Post #G10: Squash Vine Borer: Thinking through neem and considering horticultural oil?

Posted on July 15, 2020

This is a two-part followup to my prior posting on the squash vine borer (SVB).  Here, I’m finalizing my strategy for dealing with this pest.

I’ll try to avoid my usual TLDR style and get to the point.  A later section adds more detail.

I’m not going to try to get systemic protection using a neem “soil drench”.  I am going to use neem oil as a horticultural oil spray, hoping to smother the eggs.

Details follow.

What’s my problem?

The problem you face with SVB depends on how surprised you were by this.  At one end of the spectrum, some people first find out there’s a problem when their squash plants start rapidly dying.  At the other end are old hands who anticipated this and set up (e.g.) moth traps and stem wrapping well ahead of SVB arrival.

I’m in the middle.  For me, the SVB is a tough problem because a) the larvae emerge continuously over a period of weeks, but b) each critter is only vulnerable for (maybe) a few hours.  The upshot is that I have to keep some sort of poison on the stems of my squash plants, continuously, for a period of weeks.*

* I could not find a source that would state how long the typical SVB season is around here.  Conventional wisdom says it’s pretty short, so that it’s safe to plant squash now, for fall harvest.  As of yesterday, the SVB moth had been in my garden for 10 days.  Given the variable time to egg hatch-out (8 – 14 days), that means I need to keep poison on the stems of my squash, continuously, for 16 days.  So far.

Some possible solutions

Of the options available, I chose spinosad, using a concentrate that mixes up to be a 0.008% solution (or about 8 times stronger than pre-mixed Captain Jack’s Deadbug Juice).  It’s a moderately-persistent poison.  It’s known to kill the larvae, to some degree.  It’s reasonably bee-safe if applied with sense.  It requires spraying the stems on-order-of once a week.

I didn’t go with BT due to the short lifetime.  My understanding is that the half-life of BT toxin exposed to sunlight can be as short as one day.  But you really can’t quite tell.  So trying to use BT to generate the needed weeks of continuous coverage meant a) a lot of spraying, b) some guesswork as to how often you needed to spray (due to the uncertain half-life of the toxin).  Plus, I couldn’t find it locally, and I could not find any formal test of its effectiveness in this situation.  (But that’s probably more because commercial growers likely don’t use it for this.  So the lack of formal testing in this use doesn’t mean it doesn’t work, it just means nobody bothered to do a formal test of it.)

Now I’m getting ready to dump some neem at the base of my squash plants, in the hopes they’ll absorb it through their roots, and I’ll get systemic (in-the-plant-tissue) insecticidal protection.  The idea is to take care of any larvae that make it past the spinosad.  I’m having some second thoughts on that.  That’s discussed in a section below.

But this morning, it occurred to me that maybe I’m missing something obvious.  Can I kill SVB eggs with horticultural oil? 

So that’s the main point of this posting.  Is that a crazy idea or not?

Well, can I?

First off, near as I can tell, absolutely nobody mentions doing this.  But that may well be because no commercial grower would (or could) use horticultural oil on squash plants.  Indiscriminate use of horticultural oil, in the heat of summer, on delicate squash leaves, is almost certainly a recipe for killing your squash plants.  Plus, you can get toxic interactions with other pesticides, particularly those containing sulfur.

There’s a reason these oils were known historically as “dormant oil”.  You would only spray them when the plant was dormant.  I think that’s because they are just about as good at smothering your plants as they are at smothering insects.  So spraying a heavy oil, on a plant, in growing season, would kill it.  Subsequently, lighter oils were developed that could, in some limited circumstances, be sprayed on plant foliage in growing season without killing the plant.

In short, this is not something to jump into with both feet.  This is something to test, then proceed.

But I sure would like to kill those SVB eggs before they hatch.  And, for sure, horticultural oil is marketed as being effective at killing (e.g.) caterpillar eggs.  And the reported mode of action is that they smother them by filling the pores that the eggs need to respire.

So let me take this a step at a time.

First, horticultural oil is just that:  oil.  A variety of oils have been used, and they may contain surfactants and such to help them stick.  Some formulations will have (e.g.) fungicides or insecticides mixed in.  But in a basic horticultural oil spray, there’s no chemical pesticide there.  It’s just oil.

Second, it will kill some types of insect eggs.  No less an authority than the Cornell University Cooperative Extension Service (.pdf) says so.  Caterpillar eggs are specifically mentioned, but it will not kill (e.g.) gypsy moth egg masses.  But I’m guessing that these one-at-a-time, fully-exposed SVB eggs are plausible targets.

Third, I can find no direct information on how quickly it kills insect eggs, or on just exactly which eggs it kills.  That said, the oil only works for the time it takes to evaporate.  Every source seems to indicate that these oils evaporate fairly quickly, particularly lighter oils for using during the growing season.  From that, I have to infer that these product work fairly quickly to smother insect eggs.

Fourth, I really don’t need to spray the leaves.  And I don’t intend to.  I only really need to spray the first couple of feet of the main stem.  And the soil directly around the main stem.  Sure, I’d like to hose down the plants, just to be sure.  But I think I’ll address the vast majority of SVB eggs by targeting stems.  And stems don’t respire the way leaves do.

Fifth, if it works, one spraying will kill all eggs present at that time.  So this doesn’t have to be present for that narrow window of opportunity between when the egg hatches and when the insect burrows into the stem.

