Source: U Wisconsin Vegetable Entomology.
Yet another gardening post. If you have no interest in growing cucurbits, stop now.
This is a rewrite of an earlier post (G09), mostly to summarize the results of this season. And to shorten it up and tighten up the writing. It’s a summary of everything I think I have learned about the squash vine borer (SVB). All in one place. Off the top of my head, based on what I’ve read over the past week, and what I’ve observed in my garden. So I can remember it next year. Citations as to source only if and as I feel like locating them.
Key takeaways
Some of these may only apply for this year, and my climate (Zone 7). But here goes. Read further into the posting if you want details.
When does the SVB arrive? Just after the Japanese beetles. Probably. Japanese beetles emerge after 970 growing degree days of warmth (defined below in posting). The SVB emerges after 1000 growing degree days, but some sources put that as 750 to 1000 growing degree days.
Length of the SVB season: Seven weeks. Here in Vienna, VA (Zone 7), I had SVBs in my garden for seven weeks. I saw SVBs more-or-less daily from 7/5/2020 to 7/26/2020, and then occasionally through 8/25/2020.
One treatment that works: Spraying cucurbit stems every five days with 0.008% spinosad solution. I had nearly-zero SVB damage with this approach, despite growing many dozen cucurbits of various varieties. It ends up being a lot of work due to the length of the SVB season, coupled with the need to spray the full length of the stems as the season progresses.
Prevention? I didn’t try any this year, but based on what I saw, merely wrapping the last inch of the plant stem in (e.g.) tin foil will not be adequate. As the SVB season progressed, the SVBs laid eggs all up and down my cucurbit stems. I think my approach for next year is going to be to time my plantings, and pick my plants, to try to avoid this problem entirely.
Background
In case you don’t know what this is, the SVB lays eggs that hatch into larvae that eat their way into the stems of your cucurbits, and kill them. So if you grow summer squash, winter squash, pumpkins, and to a lesser degree cucumbers or melons, just a handful of these can destroy your crop.
I’ve never run across them before, in 25+ years of off-and-on gardening in Vienna, VA. But I surely have them this year. First noticed one on 7/5/2020. Had multiple daily sightings for the next few weeks, and sporadic sightings after that. They only live about five days, so at this point I presume that I’ve had a succession of them, even though I usually only see one at a time.
Arguably, I have attracted them this year because I made myself a target. I ended up planting a large garden that’s mostly cucurbits. Not intentionally, just because other crops failed. At the time I wrote this, I had 16 summer squash plants, 12 winter squash plants, 6 pie pumpkin plants, maybe 12 field/other pumpkin plants, and a couple of dozen cucumber plants.
An excellent and compact scholarly reference can be found at this URL: University of Florida. That said, there is disagreement across sources on some key details.
First things first: Kill them when you see them.
It’s not so easy to kill these by hand. They’re fast and skittish. They land on squash leaves, which are fragile. After fumbling my first few attempts, I settled on a “hand clap” technique.
They will often sun themselves in mid-afternoon on the leaves of your squash. I find that 3:30 PM is a good time for hunting. Don’t try to brush them to the ground and stomp them. Don’t try to pick them off. They’re too fast. Slowly place one hand under the leaf, then clap your hands together to kill the moth. Wear a glove if you’re squeamish.
Timing: when do they show up, how long do they stay, how long do the eggs take to hatch.
This year, in my garden (Zone 7), I saw my first SVB moth in early July, just after the Japanese beetles showed up. I don’t think that timing is a coincidence (see below). When you start seeing Japanese beetles, you should probably keep an eye out for the SVB moth.
SVBs are almost unmistakable when they fly due to their bright orange color. At that time (early July, Zone 7) there were no other insects in my garden that color. It also has a very direct, beetle-like line-of-flight (despite being a moth). Disturb one, and it will zip away so fast that you’ll lose sight of it in a second or two.
Serious growers monitor for the presence of SVBs using pheremone-based traps. Growers count the number of males caught to judge the level of infestation.
I believe that these traps are sold as monitoring devices, not as pest control devices, because they only lure the male SVB. (Others suggest using them as a pest control because they can break up the mating cycle by eliminating males. I don’t know which view is correct.
The proper way for the home gardener to be ready for them is not to look at the calendar, but to track cumulative growing degree days (GDD) of warmth. (See below for explanation of GDD and sources of current GDD counts for your area). Japanese beetles take about 970 growing degree days of warmth (see below) before they emerge (per this reference). The SVB over-winters in cocoons in the soil, and emerges somewhere around 950 to 1000 GDD. (But some scholarly sources put the SVB emergence at 750 to 1000 growing degree days, so there is some uncertainty there). That’s why I think the presence of the Japanese beetle gives you fair warning that you need to start looking out for the SVB.
