge insect profiles: VegEdge has a fairly complete list of the insects that affect each major vegetable crop group in Minnesota. FruitEdge serves the same purpose for fruit crops.
*Briar et al., 2018. Potential biopesticides for crucifer flea beetle, Phyllotreta cruciferae (Coleoptera: Chrysomelidae) management under dryland canola production in Montana. Phytoparasitica. 46:247-254.
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