European borer takes to American corn...

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Most of us have too much black-eyed Susan (Rudbeckia), so it's an added benefit of corn borer control that we might thin the black-eyed Susans' ranks. This plant might even be worth letting go entirely! Blackeye Susan 'Goldsturm' was a drip-dry plant when first it became a U.S. favorite in the 1970's. It's run into trouble with leaf spot fungus (Search this site for Rudbeckia leaf spot) and this insect problem. 

...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 and 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 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, 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.)