Assuming that I noticed them on the first day they were here, they were in my garden daily from 7/5/2020 to 7/26/2020, and then occasionally through 8/25/2020. So, a three-week period of intense infestation, followed by four weeks when the level of SVB activity trailed off to nothing. I don’t think that pattern was a fluke, because that’s an excellent match for the well-documented SVB season in New Hampshire. Here’s a count of months trapped in one New Hampshire location.
Source: Managing Squash Vine Borer in New Hampshire (.pdf).
On that last point, this directly contradicts common social-media information that says that once you’ve seen the squash borer arrive, it’s safe to plant a second crop. That may be true in some areas, but clearly is not true in other areas. In the Deep South (e.g.) Florida, they routinely have a second emergence of these during a single summer. But while a second generation is typical in warmer climates, it is not in colder climates. We’re in the middle. I never found solid information on the typical SVB season length in this area. My own observation is seven weeks — three weeks of intense infestation, and then off-and-on visits by the SVB for another four weeks.
And that’s not just on social media. The University of Maryland extension service has advice that would have been dead wrong for this year, in Zone 7. They say: “Plant early to lessen injury. Use transplants instead of seeds. Or, plant squash seed mid-June.” If I’d done that, those mid-June plants plants would have been exposed to the peak of the SVB season.
The females lay eggs that take 8 to 14 days to hatch (per scholarly reference above). (Other references put it at 9 – 14, and note that it depends on temperature — warmer = faster.) FWIW, each female can lay about 200 eggs. And so, as if the season isn’t already long enough, you’d have to keep pesticides of some sort on your squash for an additional two weeks, to account for the time for the eggs of the last SVB moth to hatch.
Extended aside: Growing Degree Days (GDD)
Growing degree days in a year are calculated as the cumulative time during which the air temperature exceeds 50F. In Virginia, Virginia Tech would be the place to get information. A nice general reference is available from the American Public Gardens Association.
That 50-degree reference point is more-or-less a standard, but you can run into specialized variants of plain-old GDD. The only one that I saw reported was Corn GDD. There, they cap the temperature at 86F, because corn stops growing appreciably when it gets that hot. I presume that for insect pests, I want to see GDD, and not corn GDD. You can also see it calculated with bases other than 50F. Finally, the true-and-accurate calculation requires hourly temperature data, but as far as I can tell, virtually everything you can find on line uses an approximation based on the mean of the minimum and maximum temperature for the day.
According to this site, we (Vienna, VA) passed 1000 GDD back on June 22 this year (2020). That’s almost two weeks before I spotted my first SVB. I can infer from the values that they are using a simple approximation (average of max and min temperature, less 50, as described by NOAA) in their calculation. Yet, this site, from Cornell University puts the 1000-GDD threshold at June 24 for Vienna this year.
That’s well before I spotted any significant number of Japanese beetles. I’m taking all of this as an indication that you can’t really do (or get) a precise growing degree day calculation that will tell you, with certainty, when to expect the SVB.
Plus, the things are good fliers. Apparently, can fly up to a mile from where they hatch, to your garden. So they may arrive some significant time after they hatch. Near as I can tell, using the presence of Japanese beetles as a warning flag is probably more reliable than doing the growing degree day calculation.
What plants are at risk?
Here, I am supposed to offer some sort of reassuring list. All sources agree that summer squash are the prime target, with zucchini being more at risk than yellow squash. Squash that have big, hollow, liquid-filled stems. After that, some pumpkins are targets. Larger-stemmed and hollow-stemmed cucurbits are preferred to smaller-stemmed and solid-stemmed cucurbits. Apparently solid-stemmed winter squash such as butternut are poor targets, and tolerate SVB well. In theory, cucumbers and melons are rarely affected, but there are plenty of case reports of gardeners losing entire cucumber crops to SVB.
Here’s my observation. That’s probably a list of what they’ll hit first, versus what they’ll hit later, if they run out of better targets. But if you have enough SVBs in your garden, they’ll eventually hit every cucurbit in your garden, at least as far down the list as cucumber.
Based on my observation, I believe there’s an order of preference that corresponds to the lists you’ll find in various references. I.e., nothing is safe, it’s just a question of what’s more at risk. So, in my case, sure enough, the first thing I saw them ovipositing on was, in fact, zucchini. Later I caught one napping on some pumpkin leaves. I’ve seen them buzzing my butternut (winter) squash. The only thing I haven’t directly seen SVB on is cucumbers (and I have a lot of cucumbers planted).
