Expert: Long Island Gardener - 7/7/2011

Question
QUESTION: I 'm a novice gardner, learning as I go. I have had petunias the last couple of years in window box, and never had any problems. This year the flower is being eaten down to almost nothing and there are little black things (mostly in the middle of the flower) but they do not look alive. They come off when I water. So I would like to know if anyone knows what it is and how to get rid of them?

ANSWER: Angela, without a better description of the "little black things" located in the center of the flower, I can only offer my best guess.  I could be right, I oould be wrong.  Please clarify as to their size (of a period? of a pinhead? of a penny?) and whether or not they move, etc.

That said, your question becomes our annual "Black Dots on the Petunias" question. Odds are, one of these issues below is your problem.

Larvae of the Hornworm Catterpillar can't resist taking chunks out of an unprotected Petunia leaf.  These bugs are crazy about Petunias.  The 'black dots' are forensic evidence: Their droppings.

Female Moths are detect the Petunia scent -- these flowers are strongly night-fragrant.  The moths lay their eggs on the leaves.  You might see them as barely visible green or yellow 'pearls', each smaller than a pinhead.

Then there's the tiny black insect called the 'Potato Flea Beetle' ('Epitrix cucumeris' to botanists).  Depending on how big these 'dots' are... anywhere from the size of the period at the end of this sentence to a maximum 1/8th inch full grown.

Adult female Beetles deposit eggs in the Soil at the base of the plant in June.  Larvae hatch out of the eggs 10 days later and eat the plant roots.  Then in early July they emerge as adult Beetles and attack. Once grown, Potato Flea Beetles JUMP from plant to plant, thanks to strong hind legs, and spend many a Summer day chewing holes in the leaves of Tomatoes, Petunias and other garden plants.

You can read more about it at the Connecticut Cooperative Extension website's cheat sheet on Petunias:

www.ct.gov/caes/cwp/view.asp?a=2823&q=377880

and on Potato Flea Beetles:

www.ct.gov/caes/cwp/view.asp?a=2815&q=376696

as well as some lovely photographs of damage to potato and tomato leaves suitable for framing at IPM Images:

www.ipmimages.org/browse/subimages.cfm?SUB=7477

You think you've got problems?  This bug drives potato and eggplant farmers CRAZY.  Warning: If you're growing Tomatoes this year, THEY'RE NEXT!

University of California posts a closeup of one of these bugs, which they describe as measuring a mere '.065 inch' -- that's .65 HUNDREDTHS of an inch big.  Around the size of the period at the end of this sentence.

Although Cornell University continues to maintain that this insect can be controlled by natural predators, the North Carolina State authorities state, 'There are no natural predators or parasites that can effectively control flea beetle populations.'

Any organic insecticidal soap will give these bugs a stomach ache they'll never forget.  Safer has a great line.  You can buy it at many garden centers and discounters.

THE LONG ISLAND GARDENER

---------- FOLLOW-UP ----------

QUESTION: Ok I haven't seen the little black dots ever move and they are about the size of the period at the end of the sentence. I haven't noticed anything bigger crawling around in them though. Can either of these insects infect my knock out roses which are planted below the window boxes that my petunias are in?

ANSWER: Thank you for your followup, Angela - and I hope you will followup again, because I love a good question, and this is one of them.

The trouble is that I'm not so sure these are insects, mainly because you say you have never seen them move, yet they wash off with water.  Maybe I'm wrong, but perhaps you have some evidence that led you to conclude they ARE insects, but you haven't told me what that is.  I need to know more about the black dots and why you know these are insects, if that is the case.

If this is mineral, rather than animal or vegetable, there is another possibility -- but you have not mentioned your location, so I am guessing here: Peroxyacetyl Nitrate -- aka "PAN"?  Here's how one gaggle of scientists studied it, and what they found:

www.actahort.org/books/440/440_42.htm

PAN damage to plants has been studied by environmentalists, and they've identified several plants that are sensitive to this pollutant -- Petunias being one species, along with Tomatoes.  Roses are not on the lists I have seen, but that could mean they were not tested.

But these black dots could also be snail droppings.  This is my best guess -- and I'll tell you why.

Snails and slugs are primarily nocturnal, and very good at hiding.  While they would have a lot of trouble crawling up a house and finding their way to a windowbox, I'm guessing -- since you have not identified your location -- that you just purchased these new this year; if even a single pot had a snail or two, the uninvited mollusk would be spending its nights devouring your Petunias.  And snails LOVE Petunias.

Then, at the crack of dawn, away they go, slithering into crevices and nooks and crannies for a good day's rest.  And this damage sounds just like that.

To save your Petunias from slugs, this is one of those situations where a shallow dish of Budweiser placed on the floor of the window box will bring them out of the woodwork, while you sleep.

Think about it.

---------- FOLLOW-UP ----------

QUESTION: Ok I'm Kansas City, Missouri. And I did notice the black dots after I added another plant to the pots, which I had purchased at Walmart. But I will definitely try the beer thing, I hope Sam Adams works just as well. Thank you for your help.

Answer
The advantage of "the beer thing" is that you can see the evidence.  Keep me posted on whether you catch any snails or slugs in there.  And oh, about that beer...

Colorado State University did a taste test of beers that slugs like most.  Professor Whitney Cranshaw ran his trial for 3 days. Alcohol-free Kingsbury Malt Beverage was in the winning glass.  At the bottom: Gallo pink Chablis wine, tap water, and a cocktail of sugar-water and yeast.

Anheuser-Busch beers apparently hit the ball out of the park just behind the teatotallers' Kingsbury, with Budweiser and Michelob in 2nd and 3rd place.  Coors Lite came in 6th and Coors placed 11th.

Rounding out the top 10 were: Bud Light, Old Milwaukee, Coors Lite, Schaefer, Miller, Strohs and (tie) Lite and sugar water/yeast. Bringing up the rear were Coors, Pabst Blue Ribbon, Rainier, Gallo pink Chablis wine and tap water.

Here's where you can read the entire study:

www.colostate.edu/Depts/AES/Pubs/pdf/tb97-1.pdf

Good luck with this!

THE LONG ISLAND GARDENER