Bright Light at Night = Caterpillar Delight
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Q. "I have a question about the possible effects of having a street light above your garden. I've been gardening successfully for five years, but last year we moved to a new house and I've been having pest issues in my raised beds ever since. Last year I had swarms of aphids, hornworms, and cabbage loppers. It was a constant battle despite this being an entirely new garden; and the pests greatly diminished our yields and even killed some plants.
"On a recent episode of YBYG you mentioned a pest that was attracted to nighttime light, and I wondered if that might be part of my problem. For some strange reason, there is a short city street lamp in my backyard. It goes on every evening like any other city street lamp. Is there any chance that this is part of our problem? What else would bring on so many sweeping plagues of pests? Thanks for all that you do; I learn new things every week!"
- ---Coleen in the Fishtown neighborhood of Philadelphia
A. Well, thank YOU Coleen. And yes, there is a very high probability that your little streetlight is causing at least some—and maybe all—of your pest problems. Despite their common names, tomato hornworms and cabbage loppers are caterpillars, and their adult forms are night-flying moths. Having a bright light right overtop of their preferred food plants is like the giant neon displays of Pat's & Geno's in South Philly luring grease-loving cheesesteak cravers through the dark night.
I am torn between two potential first choices of tactic here: 1) Try and get the city's ID numbers off the light, call the streets department and ask them if they'll just turn it off; or 2) since you say the light is on a short pole, go up during the day and unscrew or remove the bulb, especially if the light is an annoyance that's shining in a lot of people's windows.
(If it's lighting up something like an alley for nighttime safety, then maybe you could take the bulb to a hardware store and see if they can fit it with a motion activated sensor. Then it would come on when it was needed but not be attracting every moth for miles around all night long.)
But of course there's always the possibility the city will say no, and/or you can't get at the bulb. And even if you do manage to shut the light, there's probably a leftover life cycle or two of pests that are hanging around waiting for this abundant food source to reappear. So I would get some floating row covers, the basic form of Bt (BTK) and a sprayer and be ready to do caterpillar prevention and defense this season.
There's no disadvantage to the row covers; you can grow crops like cabbage and broccoli under their protection all season long, because the plants don't need to be pollinated by insects. The covers let in sunlight and rain, but not miserable moths looking to lay their eggs. And row covers are great at improving plant growth, especially early in the season.
Then I'd start spraying the uncovered plants once a week with BTK—the original strain of Bt. Sold under brand names like Dipel and Green Step, it kills caterpillars that chew on the sprayed leaves, but harms nothing else. It has no effect on birds, bees, toads, pets, people, ferrets…. It doesn't even harm adult butterflies that land on the plant.
Anyway, spray any plants that are unprotected by row covers once a week and you should be able to halt destructive caterpillars before they can do any damage. And you won't be killing any caterpillars that would have become attractive butterflies. The tomato hornworm does turn into the sphinx moth, which is huge and beautiful, but it flies by night and is rarely seen. And its caterpillar form is a massively big eater; a mere handful of hornworms can devour tomato plants to death in a very short period of time.
(But I would NOT spray any carrots, parsley, dill or fennel in the garden; the very-cool looking caterpillars that feed on those plants turn into beautiful swallowtail butterflies—so if you're planting those crops, just grow extra.)
The light may also be to blame for the aphid invasion. I seem to recall a study years ago that linked extended hours of lighting to increased aphid feeding. Luckily, aphids are an easy pest to handle; just spray them off of the plants every morning with sharp streams of water from a hose. (Or use the same sprayer you'll use to apply the Bt; just fill it with plain water to go aphid blasting.) Studies have shown that sharp streams of water alone are devastating to aphids.
You can also pit 'bug against bug' by buying green lacewings; a beneficial insect whose common name is the 'aphid lion'. They come shipped to you in the egg stage, ready to hatch into little alligator-like larvae that love to eat aphids.
Oh—and one final off-the-point note. Fishtown is an old urban area whose soils have been exposed to lots of lead from house paint and car exhausts. So if you're using any original soil in those raised beds, I'd have it tested for lead. And always wear tight fitting gloves when you're working in soil that neighborhood cats can access.