Timing of monarch butterfly larval feeding in the Corn Belt

Paul Cherubini paulcher at concentric.net
Sat Sep 11 20:28:46 EDT 1999


> I've been led to believe that in the Midwest, larval feeding can occur any
> time from April or May to October or so.  But I assume that there are much
> greater numbers somewhere in the middle of that time frame.  Also, I don't
> know how discreet or overlapping generations are in any one place.

Yes, larval feeding does occur anytime from April or May to October.
Below are two recent posts from the dplex discussion list that relate to
your questions about timing. Also, the generations are overlapping and
not very discrete since female monarchs live about 6 weeks in summer and
lay eggs over much of that period..

From: Monarch Watch <monarch at ukans.edu>
To: dplex-l at raven.cc.ukans.edu
Subject: Re: Early signs of migration
Date: Thursday, September 09, 1999 8:48 PM

The number of reproductive monarchs presently in the area, indeed
all of August and early September,  has been much higher than I
given the low production of monarchs during the summer. Eggs and larvae
have been easier to find than has been the case in past years during

Subject: Nebraska Migration
Date: Fri, 10 Sep 1999 15:05:20 -0500 (CDT)
From: rthorn at lps.org (Rosemary Thornton)
Reply-To: dplex-l at raven.cc.ukans.edu
To: dplex-l at raven.cc.ukans.edu
I haven't been able to find any eggs or larvae on milkweed in our garden
since late August. Usually we can find a few eggs and monarch larvae of
different sizes into September. Not so this year although I could find
quite a few earlier in August.
Rosemary Thornton
Science Teacher
Fredstrom Elementary School
Lincoln, NE 68521

> Corn pollen dispersal occurs from about the end of June to the first week of
> August in the Corn Belt, with the majority of pollen drop occurring for two
> weeks or so in the middle of July.  If larval feeding occurs during this
> time, then there is risk. Of course, we have to rule out preferential
> avoidance of pollen, look at the abundance of milkweed in or near
> cornfields, how far pollen is dispersed, look at the dose-response
> relationship, etc. etc.

Iowa, a state encompassing a total of about 55 million acres, is the top
producer of the corn belt states. Remarkably, about 30% (16 million
acres) of the entire land area in Iowa is planted in corn and about 30%
(5 million acres) of this massive corn crop is planted in Bt corn. So
this means about 10% of the entire land area of Iowa was planted in Bt
corn this summer. Impact of 5 million acres of Bt corn on the monarch?
Well the following recent posts to the dplex discussion list suggest
monarch numbers in Iowa are pretty high this year: 

Subject: Re: Early signs of migration
Date: Fri, 10 Sep 1999 06:18:56 -0500
From: "THE NEWBROUGHS" <newbs at netins.net>
Reply-To: dplex-l at raven.cc.ukans.edu
To: <dplex-l at raven.cc.ukans.edu>

One overnight roosting spot that I am aware of has had thousands of
monarchs roosting on trees each night since Sunday of this week!  This
located near Tripoli, Iowa, in northeast Iowa.

Happy tagging, all!!


Subject: Migration
Date: Sat, 11 Sep 1999 21:32:21 -0500
From: Jim Langhus <langhusjd at mflmarmac.k12.ia.us>
Reply-To: dplex-l at raven.cc.ukans.edu
To: Dplex-L at UKANS.EDU

Greetings from Northeast Iowa;

The migration flood gates are open now.  When we woke up this morning,
there were a large number of monarchs in the air and by 10:30 AM, you
could see up to 10 per minute crossing the road ahead of you.  We began
tagging and by noon had tagged 50 nectaring on the roadside flowers. 
Then we returned home to find a large number gathering in the trees
behind our house and tagged a bunch more.  We were then called by some
friends and ended up with a total today of around 285 monarchs tagged.  

The Langhus'
Monona, IA

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