Expert: Long Island Gardener - 12/5/2011

Question
Good Morning

I live in southern Victoria, Australia. We have a similar climate to coastal California.

25 years ago we grew petunias very successfully, about 20 years ago they started wilting and dying & we gave up growing them. The roots appeared to be being eaten just below the surface.

A couple of years ago we completely replaced the soil & tried again. Successful for a year - but we now have the same problem.

Any help you can give would be most appreciated.

Answer
The yellow-and-black Spotted Cucumber Beetle ('Diabrotica undecimpunctata') prefers Corn and Cucurbits, but in a pinch it will pick Petunias.  Its larvae find susceptible roots delicious.  Not to confuse things, but when it attacks underground Down Under as well as Up Over, it is called the Southern Corn Rootworm.

A Beetle lays its eggs somewhere around the plant.  When the eggs hatch, larvae get to work right away devouring entire root systems, then head up to attack the stem.  If the loss of chunks of tissue are not enough to kill the infected plant, bacteria injected by the Beetles as they chew are usually enough to finish the unlucky plant off:

www.eppo.org/QUARANTINE/insects/Diabrotica_undecimpunctata/DIABUN_ds.pdf

You are, by the way, urged to contact the Exotic Pest Hotline to report the presence of Diabrotica, once you have confirmed the Beetle's presence in your Petunia patch:

flyaqis.mov.vic.gov.au/padil/beetles.html

Here's a picture of one of the larvae dining on the root of a very sad Corn plant:

extension.entm.purdue.edu/fieldcropsipm/insects/corn-rootworms.php

Now, what can you do about this?

Interestingly, the experts at Purdue University extension say that

"soil texture is an important factor in regard to larval survival.  Newly hatched rootworm larvae must move through the soil in search of corn roots and, as they move, they come in contact with soil particles.  These particles, if coarse and abrasive, may scratch the cuticle of the larvae, resulting in death by desiccation.  As a result, rootworms may be less of a problem in sandy soils."

They go on to point out that wet sand is not as abrasive as dry sand.  So this is imperfect.  The University of Illinois recommends "wire-or-cloth screen protectors shaped like cones will keep beetles off home plantings until plants get established."

Resist planting Petunias annually in the same place as last year -- crop rotation -- so you can rotate plants and discourage repeat Beetle visitors.  I'll continue to look for more IPMs in the meantime.  Thanks for writing!