...and just about everything else
Just cut back the flower stalks from some
Rudbeckia 'Goldsturm' and found borers in all the larger
flower stems. You could see the frass at the lower portion of the
flower stalk. I pulled out the bored stem with root and found the
borer at the base of the stem right by the root. I was trying to
find out if this is a major concern or did it just happen to be a
bad year. Black- eyed Susans have been so bullet proof up until
now. - P.S. -
It's a Stumper
Some questions have no answer, and all we can do is commiserate.
We'll help you with the main question but can't touch this one:
Whyizzit a European insects is the "corn borer", even though
corn is not native to Europe but is a relatively recent arrival?
Did a European bug that formerly lived on other plants switch to
corn as a result of the Columbian exchange at the end of the
1400's?
Corn borer: What it will attack
Identifying this moth and
caterpillar
Controlling the pest
Predicting its development, the better
to beat it
Corn borer's wide ranging
tastes
The most likely culprit is the European corn borer (Ostrinia
nubilalis), a moth whose last set of caterpillars each year
rests throughout winter in the stubble of their host plants. There
is a native stalk borer that may attack aster relatives such as
Rudbeckia (as well as species of Lilium,
Monarda, Centaurea, Aquilegia and
others) but it overwinters in the egg state.
Since coming to the New World about 100 years ago corn borer has
become a grievous pest of corn and also has shown itself to have
very wide-ranging tastes. It will dine on several hundred species
in about forty plant families. Host plants include: Aster, daisy,
mum Cosmos, Dahlia, Echinops,
Rudbeckia, Helenium and others in the aster
family; beans and other pea family plants; apple, pear and peach
trees (borers burrow into new green shoots; they're a suspect on
all rose family species); the nightshade species tomato, potato,
datura, flowering tobacco and bell pepper; mallows, especially
cotton, hibiscus and hollyhock; and also gladiolas, beets, celery,
sycamore, and poplar. It seems this insect's only criteria in
looking for host plants is to find those with stems thick enough to
support its young.
Identifying the
culprit
It does have some preferred hosts, however. Corn is definitely
one of them. Where this pest becomes a very big problem in other
species there is usually heavily infested corn nearby.
The moth is a tan night-flying species just 1/2 inch long which
holds its wings in a delta shape when at rest. Moths appear in
early summer (see the box below for timing) to lay eggs on the
leaves of select plants.
About a week later the young eclose (the insect equivalent of
hatching), disperse on their birth plant and nearby plants to chew
pinholes in leaves. Within a couple of weeks each caterpillar bores
into and down through a stalk or fruit stem.
The infested stem, which may harbor but a single caterpillar,
will wilt above the caterpillar's feeding site. A curious gardener
who cuts such a stem just below the wilt point will usually find
the grub-like caterpillar inside the stem.
Crush that caterpillar!
The caterpillars are pinkish-tan to gray with brown spots on
each segment. Those which were eggs in early summer have time to
progress through their five molts or "instars", pupate, fly and lay
more eggs.
That second generation may feed for a while and overwinter as
caterpillars, but in some regions it has time to complete its turn
and lay the eggs of a third wave. In Minnesota, northern Europe and
the Upper Peninsula of Michigan there's just one full cycle per
year but in the Mediterranean, Ohio and southeast Michigan there
can be two in warm years. In Florida, there may be three or
four.
By the season's last instar the caterpillars may be quite gray
but still spotted. To see the caterpillars, moths and eggs, go to
http://www.ent.iastate.edu/pest/cornborer/ and
http://entnemdept.ufl.edu/creatures/field/e_corn_borer.htm.
Be sure to click on the image thumbnails to see enlargements.
Control the
pest
In a garden, there are several steps you can take to control
corn borer infestations. First, there's clipping off infested stems
as you see them and destroying the larvae inside.
Next, watch as you have in fall for the overwintering sites and
destroy those larvae, too.
Finally, you can monitor particularly troubled host plants
during egg laying periods and spray an insecticide to kill larvae
as they emerge from the eggs. (See the chart below for help with
timing these approaches.)
Aim for caterpillars, moths too mobile
It's not effective to try to kill the moths, which are highly
mobile. More fly in to replace any you eliminate, and as they are
not big eaters they're tough to kill with toxins. The most
vulnerable stages of this pest's life are its earliest instars --
right after eggs eclose -- and in fall when larvae cannot move much
but can be exposed by cutting down host plants.
These caterpillars are very vulnerable in
fall!