Given that orderly approach to my plants, I suspect that the moths can tell if another moth has laid eggs on a plant. And so, as a species, they’ll just run through your garden, from most to least desirable target. Each moth only lays maybe 200 eggs max. So it’s first come, first served, on your cucurbits. First moth hits the best target, which is zucchini. I haven’t seen them on the zucchini since the first day. The latecomers then hit what’s left. To the point where this morning’s moth was on the sole outlier (a pumpkin that I planted, on a whim, in my sunflower patch).
Plants also vary in the susceptibility. As I understand it, unchecked, the SVB will kill summer squash outright, and in a short time period. The SVB will typically just damage others and reduce their yield.
I think that’s in large part due to secondary root systems on vining curcubits. Summer squash are both desirable to the pest, and vulnerable because they have a single short stem and a single root system. By contrast, vining squash put down secondary roots (where possible) at each node on the vine. These secondary root systems prevent the borer from killing ground-grown vining curcubits outright. But note that If you are growing (e.g.) winter squash on trellises, you don’t get that protection from secondary roots farther down the vine.
When will damage appear? It takes the eggs 7 (possibly 9) to 14 days to hatch. Then the borer has to grow to a size that affects the plant. So, best guess, something like three weeks after the first SVB moth appears. Possibly later. The SVB borer larvae spend four to six weeks inside the squash stem, before exiting and entering the ground below to continue their life cycle.
What will it look like? Classically, for summer squash, the plant just suddenly wilts and dies, as the presence of the borer in the stem disrupts the flow of fluids. But I have now watched enough YouTubes on this to know that many gardeners don’t see that classic damage. Many first learn of the presence of the borer due to a soft spot on the squash stem. On pumpkins, damage may come in the form of reduced yield rather than outright loss of the plant.
Prevention.
By prevention, I mean all the methods you can use to keep the SVB moth from laying eggs on your cucurbits. Everything after that is listed under “treatment”.
The SVB caught me by surprise this year, and I kept thinking that the season would be short. So I didn’t do any prevention. This is my summary of other people’s prevention ideas, and some comments on those.
Kill the larvae underground, before they emerge. If you lost plants to the SVB this year, these are things you might consider doing now, if you think that some larvae may have successfully matured. These techniques are probably more applicable to commercial gardeners than home gardeners. Presumably, most of the SVB moths in your garden grew up somewhere else. That said, if you have a mind to, these techniques were mentioned as way to kill SVB larvae that left your plants and entered the soil beneath. I am not sure of the effectiveness of any of these methods.
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- Till the soil in the fall to a depth of six inches. The idea is go bring any cocooned larvae to surface, where winter cold will kill them.
- Perform a spinosad soil drench where infected plants grew. The spinosad would kill any larvae present. (N.B. Spinosad doesn’t kill earth worms at dosages you should use, based on this 2002 USDA report (.pdf). Spinosad is toxic to bees when it is wet, so you need to avoid bees when you spray).
- Apply beneficial nematodes to the soil.
Rotating crops is not necessary, for the home gardener, to deal with SVB. For the obvious reason that you can’t rotate them out of range of any emerging moths. The advice to rotate crops, in this context, is advice for commercial farmers, not for home gardeners.
For summer squash, time your planting to avoid the SVB entirely. For summer squash, in this climate, I think there’s a lot in favor of just growing what you are going to grow, for the year, prior to mid-July. And when the SVB, cucumber beetle, and powdery mildew show up, just call it a season.
I’ve seen a number of seemingly respectable sources adopt that strategy. Plant early, then tear the summer squash out and replant that area mid-season. Having dealt with all three pests this year, that’s my plan for next year.
Less reliably, some sources suggest re-planting your summer squash in mid-to-late June, for a fall harvest. That may work in some climates, but I think that’s questionable here. This year, the SVB season extended well into August in this area. (And, I still have cucumber beetles in my garden as of 9/17/2020.)
My best guess is that, if it is successful, planting for a fall crop would reduce yield by at least 25%, compared to a summer crop, due to lower levels of sunlight. So the bottom line is that if you lose 25% of your summer squash to pests and disease anyway, if planting for a fall harvest works in your area, you might consider that as the lower-hassle option.
