Post #G21-032: Cucumber beetles, bee-proof sticky traps, neem oil.

Posted on June 11, 2021

This post follows on a series of posts regarding cucumber beetles.  You can read the background in Post #G21-027.

(Source for photo:  University of Kentucky).


This year, I have a plan of sorts for dealing with cucumber beetles.

First, I continue to do what I did last year.  I go into the garden every morning and kill the cucumber beetles that are chowing down inside my squash blossoms.

That’s more of a vendetta than an actual plan.  Satisfying but ineffective.  I need something more systematic.

The typical recommendation for dealing with cucumber beetles is to use some sort of non-specific contact poison.  That would include Sevin (carbaryl), pyrethrin (short-lived natural compound), or permethrin (longer-lived synthetic pyrethrin analogs).

I’m not going to do that.  The main problem I have with that approach is that those substances kill more-or-less all insects that come into contact with them.  Secondarily, the short-lived ones are a lot of work, and the long-lived ones leave residues.  If I want pesticide residues in my vegetables I’ll buy them from the grocery store.

Last year I tried a pheremone-based lure and sticky trap, and I’m not going to try that again.  At least not in that raw format.  Those traps did attract cucumber beetles.  And they caught every other manner of insect as well.   They were far too non-specific for use in a small home garden.

That seemed to leave me with four plausible options.  These either attack the beetles with physical means, or rely on relatively short-lived toxins that have to be eaten by the insect in order to poison it.  These short-lived toxins have a reputation for being bee-safe when used correctly.

These options were:

  • Yellow sticky traps screened to prevent larger insects from reaching them.
  • Possibly, diatomaceous earth sprinkled on the leaves.
  • Possibly, insecticidal neem oil solution applies to the leaves of the cucumber plants.
  • Possibly, high-strength spinosad solution applied to the leaves of the cucumber plants.

I’m working through those, and I’ve come up with these additional ones:

  • Peppermint-garlic spray, to make the cucurbits harder to find.
  • Re-think the bee-proof trap using pheremone-based sticky traps or other types of traps, screened to prevent larger insects from reaching them.
  • Plant cucumbers that are resistant to bacterial wilt.

The rest of this post described what I’ve tried and not yet tried, and how that has worked out so far.

1:  The mythical cucumber beetle season and a strategy of timing your plantings.

I summarized what I could find out about cucumber beetle season and life cycle in Post #G21-030Here in Zone 7, from the gardener’s perspective, there should be three generations of beetles.  The first consists last year’s adults that  overwintered nearby.  Those beetles arrive around the end of May.  The subsequent two generations are the children and grandchildren of those adults, and should show up June/July and September/October.

But guess what?  I don’t think I’m seeing any clear and obvious end point of one generation and starting point for the next.  What I see, in my morning routine of inspecting the squash blossoms, is day after day where some beetles are present.

What I’m not seeing is any prolonged absence of cucumber beetles.  No beetle-free period between the three generations.  So while there may technically be three generations (one overwintered, and two new ones) in a typical year, I’m afraid that what I’m actually going to see is one summer-long cucumber beetle season.

My problem is that any number greater than zero is too many.  It’s not the physical damage to the plants that is the problem.  It’s that these beetles spread bacterial wilt, and that’s what killed off my cucumber crop last year.  All it takes is one feeding, by one infected beetle, and I’ll lose a cucumber plant.

This makes a hash of any strategy based on the timing of my plantings to avoid cucumber beetles.  A strategy mentioned by both Virginia Tech and University of Maryland was to plant cucumbers mid-June in order to avoid cucumber beetles.  But if I’m going to have cucumber beetles in my garden for the entire summer, my cucumbers will always be at risk of being killed by bacterial wilt.  The only solution would be to tear up what I’ve planted and re-plant with cucumbers that are highly resistant to bacterial wilt.


2:  Bee-proof sticky traps:  Total failure so far.

The construction of these is detailed in Post #G21-029.  The idea is to use a standard yellow sticky trap, but surround it with 1/8″ mesh hardware cloth to exclude bees and butterflies.

Ten of those have now been in my garden for more than a week.  I think I can confidently declare them to be a total failure for catching cucumber beetles.  I definitely still have striped and spotted cucumber beetles in my garden.  But none on my sticky traps.

On the plus side, these didn’t catch any bees, butterflies or other obvious beneficial insects.

I’m not sure why these failed.  Possibly the screening dulls the yellow color and prevents them from attracting beetles.  Possibly they need some sort of floral lure to be able to compete with the real flowers.  Possibly the cucumber beetles just won’t go through that mesh.

The verdict is that this particular setup is a failure.  That doesn’t rule out alternatives, so I will keep experimenting with these.

