Expert: Long Island Gardener - 8/2/2008

Question
I am growing petunias in pots on my terrace on the 25th floor in NYC.  The
flowers on one of the plants are being eaten.  I see little black dots on the
leaves.  What is eating them and what should I use?  Thanks for your help.


Answer
Ah, the annual Black Dots on my Petunias question.  This could be symptom of a couple of things.

One is the Larvae of a Moth, Hornworm Caterpillars.  They're wild about Petunias.  The 'black dots' -- if motionless -- would be droppings of the caterpiller.  Female Moths are drawn to the night fragrance of Petunias.  They lay their eggs on the leaves -- faintly colored green or yellow 'pearls' smaller than a pinhead.

But this could also be another culprit:  The tiny black insect called the 'Potato Flea Beetle' ('Epitrix cucumeris' to botanists).  Depends 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.  These insects JUMP around, 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.

TINY Beetle. BIG appetite.

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

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

and on Potato Flea Beetles:

http://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:

http://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 the Cornell University position suggests that this insect can be controlled by natural predators, the North Carolina State authorities maintain, '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.  Let me know if you can't find it.  I'm in NYC every day.

Damage may have peaked at this point.  Don't let this one go.  They'll be back next year, bigger, stronger and hungrier.

THE LONG ISLAND GARDENER