Source: University of Kentucky
I saw my first striped cucumber beetle of the year, at the end of last week. It feels like it’s too early in the season, but I’m reasonably sure I didn’t hallucinate it. Last year, they showed up in my garden at the end of May (Post #G21-027, Cucumber Beetles). So they’re right on time.
Those little b@stards utterly defeated me last year. They spread bacterial wilt, and that kills cucumber plants. I tried “wilt resistant” cucumber varieties, but none survived. I tried a program of suppressing the beetles by squishing a few every morning. That was satisfying but ineffective. I tried (and failed) to gin up some bee- and butterfly-safe sticky traps as yet a different pesticide-free approach to controlling them (Post #G21-032).
I gave up. Near as I could tell, the only effective solutions to cucumber beetle infestation involve use of highly toxic and non-selective insecticides (e.g., Sevin dust, Bonide dust). I didn’t want to go in that direction. And, really, if I wanted pesticide residue on my produce, I’d just buy my vegetables at Safeway.
A rocky start for the 2022 cucumber season.
This year I’m going in yet a different pesticide-free direction: I’m growing cucumbers under insect-proof netting. In theory, that physical barrier will keep the cucumber beetles off my plants. But it also keeps the bees away. Which is why I’m growing parthenocarpic varieties this year, meaning, cucumber varieties that will set fruit without pollination.
So far, that’s been something of a series of failures and recoveries.
First, my seed starts failed. What I thought was a poor germination rate turned out to bird damage, with birds having pecked all the seeds out of the peat pellets in which they were growing. The upshot was that I provided my backyard birds with the worlds most expensive birdseed.
I re-planted and kept the seed starts indoors. Lo and behold, I had a 100% germination rate.
Second, I decided to do a neem oil soil drench on the beds that I was setting up for the parthenocarpic plants. The idea is that if there were any eggs or papae of the relevant insect pests in the soil, I wanted to kill those off before I constructed my “insect-proof” bed on top of that. Otherwise, they’ll just emerge from the soil and eat my plants.
That turned out to be — probably — not a good idea. Almost certainly not an effective idea. But maybe, mostly harmless. I discuss the neem oil soil drench below.
Finally, on top of the neem oil soil drench, I decided to use black woven plastic as the ground cover. This would cover the entire top of the bed, except where I melted holes through it to plant my seedlings. The idea is that if insects tried to emerge from the bed, they’d get trapped against that plastic surface and, ideally, perish there.
Source: Amazon.com
I had my doubts about using this stuff, but I seem to see people using this in just about every YouTube gardening video I watch. And commercial farming operations sometimes use it. It seemed like the thing to do, both to keep insects from emerging from the soil, and to keep from having to do weeding within that insect-proof bed.
If you find yourself saying “doesn’t that black plastic get hot”, the answer is, yes it does. So that when I planted my already-vining cucumber starts in it, it managed to fry those plants over the course of one day, even with white floating row cover on top to keep the bugs off.
I guess that black woven plastic works well enough for plants that are fairly upright. But in a warm southern climate, it’s a huge mistake for anything that will actually recline onto the plastic at a relatively young (and shade-free) age. It just gets too hot. (Which, I guess, should have been obvious, but nobody on YouTube seemed to mention that.)
I replanted with yet more parthencarpic cucumber seed starts, placed a mulch of straw on top of the black plastic, covered the entire mess with floating row cover. And tried to get on with my life.
Neem oil soil drench: By-gosh-and-by-golly insecticidal treatment.
Based on the number of bugs I had last year, there’s a good chance that I have eggs or larvae for cucumber beetles and squash vine borers in my soil. So, following yet more internet-based advice, I decided that it would be wise to do a neem oil soil drench in these beds, before planting the parthenocarpic cukes and summer squash. In theory, this will kill most of the insect eggs and larvae in the soil in these two small beds without harming (e.g.) earthworm populations.
(People use a neem oil soil drench for two different uses. One is to kill certain types of immature insects in the soil. That’s my intent here. Others use it under the theory that plants will absorb it, and it then becomes a systemic insecticide within the plant, so that insects feeding on the plant will die. That’s not my intent, and a single dose of it is unlikely to provide any systemic insecticidal effect because the active ingredient (azidarachtin) breaks down quickly in soil.)
On the down side, this dosage and application rate for this step are determined by guesswork, near as I can tell. Commercial farmers don’t, in general, use neem oil, or use it in this particular way. And because of that, a) there’s almost no systematic research on effectiveness, and b) there’s almost no systematic guidance on the right dosage.
