Ask a Question forum: Caterpillars come back every year to damage petunias

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New York, NY
AnjJ
Jul 29, 2017 8:37 AM CST
I live in NYC and am a terrace gardener. I love to grow petunias, particularly trailing ones as they can be seen from the street from my 4th floor terrace. For years they've been plagued by caterpillar damage. I use BT (Bacillus thuringiensis), which works very well, but they always come back, every season. I have to spray BT nearly all summer. It kills only caterpillars ingesting it; it does not kill the eggs.

Why do they keep coming back? Do they lay billions of eggs that continue to hatch all summer, and that can survive the winter? I once tried changing out all of the soil, but it didn't help (it's possible I wasn't diligent enough in getting rid of every grain of the old soil).

How do I get rid of this problem for good? Thanks for any help you can provide!
Name: Deb
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Bonehead
Jul 29, 2017 9:00 AM CST
Before staging all out war, you may want to identify what specific caterpillar you are getting, to figure out what it turns into. You might be hosting a beautiful helpful butterfly, or some sort of damaging moth pest. I would take some photos of your crawlers and post it (either here or on the butterfly forum) to see if someone can help you out. Once you know what species you are dealing with, you can plan accordingly. In any event, be thankful they are soft fuzzy caterpillars rather than slimey slugs or other icks. You could also post pictures of your petunias in bloom - sounds lovely!
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New York, NY
AnjJ
Jul 29, 2017 11:37 AM CST
I don't normally see them, because by the time I see them, the petunias are destroyed. However, in the past, they were plain slimy green, not fuzzy.
Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Jul 29, 2017 12:10 PM CST
Are they eating the flowers or the leaves? There are a lot of green caterpillars but did they look like this one?

http://bugguide.net/node/view/...
New York, NY
AnjJ
Aug 6, 2017 10:53 AM CST
They were smaller and fatter but possibly less mature. They primarily eat the flowers, but they will eat the leaves after they've destroyed all the flowers.

The BT does work but they keep coming back. Any ideas on how to get rid of them permanently would be appreciated.
Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Aug 6, 2017 11:52 AM CST
If we assume that's the caterpillar, the number of generations apparently depends on the location. In New York it could be either one or two a year. They pupate in the soil (or in your case I guess the potting medium). Do you keep the petunias over winter? Yes you should completely change the potting mix and clean the containers. Are there flowers growing at the base of the building also? It's possible they are pupating down there also, if so.
Name: Philip Becker
Fresno California (Zone 8a)
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Philipwonel
Aug 6, 2017 5:28 PM CST
AnjJ ! Hi !
They sort of sound like cabbage loopers. White little butterflies, you see in daytime. They love my collards. There around from early spring to late fall, laying eggs, on leaves. Green worms hide very well on top, or under leaf stem veins. Hard to see, till there big.

To save yourself some trouble. Do as i do. Save a gallon jug. Mix up gallon of BT. And have a Windex type spray bottle, next to BT mix.

Every week or two, or when you see damage. Spray with BT.
I also add 1 Tbl. Reg Dawn dish soap in gallon. It keeps aphids.white flies .and spider mites in check.
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Name: greene
Savannah, GA (Sunset 28) (Zone 8b)
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greene
Aug 6, 2017 6:12 PM CST
Everyone loves butterflies but lots of people hate caterpillars; can't have one without the other.

AnjJ said:...slimy green...


Maybe you are seeing slugs and are calling them caterpillars? For the most part, caterpillars are not slimy.

Photos would be helpful. Thank You!

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Name: Sandy B.
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Weedwhacker
Aug 6, 2017 6:34 PM CST
I had the same thought about the "slimy green" description... caterpillars aren't actually slimy, although neither are snakes and a lot of people perceive them to be. A photo would be extremely helpful.

My petunias were decimated one year by caterpillars that I considered very ugly ... until I got rid of them and then learned that they were the babies of Painted Lady butterflies. *Blush* I'm a lot more careful and tolerant of caterpillar damage now!
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Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Aug 6, 2017 6:44 PM CST
AnjJ said that Bt works but they come back (more than one generation presumably). If Bt worked they must be caterpillars. I assumed they just had a slimy look rather than actually being slimy. Certainly a picture would help but there are so many green caterpillars. The tobacco budworm in the link above does eat petunia flowers.
Name: Sue
Ontario, Canada (Zone 4a)
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sooby
Aug 6, 2017 7:07 PM CST
@AnjJ here are some pictures of the tobacco budworm eating petunia flowers, does this look like it?

http://www.ag.utah.gov/utahpes...

https://www.extension.umn.edu/...

https://blogs.k-state.edu/kans...
Name: stone
near Macon Georgia (USA) (Zone 8a)
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stone
Aug 7, 2017 4:27 AM CST
Weedwhacker said:
My petunias were decimated one year by caterpillars that I considered very ugly ... until I got rid of them and then learned that they were the babies of Painted Lady butterflies. *Blush* I'm a lot more careful and tolerant of caterpillar damage now!

Hurray!

At my house, I leave the cudweeds for the american lady butterflies to raise their families....

Thumb of 2017-08-07/stone/6ca973


Sorry, don't know the petunia caterpillars.... I googled them.... Seems like a lot of moths and butterflies like petunias....

The only advice I could offer.... Find something else to plant...
Like maybe four o'clock, or zinnia... Marigold....

Like, what is your goal in planting petunia?

To attract butterflies and hummingbirds?
Those poisons are harmful to these critters.

Or are you just interested in a splash of colour?
I reckon that we could all suggest alternatives that are less trouble.

Seems like a bit of crop rotation should solve the problem.

Butterfly coming back for petunias would find something unusable...
[Last edited by stone - Aug 7, 2017 4:41 AM (+)]
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New York, NY
AnjJ
Aug 7, 2017 5:32 AM CST
In using the word slimy, I was trying to make a distinction from fuzzy, in response to The Bonehead. They are smooth and green. Can't take photos because I haven't seen them in years. Philipwonel, thanks for your suggestion, I do keep the BT on hand because it definitely works. Just wish I could find a way to kill the eggs as well. Thanks all.

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