Source: Clemson University, original photo credit David Cappaert, Bugwood.org
I saw my first Japanese beetle of the season today.
Is it really that time already? Yep, sure is. The Japanese beetles are out, which means more garden pests are soon to follow. In particular, this means that the squash vine borer should be arriving more-or-less now, in this region.
Insects emerge almost like clockwork, based on the cumulative springtime warming of air and soil. In particular, Japanese beetles emerge right around 1000 growing degree days of warmth.
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. The most commonly-used basis is 50 degrees because that’s the temperature at which most plants and insects begin to grow.
Last year, in Northern Virginia, we passed 1000 growing degree days on or about June 11, 2021 (Post G21-033). And this year, we passed that mark on June 11, 2022, per the Cornell University degree-day counter.
Source: Cornell University degree-day counter.
In short, the only thing that’s surprising about this is that I’m surprised.
To trap or not to trap?
For me, it’s now time to buy and hang a couple of Japanese beetle traps. I use the Bag-a-Bug brand. It’s not that I think these are necessarily more effective than any other. It’s just that, as an economist, I get a chuckle out of the carefully-calibrated prices of the traps, lures, and bags. All the parts are available, so, technically, it looks like you could keep re-using the same trap year after year. In practice, the prices are such that ever-so-slightly cheaper to chuck everything and buy new traps every year. There’s no way that could possibly be coincidence.
There is considerable controversy over whether or not to use Japanese beetle traps.
The controversy isn’t really about environmental impact. This is, after all, an invasive non-native species. To the best of my understanding, birds around here will only eat the adults if there’s absolutely nothing else available. So I don’t think anybody sheds a tear for a dead Japanese beetle.
Nor are the traps toxic. These are lure-and-pheremone traps. They attract Japanese beetles, then physically trap them. As far as I have ever seen, they are absolutely specific to Japanese beetles. Nothing else gets killed, and there are no pesticides involved.
Instead, the controversy is all about how smart (or dumb) it is to have these in your yard. Maybe all you do is lure even more Japanese beetles into your garden, and so increase your beetle damage. Completely respectable sources of gardening information urge you NOT to use Japanese beetle traps.
And yet I persist. I looked at the research showing that traps increase crop damage, read up a bit, and decided that putting Japanese beetle traps directly in your garden is, in fact, not very smart. To me, that’s what the research seemed to show. Hanging widely-spaced traps in the middle of an orchard (which is the method used in the research typically cited) turns out to be a fairly bad idea. That did, in fact, increase beetle damage relative to adjacent areas without traps.
Instead, I think you want to place the traps downwind and away from your garden. Not in your garden. Not upwind of your garden.
The idea is to place these so as to intercept beetles flying upwind, lured by the smell of your delicious garden plants. At this latitude, prevailing winds are out of the northwest, so my traps get placed south-east of the garden itself, as far away as I can place them in my back yard.
You don’t use these traps to lure beetles away from your garden, or away from your plants. (Which would mean placing them upwind of your garden — don’t do that.) Instead, the idea is to intercept beetles that are flying upwind, toward you garden.
In any case, I know what the scholarly research says. I just happen to disagree with their blanket conclusion that these traps increase damage on your plants.
My observation is that, placed downwind and away from the garden, Japanese beetle traps work exceptionally well. It seems to take about a week to go from beetles everywhere, to nary a beetle in sight. Maybe that’s just by chance, but that observation — plus a couple of quart bags full of dead beetles — says otherwise.
Finally, there’s a reason that I’m on the lookout every year. As soon as everybody realizes the Japanese beetles are here, the stores (and even Amazon) will sell out of Japanese beetle traps. Snapped up by all the people who, like myself, ignore the expert advice on this issue.
So I’m off to the local big box store to pick up a couple of traps. If this year is like last year, in another two weeks there won’t be a trap left on the shelves.