On farms, probably the most important step is one you can mimic
right now, P.S. The farmer plows under the corn stubble, now or
before next spring so the larvae holed up there are killed or at
least rendered homeless and vulnerable. You can dig out that
Rudbeckia, reset a few clean pieces and throw all the rest
out being sure to hot compost, burn or pulverize all
caterpillar-infested crowns. Come on, don't waffle -- everyone has
too much Rudbeckia and if you re-set just a few divisions
each will be bigger and more floriferous than before and so
vigorous they will fill a whole bed within one season!
If you find it necessary to apply pesticides during egg-laying
season, start with products that are toxic to the narrowest range
of organisms, or that degrade into harmlessness most quickly. That
may mean starting with insecticidal soap or Neem oil, or hot pepper
spray. (Corn borer will host on bell peppers but not hot peppers so
that ought to tell us something).
Be smart if you spray to control these
pests
Change what you use from time to time because with so many
potential hosts and such a long window of egg-laying time you're
going to be applying rather a lot of pesticide. Long-term,
wide-spread use of any single substance is a formula for breeding
pesticide-resistant insects. It's also almost certain to upset the
balance of beneficial and pest insects you probably didn't even
know you had going until it's disrupted and "new" pests begin to
make themselves seen.
Not so nice in this case to be "down on the
farm"
If you garden near a corn field, you may not have much choice
but to make corn borer control a year-round occupation and employ
many diverse tactics. Because many farms are not rotating other
crops into corn fields so regularly as should be done there are few
breaks in host plant availability during which corn borer numbers
can fall off. Worse, constant exposure to the pesticides used on
corn crops, including the Bt built right into genetically modified
corn, has created strains of corn borer that are resistant to those
toxins.
I'll cross my fingers that the garden you're cutting down right
now is just having one bad year and corn borer won't be with you
for any long stay.
Predicting your
corn borer's life cycle
Here's a chart of two-generations of corn borer life that can
apply in much of the Great Lakes region and Upper Midwest USDA
zones 5-6. Any pesticide applications should be made between the
events noted in bold. If you live in a warmer or colder region
DON'T use the calendar dates* here, use the growing degree days
plus your own State's Extension service degree day calculator** to
learn when your garden reaches that mark.
Corn borer's "Coincide"
life stage
|
Approxim. calendar dates*
|
Growing Degree Days (base 50°F)
|
"Coincide" indicator plant
|
| Pupating in host plant stubble |
10/15 - 4/1 SE MI; 10/29 - 4/1 NE WI |
Up to 750 GDD |
Until bearded iris in full bloom |
| Moths emerge; egg laying |
6/25 SE MI; 6/16 Sioux Falls ND |
750 GDD |
Snowball hydrangea begins to bloom; Catalpa and hawthorn
trees in full bloom |
| Eggs begin to eclose |
7/1 SE MI |
800 GDD |
Hawthorn & Catalpa petals fall |
| First egg laying ends |
7/15/ SE MI |
900 GDD |
Snowball hydrangea ends bloom |
| Most larvae too large to poison or inside stems |
8/1 SE MI |
|
|
| Second flight moths emerge |
8/13 SE MI; 7/21 central MO |
1860 GDD |
Snowball hydrangea spent flowers fade to green; Mt. ash
fruit begins to ripen & color |
| Eggs begin to eclose |
8/20 SE MI |
1910 GDD |
|
Egg laying ends; larvae rest in plant
stubble
|
9/5 SE MI; 10/29 Green Bay WI; 8/15 central MO |
2500 GDD |
Sedum 'Autumn Joy' in full bloom
|
*Calendar dates are the least reliable prediction tool, varying
as they do from region to region and year to year. I provide my
area's dates and some other region's approximate dates, to
illustrate the wide variance.
**For more on this most accurate means to predict insect
activity, go to http://ccesuffolk.org/growing-degree-days-gdd-/
and download the Cornell University bulletin Using Growing Degree
Days for Insect Pest Management
To learn when your area is likely to reach any of the growing
degree day levels in our chart, or see a real time report of
growing degree days in the base 50 system (DD 50) for your
particular area -- to know what stage "your" corn borers are in --
search for current growing degree days your city, your State on an
Internet search engine, a weather website or at your State's
Agricultural University Extension website. As an example,
weather.com http://www.weather.com/outdoors/agriculture/growing-degree-days/
gives you a calculator that has you set the base temperature
(select 50F) and start date (use January 1) then enter your zip
code to learn when your area is likely to reach the growing degree
day level listed above. (Such as 2500 GDD level, when all eggs are
probably laid so it's about time to start controlling the
caterpillars while they're young and weak.)