This is an easy pest to identify. If leaves of your plants have small, individual holes, you likely have flea beetles. You can usually find the adults on the leaves. They are tiny, round beetles, often black, brown or with faint stripes. They have over-developed hind legs and will “jump” away from you when you try to touch them. The jumping, and the hind legs are why they are given the common name “Flea Beetle.”
It’s also easy to identify them because they are usually limited to a few common host plants. The Nightshade family typically gets one species, whereas brassicas (including the turnip, mustard, radish side of the family) are often hosts as well.
But, take a deep breath – They really aren’t that bad.
Despite the leaf damage, you can do very little and still get the necessary control. However, there are usually two main concerns: potatoes and leafy brassica greens (like kale and arugula.) In those cases, a little more care is necessary to prevent damage making your produce unmarketable, or simply unwanted. Furthermore, in some areas, especially the Eastern US, flea beetle damage can be so quick and damaging that young plants can be killed or at least severely inhibited. While I mentioned two main host plants above, across North America there are several species and the host ranges are similarly specific, but affect other crops.
Flea beetles can have one or two generations each year. Adults overwinter under cover of leaf debris in wooded or bushy areas. Early in the spring (10C) they will begin to look for and feed on early plants in their host range. By late spring and early summer they will begin laying their eggs at the base of host plants. Those eggs hatch into larva that feed upon plant roots (or tubers) before pupating then emerging from the soil as adults.
The in-soil life stage makes these pests easy to control with bio-control:
Stratiolaelaps scimitus is an in-soil predatory mite that controls all sorts of soft-bodied plant pests. It is inexpensive, native and lives permanently in the soil. While they can spread out over the years, a top up in most crops hasn’t been necessary even 20 years after application. The added bonus is Stratiolaelaps is the industry standard control of fungus gnats, overwintering spider mites, pupating thrips, black vine root weevil, other grubs, springtails, early wireworm larva, and countless other pests.
A trial in North Carolina proved that Stratiolaelaps was a control of the larva, but they also trialed Dalotia coriaria, a predatory rove beetle. (You can also buy them here.) They found the Dalotia to be a “promising control” of both adults and larva, but the trials concluded when the farmers chose to spray chemicals for the adults. The ultimate finding, from the farmers perspective, was that adults coming in from neighbouring areas meant that spraying early in the season was still most cost effective. However, the finding from the State Extension office was that Stratiolaelaps and Dalotia were suitable controls for the pest.
You may want to follow the lead by the farmers who sprayed. Fortunately, with a small host range, you can apply chemicals to only some plants – reducing their non-target impacts. However, it is never truly worth spraying. Their natural predators include parasitic wasps which parasitize the adults. Every time you spray you kill off a population of the predators that would normally do their job and make next year better (if not the current year.)
So, how do you get early season control (in adult stage) without the use of chemicals?
Potatoes and Radishes. When you know flea beetles will find your crop, give them something else to eat. For example, if you have brassicas, plant a variety of radishes to try to attract their attention. Apply Stratiolaelaps to the soil of the radishes to target their effect instead of broadcasting the soil mites around the rest of your site. Similarly, different species of potatoes seem to be favoured. Plant a variety to similarly attract the adults. You could also spray or otherwise kill the adults on these trap plants (like with sticky cards or soapy water) but again, you’d be killing the parasitoids inside of them that may spread to other populations if left to do their job.
The Brown Lacewing, Micromus variegatus is a North American predator in both adult and larval stages and is a winter predator. It’s active all year and is actively feeding at temperatures as low as 4C. Applying these to your growing area in the fall will ensure they follow adult flea beetles to overwintering areas, reducing their adult population ahead of spring planting. This is not a “control,” since the Brown Lacewing is a generalist and will feed on over wintering aphids, and other pests and in the spring will also be interested in leaf hoppers, soft scale, mealybugs and others with a special love for aphids. But to have a predator working for you in the dark months is certainly worth considering. Also available here.
Both Beauvaria and Metarhizium should be tried against flea beetle. Both foliar expressions or drenches (for the larva) should be very effective and have limited non-target damage. Both of these are parasitic fungi that infect the soft-bodied stages of many pests. Simply through contact the fungus eventually kills the host. There are many strains or species of these two fungi and some are always present in the soil to some extent, but usually not enough to control the best. So each online to buy and apply these at a rate recommended for control.
The Bottom Line
Ultimately, this pest gets lots of attention because of the sometimes large amount of damage they can do to leaves. But unless you’re going to sell those leaves and therefore need an immediate intervention, you can simply relax since the damage usually comes and goes (new leaves come up without damage) as the season goes along. But if you need early season control, or control to ensure your tubers are ready for sale, then there should be no other first step than applying Stratiolaelaps to the soil. This will both protect the roots now, and ensure fewer adults will be present next season. Your next steps should be to control adults to whatever level of “control” you are comfortable with.
Avoid spraying and be a bit more tolerant of leaf damage. There is nothing more empowering as a gardener than doing nothing when you think you need to act and then finding out a week or two later that the problems take care of themselves.