Dealing with Flea Beetles
Gardening with organic mulch has many benefits, but it also provides an ideal habitat for flea beetles. Here are some effective ways to deal with them.
I keep a 2,500 square foot vegetable garden that is perpetually mulched with whatever organic material I can find that will work as an effective mulch. The mulch is great for suppressing weeds, maintaining soil moisture levels, and it feeds soil organisms as it gradually breaks down - which are all great for plants. It's great stuff, but it has one drawback in that it creates an ideal habitat for flea beetles.
What are flea beetles?
Flea beetles are very small beetles. They come by their name honestly because they are the size of fleas, and they jump like fleas. They come in a range of colors (black/brown/blue/gray/striped) and can be spotted by the naked eye as little dark flecks on the foliage of plants. Flea beetle damage is easy to spot because affected plant leaves look like they have been blasted with bird-shot - such that they become full of tiny holes.
Why are they a problem?
Flea beetles eat the leaves of plants. There are many types of flea beetles that feed on many types of plants - but the worst kind feed on the cotyledon leaves of newly emerging seedlings. Plants are particularly vulnerable at this stage of growth, and flea beetle damage can either set them back, stunt them, or kill them. It is also the case that plants weakened by flea beetle damage can be vulnerable to other pests - so a combination of flea beetles, slugs and snails can be devastating, and whole crops can be lost in a matter of days if the situation is left unchecked. As plants mature, some can shrug off a little flea beetle damage and simply “soldier on” while others become infested, and the foliage - while still edible - becomes very unappetizing. All that feeding also provides the energy for the successful breeding of larger future populations of flea beetles - which is not a good thing!
What do they attack?
As stated above - there are many types of flea beetles, and some are fairly specialized in terms of the plants that they attack. Various sources indicate that, depending on the variety, they can attack melons, squash, beans, corn, sunflowers, lettuce, potatoes, tomatoes, eggplants, peppers and all crucifers (e.g. kale/cabbage/broccoli). With that having been said, in my garden, as in many, they usually focus on the cruciferous vegetables, and they also attack my potato plants- so I guess I am lucky in the broader scope of things. I've never found the damage they do to potato foliage to be a real problem, so I don't worry about it. My crucifers, on the other hand, do pose a problem, and I have lost whole crops to these tiny pests. This year, for instance, I planted a bed of Brussels sprouts - and all but one of them were wiped out within a week or so of having sprouted.
How to deal with them
Some sources suggest companion planting with things like thyme, catnip, sage, mint, hyssop, nasturtium, or basil. I have to admit that I haven’t tried all of these - but I also have a funny feeling that most of people suggesting these things haven’t either - because no gardener in their right mind would ever suggest companion planting with mint due to its invasive nature. I have tried basil, thyme and sage. While flea beetles do leave these plants alone - I’ve not seen any sign that they care if a kale plant is right next to one of them. It is also the case that it is simply not practical to have these things all over the garden. Think about it - let’s say that a catnip plant actually has an effective range of 2 feet (which I sincerely doubt) for repelling flea beetles - like a force-field. That means that you would need eight for every 4x8 bed - and of course they would need space to grow, so there would be less space for the actual vegetables for which the garden was intended.
Again, I haven’t tried all of the companion planting ideas listed above, so maybe some will work in some way - but based on scanning the internet - all I see is people saying that they work, and I don’t see any actual experiments with controls to prove it - so I think most of these are probably a waste of time for anyone with a real flea beetle problem. Perhaps next spring I should sacrifice some of my precious growing space to science and try another one of them.
Sticky traps also come highly recommended - but having used them, I’ve noticed that they only catch the few bugs that are unlucky enough to crawl on them - while the other 99% devastate my greens.
In my experience, there are six effective strategies for growing crucifers when flea beetles are present in a garden - and often the best results are achieved when they are used in combination.
Early planting: Getting things started under cover (e.g. hoop-house/dome/cold-frame), about a month ahead of when it would normally be possible is a good way to get plants through their vulnerable stages while it is still too cold for flea beetles to be active.
Transplants: Similar to the approach above, by starting plants in a pest-free environment, they can mature a bit before being subjected to flea beetles - and with a little luck, they can shrug it off in a way that requires little intervention on the part of the gardener.
Spacing: Crucifers that are planted too tightly provide a safe place for flea beetles to hide, and also makes weak plants that are more vulnerable to attack due to each plant not being able to gather optimal amounts of sun, water and nutrients. When spacing - think of the actual spread of the mature plant, and space to that diameter so that no leaves will be touching. This does a great job at preventing a lot of pests, and it also results in big healthy plants.
Beneficial Predators: It always makes sense to do whatever can be done to invite beneficial insects and other predators into the garden. If pesticides are necessary. keep them to a minimum (more on this below), and with a little luck, there might be an army of flea beetle predators in the garden some day. I’m not there yet, but I always do what I can to keep my garden hospitable to a range of pest-eating visitors.
Row covers: Fine mesh row covers are a popular suggestion for thwarting flea beetle attacks. The idea is that the mesh lets the sun and rain in, but keeps the flea beetles out. The challenge is that flea beetles are tiny and they crawl on the ground. They also lay eggs in the ground. A row cover can conceivably contend with the 1st problem - but it cannot contend with the second. Thus, a row cover can work - but this is only providing two very important conditions:
The cover is tight to the ground so that the flea beetles cannot crawl under it.
There are no flea beetle eggs and/or larvae already present in the soil from the previous season.
Pesticides: I know. The “P-word”. We all want “pure” organic gardens, where balance, soil health, symbiotic relationships, etc. result in minimal pest damage and great gardens. I think this is possible - but of course, as with everything - there are degrees of freedom. Sadly, flea beetles do not care about our noble convictions. Moreover - every single plant in your garden is already engaged in chemical warfare with pests - that’s why the flea beetles only attack some of them. Swiss chard and spinach are often pest free - and this is because they are full of oxalates - natural pesticides that plants make to deter insect attacks.
So since there is already a chemical battle happening - why not simply join in - but with a few basic principles so that the battle can still be waged within the constraints of our noble convictions:
Don’t use anything that can persist and gradually toxify the soil and surrounding environment.
Don’t use anything unless it is necessary, and don’t use anything for any longer than is necessary.
Manage - don’t eradicate. Every pest has a predator. Always leave some pests around so that their predators will set up shop in your garden. When done correctly, a healthy population of pest predators might solve the problem completely (personally I’m still waiting for this to happen with flea beetles).
Given the principles laid out above, I use something relatively benign, that breaks down quickly. My go-to is a pesticide that is a combination of potassium salts of fatty acids (insecticidal soap) and pyrethrins (natural pesticides found in chrysanthemums). Whenever I see that flea beetle damage is becoming a problem, I spray every two days, over a six-day period - during a time of day when it is warm enough for flea beetles to be active, and when I can see them on the plants. This approach works to effectively reduce the flea beetle population in my garden to levels are not a problem for my plants.
Final Thoughts
As with all pest problems, procrastination and indecision are your worst enemies. If you have a flea beetle problem, they will probably not go away without some type of intervention. Do not delay action! Start trying things until something knocks the pest population back and the plants can begin to recover and become productive. Have any other good solutions - please let me know!
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Here’s how I usually deal with flea beetles when they are out of control:
Love to read your informative articles that are well written, to the point, even fun. You are a realist and and experimentalist! Thank you