As a result, if you read three different neem oil soil drench recipes, you will see three different ratios of neem, soap, and water. Worse, none of them will tell you how much of the resulting neem/water mixture should be applied per square foot of garden. The result is that in terms of the true bottom line — grams of insecticide (azidarachtin) per square meter of soil — the popular gardening literature gives you literally zero guidance.
Mix the recipe, “drench” the soil. That’s all the precision you get.
I have to say, it makes me uneasy to dole out any pesticide so casually, even one that’s a plant by-product, with a relatively short half-life in the environment, and with a reputation for being mostly harmless.
Source:
I chalk this reticence up to my childhood. When I was a kid, in the 1960’s, anybody could pick up a can of DDT at the hardware store. It was a miracle in a can. I recall my dad kept some around the house, and that at some point I used it for getting rid of anthills around my mother’s clothesline in the back yard. It wasn’t banned outright in the U.S. until 1972 (reference), although home use of the product was banned a few years earlier (reference NY Times archives). That was part of a long series of moves to restrict DDT use before the outright U.S. ban. And even after 1972, there were exceptions that allowed DDT use for certain public health reasons and for a handful of minor crops for which there was no known alternative at that time.
So maybe it’s a case of “no saint like a reformed sinner”, but the idea of spreading an indeterminate amount of insect poison around, because it’ll do some good — I’ve seen that movie before and it didn’t turn out well.
On the up side, neem oil generally gets a pass as being mostly harmless. Incidental exposure is generally harmless to mammals and birds, and it breaks down fairly rapidly in the environment (reference). It needs to be ingested to be effective, so if you keep it off the flowers it won’t harm bees. And so on.
On balance, the only reason I’m willing to try this is that a gardener I trust (and follow in YouTube) uses neem oil to control certain sole-borne pests (reference). His advice is generally pretty sensible, and I guess if there’s no obvious harm in his Zone 7 garden, I should not expect any in mine.
Here’s a good overview of the current use of neem as a pesticide (reference).
Of the systematic research showing that a neem oil soil drench was effective against some larve in the soil, I can find the following concentrations and effectiveness estimates. This is based on the first two pages of a Google Scholar search for the keywords: neem oil soil drench, restricted to open-source (no paywall) studies in which neem oil was used in isolation against soil-based insect pests.
That said, now that I’ve done it, in some sloppy fashion, I’m not sure I’d do that again. Here’s my takeaway after spending an afternoon trying to pin down what an appropriate dosage of neem oil might be.
Neem (mainly, the azidarachtin in neem) is pretty much a mystery, even to those who study it. In reading what scholarly literature I could find, it’s evident that neem has a profound effect on some insects, at some points in their life cycles. And no effect whatsoever on others. It has different routes of operation, sometimes as a hormone disrupter, sometimes smothering insects as with any agricultural oil, and sometimes by reducing insects’ drive to feed on plants. Some insects are incredibly sensitive to it, so that only a trace dose of the active ingredient is necessary. Others require a lot of it before an effect is noticed. And, because it’s a natural ingredient, the composition and amount of active ingredients in the neem oil vary in an unknown way from batch to batch.
And to top it off, it’s hardly the miracle pesticide that some tout it as being. In scholarly trial after scholarly trial, neem would reduce (but not eliminate) the insect pests being targeted. That, combined with a short half-life in the environment, means that it’s not a one-and-done insecticide, but instead is something that some people tend to spray routinely. Almost as a kind of good garden housekeeping.
At the end of the day, I took my best guess, crossed my fingers, and went for it. My soil drench consisted of a few tablespoons of raw neem oil per gallon of water. I mixed that in a clear container so that I could titrate the amount of Dr. Bronner’s soap to be added, to emulsify the oil. (Add just enough soap to turn the entire mixture milky, with no visible oil on the surface.) And I used raw neem oil, not the “hydrophobic extract”, which does not contain the insecticide azidarachtin present in the neem plant.
I guess the dumbest aspect of this is that it was straight-up monkey-see monkey-do pest control. Not only do I now know what the right “dose” is, I’m not even sure neem will attack the insects at issue (as a soil drench).
Worst of all, I have no way of knowing whether it did anything or not. If no cucumber beetles or squash vine borers emerge from that soil, is that because neem did the job, or because there weren’t any in the soil in the first place.