So, at first blush, this isn’t quite as crazy as it seems.  As a home gardener, I can target the oil to the stems only, something that a commercial grower would find difficult (or not cost-effective) to do.  Plausibly, by not spraying the leaves, I avoid most of the risk of damage.  I also note that neem oil can be used as a horticultural oil, although petroleum-derived oils appear to be much better at smothering insects than natural plant-derived oils.  My point being that (by report) people routinely spray neem oil on their squash leaves.  So some applications of some light oils appear safe.  Ish.

So I believe I’ll try this in some limited way, and if no harm is evident, I’ll apply horticultural oil to the stems of my cucurbits.

Neem oil for systemic protection.

First, to be clear, by systemic protection I mean that the insect poison in neem oil (azadirachtin) is absorbed by a plant, to the extent that it makes the plant itself toxic to insects that eat it.

Here, I’m trying to figure all that out.  Does neem in fact act as a systemic poison, will it do it for cucurbits, how much do I need, how long does it persist, do I really want to eat the fruit containing that systemic poison, and so on.  I’m just trying to treat neem as the poison that it is, and not just wing it and hope for good results.

This is the best reference I’ve found so far on neem oil.  For example, it distinguishes the actual main insect poison (azadirachtin) from clarified hydrophobic extract of neem (sold as neem oil), which is merely a horticultural oil without the poison.  If you buy “70% neem oil” that is clarified hydrophobic extract of neem, based on this reference, you haven’t actually bought the relevant poison (azadirachtin) at all.  FWIW, I was completely unaware of that before I read this reference.  Perhaps other useful compounds are in that hydrophobic extract, but the main insecticidal component of neem is not one of them.

Important aside:  The one glowing case report that I read about neem and SVB was from a gardener who sprayed her plants regularly before the SVB showed up.  She claimed that this provided systemic protection.  But absorption through leaves is poor, and the only recommended method for providing systemic protection is via soil drench.  But now that I know that neem oil is in fact used as a horticultural oil — in the 70% hydrophic extract form — I wonder if what she actually did was smother the eggs with repeated applications of light horticultural oil.  Which in this case, just happened to be neem oil? 

With that clarified, it turns out, a) there’s a term for what I plan to do (“soil drench”), b) neem is in fact used that way, and c) neem doesn’t kill earthworms.  (I’m saying that all without citation as to source, but any Google search will reveal these facts.)  I think I’ve already established that neem is fairly bee-safe as well.

I am really not all that keen on applying a bunch of pesticide to the soil of my garden.  Even something seemingly as benign as neem.  I guess it’s some comfort to know that some people routinely do this?  And, given the options, putting the neem at the roots seems a lot smarter than spraying it all over the leaves.  My problem isn’t with bugs on the leaves, and so putting pesticides on the squash leaves seems like a much worse idea than doing a soil drench.

I’m a little uncertain as to a) whether or not this will provide systemic protection, and b) whether or not I want to eat the squash if it does. 

Will neem actually provide systemic protection?  That depends.  Apparently, it depends on soil ph and on the plant in question (per the best reference already cited above).  So, this is one of those things that everybody knows is true, that may or may not be.  You can see tons of amateur gardeners saying that soil drench, absorbed through the roots, or spraying it on the leaves, yields systemic insecticidal protection as neem is absorbed into the plant.  You can see reputable organizations (here, Oregon State U) saying that the main component does in fact act as a systemic insecticide absorbed through the roots. But in the scholarly literature, where this was rigorously tested, the results are not so clear-cut.  E.g., this one, where there was no systemic effect.  Here’s one where the necessary dose of neem, as a soil drench, varied 10-fold across plant species and insect pests.  Here’s another controlled trial where it definitely worked as a systemic insecticide in bean plants.

This Ph.D. thesis sites one instance where neem soil drench provided some control of thrips on cucumbers.  So at least I can be assured that it will act as a systemic poison in cucurbits.  But, again, efficacy varied by the exact plant and conditions studied.

I really hate doing gardening by-gosh-and-by-golly, particularly when it comes to use of insecticides.  But that seems to be the case here.  I know that, at some concentration, in some circumstances, some cucurbits will pick up enough poison from the soil to have some systemic insecticidal effect.  On some bugs.  I’m not sure that’s enough to convince me to go ahead with trying a neem soil drench in the hopes of getting systemic protection for my cucurbits.

And then there’s the issue of eating the squash, assuming this does work to provide systemic insecticidal protection.  Yeah, neem is thought to be about as safe as it gets.  And they put it into soaps and shampoos and so on.  But … it still leaves me a little uneasy to chow down on zucchini that I believe to have insect-lethal doses of insecticide inside the zucchini itself.  I could just kill the budding squash for a while, until such time as I thought the neem was mostly gone.  But I have no way to test for that.

All-in-all, this is not looking as promising as I had hoped.  I don’t know the dose, I don’t know the effectiveness, I’m a little scared of potential human toxicity issues in the resulting fruit.  I don’t think I’m going to do a neem drench.  I may use neem as a horticultural oil, hoping to kill the eggs.  But there are too many unknowns for me just to toss a whole lot of it into the soil and hope for a good outcome.

Addendum:  Neem anecdote:

From this source:

“”I use neem on everything,” he wrote. “Had squash get vine borers last year, sprayed neem oil into the stalk and the critters backed out. The plants not only survived but produced, too.”

I’m definitely putting that one in my back pocket.