  • Spray-painting the screens bright yellow is quick and cheap.
  • I could try yellow bowls of soapy water with cucurbit flowers in them, covered with mesh.  That’s also quick and cheap.
  • I could buy pheremone-based sticky traps and surround those with mesh.

I’m not giving up on the concept quite yet.  But it’s clear that the combination of off-the-shelf sticky trap and mesh does not yield a functioning cucumber beetle trap.

3:  Plant varieties resistant to bacterial wilt.

I finally gave in and bought two cucumber varieties that are advertised as being highly resistant to bacterial wilt.  Those are Little Leaf and Cross Country (presumably a cross with Country Fair, another resistant variety).  I planted those in a couple of places in my garden yesterday.  We’ll see if they survive or possibly even thrive.

As a bonus, the Little Leaf cucumbers are parthenocarpic (require no pollination).  If worst comes to worst, I can grow those entirely under insect-excluding row cover.

4:  Neem:  I tried it, but I may never know if it works or not.

I have seen two common short-lived organic pesticides been mentioned for potential beetle control: Neem and spinosad.  Neither one of these is supposed to be very effective at controlling cucumber beetles.

Here are the reasons I chose to spray with neem.

First, at least one commercial supplier of cucumber seeds suggests using azidarachtin to control cucumber beetles.  That’s the insecticide in neem oil.

Second, it’s not clear that anecdotal reports of neem’s inability to control adult cucumber beetles are based on the form of neem oil that actually contains insecticide.

There are two distinct forms in which you can purchase “neem oil”.  One contains insecticide, one does not.

One form is the hydrophobic extract.  That is more-or-less vegetable oil containing only trace amounts of actual insecticides.  This product, for example, from Bonide, is just a neem-based horticultural oil.  The other is the complete or raw or pure neem oil.  That form contains a potent insecticide, azidarachtinThis product, for example, advertises its high azidarachtin content.

People seldom distinguish between these two forms of neem when they report the results of “using neem oil” to control pests.  Accordingly, when some gardeners report poor results using neem to control cucumber beetles, you don’t really know what that means.  There’s a good chance they used the more-commonly-available hydrophobic extract (the pesticide-free version), instead of raw neem oil (the pesticide-containing version).  As a result, it’s possible that raw neem oil actually does work on cucumber beetles, despite internet reports that “neem oil” does not work well.

Third, neem oil (in either form) works well to smother and kill insect eggs.  This is why I sprayed around the bases of all my cucurbits.  By reputation, that’s where the female cucumber beetles go to lay their eggs, providing the larvae with access to the cucurbit roots that they will feed on.

Fourth, raw neem oil has a reputation for being able to kill larvae in the soil when used as a soil drench.  I’m not going to do that — I’m not going to saturate the soil around my plants with neem solution — but to the extent that the eggs hatch in the presence of raw neem oil, there’s some chance it will kill the emerging larvae.

Practically speaking, there’s no good way to tell whether or not this worked.  I simply don’t know enough about what cucumber beetle numbers to expect in the absence of neem.  I don’t have a big enough garden area even to think about test and control plots.

So this is more-or-less an act of faith.  It’s also reasonably safe, cheap, and quick to do.  Ultimately, this falls under “might as well give it a try”, rather than “let me test this formally and see if it works”.

5:  Diatomaceous Earth:  Too clumsy.

And by that I mean, I’m too clumsy.  Or, at least, I’m too clumsy to apply that stuff with a blower or a shaker.  I gave it a try, but no matter how I apply it, some parts of some leaves get absolutely plastered with the stuff, other parts are bare.  And once they’re plastered, presumably there’s not a lot of photosynthesis going on.

Plausibly, I could find a better way to apply it.  I’ve heard of people mixing it with water and spraying it on.

In any case, it’s a non-specific contact poison that has to be re-applied after every rain.  I tried it on my most beaten-up cucumber.  There’s no way to know if it works.  I might give it a more serious try if nothing else works.  But for now, this is going back on the shelf.

I think the same goes for Surround (r) kaolin clay.  While that has the reputation for deterring cucumber beetles, it has to be re-applied after every rain.  And I suspect that it must reduce photosynthesis because it surely blocks some portion of the sunlight hitting the leaf.  I’ll try that if nothing else works.


6:  Mint and garlic spray, I may try this.

The theory here is that this disguises the smell of your curubits and makes it harder for the cucumber beetle to find them.  Given that we have a lot of mint growing in our yard, this one certainly seems easy enough to try.  Grind up some mint and garlic, let it steep in water for a day or so, and spray the resulting water on your plants.

I’m not betting on this one.  The wily cucumber beetle has managed to avoid everything I’ve thrown at it so far.  I’d be surprised if it would be taken in by a little bit of mint and garlic smell on top of the smell of cucurbits

I’m keeping this one on the list in case nothing else works.  Its harmless and cheap.