Calculation: What’s the difference in sunlight available, over the (say) month-long peak production period, for spring-planted squash versus mid-summer-planted squash? The longest day of the year is June 21 or so. With an early spring planting, presumably your squash bed is covered by leaves as of June 21. By contrast, mid-summer planting will reach full leave cover only around August. Courtesy of this reference, from the National Renewable Energy Labs, we see that in Richmond VA July total sunlight rates about 20,000 (kilojoules per square meter), but September rates just 15,000 on that same scale. From which you can plausibly infer that yields of late-planted summer squash will be maybe 75% of the yields of spring-planted summer squash, due to reduced solar input.
Neither of those timing approaches works for winter squash or pumpkins, owing to the long period between planting and harvest.
Wrap the squash stem. One commonly-suggested method is to wrap the squash stem in tin foil or pantyhose. That would work for vining squash, but I don’t quite see how that would work for summer squash. For sure, based on reliable sources and my own observation, if you rely on wrapping the stem, you need to wrap a foot or two of stem, not just the bottom inch or two. I’ve definitely observed the SVB ovipositing on parts of the stem that were a foot off the ground. And late in the season, I’ve seen them over the entire length of the stem, out to the growing tips of my pumpkin vines.
Use Tanglefoot on the squash stem. That seems reasonable to me, as bees would have no reason to light on the lower stems. I have not tried this nor have I seen any estimates of effectiveness at trapping the SVB moth. Nor have I seen any estimate of what other insects this will trap incidentally.
Some sources suggest trying to trap the moth with yellow bowls filled with soapy water, as they are attracted to the yellow of squash blossoms. I’m pretty sure this is a non-selective method and by report will (or can) result in killing anything else attracted to squash blossoms (e.g., bees).
Plant a trap crop. Many sources suggest planting a trap crop of (e.g.) Blue Hubbard squash, which is very attractive to SVB. I assume that most gardeners in this area do not have the space to waste on a trap crop.
Interplant with cucumbers. One source suggests interplanting with cucumbers as a way to discourage SVB. That’s based on a single anecdote from her garden. That said, my experience is that cucumbers are just about the last cucurbit that the SVB will lay eggs on, given any alternatives. Plausibly, the presence of cucumbers serves to disguise the other cucurbits.
Trap the male moths using pheremone-based traps. As I said earlier, I’m not sure whether this works or not. Some vendors sell these as a way to monitor the extent of infestation, with no mention of control. Others sell them as one way to control the pest. Plausibly, if you are growing acres of cucurbits, these traps don’t catch enough males (or are too expensive) to be an effective control. But in a home garden, it’s possible that catching the majority of males in that small area could put a dent in the number of eggs laid on your plants. In the end, I have no idea whether these work to reduce the number of eggs laid in your garden.
Insecticide-based treatments.
For this section, I assume the SVB moth has shown up and is in the process of laying eggs on your plants. The eggs are far too small, nondescript, and dispersed to be removed by hand.
What are your options for treatment?
My prejudice: I’m not going to discuss long-lived or non-selected pesticide methods, either organic or inorganic. So, e.g., pyrethrins are effective against SVB, but they are also quite toxic to bees. In a nutshell, I’m restricting myself to methods that appear fairly bee friendly.
BT is bee-safe and will, in theory, kill SVB larvae in the brief interval between hatching and burrowing into the stem. At least one formal research study found that it was at least as good as the pesticides that farmers were using at that time. And it is frequently mentioned in popular press articles on SVB control. BT can also be injected into an infested plant to kill the borers within the plant.
For me, the main drawback was the short lifetime of BT in the garden, as little as two days when exposed to sun and rain. Because the SVB moths were continuously arriving and laying eggs, I think that an effective BT-based strategy would require spraying every-other-day. That seemed like far too much work.
Spinosad has been tested in commercial settings and shown to be as effective as pyrethrins (the current standard) for killing SVB larvae. And this has a five-to-seven day lifetime in the garden setting.
And so I settled for spraying a 0.008 percent spinosad solution on my cucurbit stems, roughly every fifth evening. (Evening, to avoid bees.) This has to be mixed up from concentrate. (The commercially-available ready-mixed “Captain Jack’s Dead Bug Juice” is only 0.001 percent spinosad.) As far as I can tell, this has had no impact on the bee population of my garden. But I was careful to spray in the evening, and not to spray blossoms.
The result was virtually zero SVB damage, despite extensive cucurbit plantings and plenty of SVB activity in my garden. All told, I think that’s a pretty good demonstration that this approach works.
