What is the European corn borer and why should I care?
The European corn borer (ECB) is a moth that lays eggs on
the undersides of corn leaves, but it’s the larval stage of its life cycle that
poses a risk to corn (Figure 1). Larvae
initially feed on leaves and when they get bigger tunnel into stalks and ear
shanks. Tunneling affects a plant’s
ability to transfer nutrients throughout the plant and can reduce yield by
reducing kernel weight and number. Tunneling
can also let in stalk rotting fungi and result in stalk breakage and ear drop.
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| European corn borer (Ostrinia nubilalis). Photo: Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org. |
ECB and Bt corn.
More than 20 years ago scientists at seed companies found a
way to transfer a gene from a soil-borne bacterium called Bacillus thuringiensis into the corn genome. This bacterial gene produces a protein that is
toxic to corn borer larvae. When a larva
eats tissue from a Bt hybrid they eat this protein. The protein breaks down to produce a toxin
that causes the larval gut contents to leak into the rest of the body cavity –
which, as one might imagine, is fatal.
Before Bt corn was developed, farmers and crop advisors had
to closely monitor moth flights and egg laying by corn borer moths and if
populations warranted carefully time an insecticide spray. Larvae are only susceptible to insecticides
for 10 to 14 days, after which time they are big enough to tunnel inside the
plant, and are protected from insecticide applications. Even the best timed insecticide application is
less effective than growing a hybrid with the Bt trait, around 70% versus 99.5%
control, respectively.
So why would someone want to plant non-Bt corn?
Although the benefits of Bt corn for ECB in Minnesota have
been well documented for 20+ years. The economic benefit not only occurs for Bt
fields, but also for non-Bt fields due to the ongoing areawide suppression
effect of Bt corn on ECB; i.e., Bt corn has “driven down” ECB populations
across the state so that the yield benefit occurs in non-Bt fields but without
the tech fee or increased seed costs.
Because seed costs for Bt corn continue to be high – as much
as $20/acre more than conventional seed, and given current corn prices, more
growers are looking at ways to reduce input costs. In the current economic environment
when every penny counts $20 is a big deal.
Agronomists advise that when selecting which field corn
hybrids to plant it is best to consider yield potential first and then focus on
other things like herbicide tolerance traits, Bt traits and disease resistance.
Each farmer has a different tolerance
for risk. Farmers also need to factor
into their hybrid selection decision whether starting out with a non-Bt hybrid,
that may have a lower yield potential, and then spending time and money to
scout and potentially treat with an insecticide ends up saving money.
In southern MN where most of the corn acres have the ECB Bt
trait, if a person wanted to plant non-Bt corn, ECB population densities would likely
be low enough to not warrant insecticide treatment, particularly within the
next few years. In the northwest, where
corn is newer to our cropping system, there are more non-Bt acres, and so there
may be less of a landscape-level effect on ECB population densities.
2019 ECB Survey
There are several objectives that UMN Extension entomologists hope to learn and share with corn growers as a result of this survey:1) Are there changes in ECB population densities over time? To what extent does the areawide suppression effect occur in the NW?
2) Have any population shifts taken place? ie. is the Bt trait still effective and does ECB in NW MN continue to only produce a single generation of larvae each year.
In southern MN, some ECB produce a single generation each year while others produce two generations. Understanding this is essential for those that plant non-Bt corn to effectively manage ECB.
3) What percentage of larvae and adult moths are infected by a fungal pathogen of ECB, called Nosema?
This ECB pathogen naturally reduces larval and moth populations, causing both direct mortality as well as sub-lethal effects, such as how long moths live and the number of eggs produced by females.
As part of the survey this year, I have been visiting both Bt and non-Bt corn fields and looking for evidence of corn borer feeding. If I see evidence of ECB, my lovely assistant (my husband) uses a spade to take out the corn plant and we bring it back to the truck to split the stalk and look for larvae. If we find larvae we put them into little plastic containers filled with an artificial diet that the larvae can use for food and shelter (Figure 2). The larvae are then brought to campus in St. Paul where they are studied further in the lab of UMN Extension entomologist Dr. Bill Hutchison.
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| European corn borer larva on an artificial diet medium after capture during the 2019 UMN European corn borer survey. Notice the holes in the diet in the cup's upper right hand corner caused by feeding. |
Stay tuned for more information about the results of this survey in northwest Minnesota and what they might mean for managing European corn borer in 2020.
References.
Bohnenblust, E. and Tooker, J. 2010. European corn borer in
field corn. Online.
Pennsylvania State University Extension Entomological Notes. October 3, 2019.
Capinera, J.L. Kalisch, J., Hunt, T. and Clark, T. 2017. Featured
Creatures. University of Florida Dept. of Entomology and Nematology. Online.
EENY-156. October 3, 2019.
Cook, K.A., Ratcliffe, S.T., Gray, M.E and Steffey, K.L.
2003. European corn borer (Ostrinia
nubilalis Huber). Online.
University of Illinois IPM program. Verified October 3, 2019.
Ostlie, K., W. Hutchison & R. Hellmich (eds.). 1997. Bt Corn &
European corn borer: Resistance Management for Long-term Success. (Online,
Univ. of Nebraska). Verified October 14, 2019.
Potter, B., Ostlie, K. and Hutchison, W. Reducing Bt corn
acreage to cut production costs. Online.
University of Minnesota Extension IPM program.
Verified October 3, 2019.