Commercial growers must wait three days between spraying spinosad and harvesting summer squash. I’m not sure about the extent to which that applies in the home garden, because commercial growers are presumably dousing the entire plant. But if you abide by that rule, an effective spraying strategy starts with harvesting any summer squash that are nearly ready to be picked, and then spraying. Because you then should not harvest any more summer squash for the next three days.
Commercial growers can only use it for a few sprayings per year on any one crop. That said, presumably they spray the entire plant, including all the foliage. I didn’t think that same limitation would apply to spraying it over a very limited area of each plant — just the stem. Presumably I’m applying on a tiny fraction of the total amount of spinosad that a commercial grower would apply.
There are many internet case reports suggesting that spraying with neem oil controls SVB. (Neem is presumed bee-safe if used reasonably). Though what, exactly, that is doing is not clear (smothering eggs, disrupting egg life cycle, disrupting larval life cycle?).
The best case report I read suggested that using neem as a systemic poison — dosing the plants on a regular basis, as they grow, well before before the SVB arrives — seems to work particularly well. That suggests to me that a primary mechanism of action may be literally poisoning whatever larvae actually make it into the stems of the plants. That also suggests that people are using “100% neem oil”, which contains natural pesticides, and not “70% hydrophobic extract” of neem oil, which is merely oil, and does not contain those pesticides.
That said, the only effective way to get systemic insecticidal action using neem oil is via a soil drench. (It is poorly absorbed through foliage). But as far as I could tell, using neem this way is strictly for D-I-Y gardeners. I could not find guidance as to concentration, frequency of use, amount to be used, and so on. The upshot is that if you want to do a “neem oil soil drench”, you have to make it up as you go along. To me, that’s not a plan for any pesticide and after considering it, I did not do a soil drench.
Diatomaceous earth gets mentioned frequently as a pesticide for SVB. I did not try it, for a number of reasons. First, I could find no formal study showing that it worked. Second, I could not even figure out what that was supposed to be targeting, the moth or the larvae. I presumed it targeted the larvae, for the brief period between egg hatch and burrowing into the cucurbit stem. Third, it’s non-selective, more-or-less killing whatever crawls through it. Fourth, it’s hard to apply to a large area, and needs to be re-applied after every rain.
Summary and looking forward
The SVB caught me by surprise this year. I reacted by spraying my cucurbit stems every five days with spinosad. While this prevented SVB damage, in the end, it turned out to be a real chore to maintain. That’s due to the length of the SVB season, and to the total length of curcurbit vines that needed to be sprayed.
Next year, rather than fight the SVB, I’m just going to try to dodge it. For summer squash, I can do that by planting early, and just tearing out my summer squash as they get infested. For winter squash and pumpkins, I’ll do that by growing solid-stemmed varieties that are naturally resistant to the SVB.
Summer squash: Next year, if I grow summer squash, I’m going to plant early and plan to pull them up at mid-summer. Near as I can recollect, I got by the majority of my summer squash yield before the SVB showed up, although the plants were still producing well at that time. I think I’ll settle for that, rather than go through this intense spraying schedule again.
In addition, I may plant a few summer quash at mid-summer and see if I can get a fall crop. But I’m not hopeful. I did, in fact, try that this year, planting a few seeds around August 1. But the germination rate was poor, and the seedlings really didn’t like the heat. I’m not sure that any of them survived, but I am sure at this point that if they did, I’m not going to get any squash from them. So if I go for a fall crop, I’m going to start the seeds indoors so that I have fairly robust squash plants to move to the garden mid-summer.
For winter squash, this really shouldn’t be much of an issue. At least not for butternut squash, which has a solid (not hollow) stem. Those should be largely resistant to SVB attack. I had a good yield of butternut squash this year, so that’s what I’ll grow again next year. I’ll see if they get by without spraying for the SVB.
And for pumpkins, just by luck, I have a strategy as well. I grew three or four varieties this year: pie pumpkins and decorative pumpkins. Far and away, the most productive was the Dickinson pumpkin (pictured below, with a pie pumpkin). It doesn’t really look like a pumpkin. It looks more like a tan watermelon. And it actually isn’t a pumpkin. It’s a very large winter squash, complete with solid stem. So that, too, ought to be resistant to SVB attack. And that’s what I’ll be growing next year.
Below, here’s a Dickinson pumpkin from my garden, alongside the biggest “sweetie pie” pie pumpkin that I’ve grown so far. Near as I can tell, they set roughly the same number of fruit per 100 square feet of garden area. No wonder Dickinson is the variety Libby’s grows for canned pumpkin.
Above: Dickinson pumpkin, standard pie pumpkin, and a yardstick.
