

Monarch Migration Update: May 4, 1999

Today's Report Includes:

Quiet Due to Generation Gap?
Every year at this time, things become very quiet on the migration-tracking front:

Why do you think only 7 sightings of FIRST monarchs were reported during the last week?

From New Orleans, Joey Donnelly's observations remind us what's happening. In his backyard, "Monarch are hatching at the rate of one or two per day. Some milkweed plants are already munched to twigs by larvae. The adults must be moving north after a day or so, because we are not noticing an accumulation."

Throughout their range, let's hope the female monarchs can find the milkweed they need! Which brings last week's question to mind:

How Do Monarchs Find Milkweed?
Discussion of Challenge Question #31

From Miss Bailey's Class in Vero Beach, FL:

"Our class thought about how monarchs find milkweed. We all think that the senses of smell and taste are used. The monarchs use smell receptors on their antennae to find the milkweed plants when they are far away from them. After the monarch lands on a plant it thinks might be milkweed, it uses the taste sensors on its feet to make sure. Latasha wrote in her journal, 'They find milkweed by smelling with their feet.' Brooke said, 'They just go from one plant to another to see if it's a milkweed plant or not. They use the senses of smell and taste.' Bobby wrote, 'Monarchs find milkweed by using their feet when they are on the milkweed.'"
Citrus Elementary School

Here's Dr. Bill Calvert's response:
In general, insects that eat plants (herbivorous insects) are thought to find their host-plant by using several different senses and behaviors. As they get closer and closer, each step involves more precision. For example:

Vision: They probably use vision to find the general habitat. (I say probably because the last time I studied these matters the host finding mechanism was still largely speculative. But I go on...)

Olfaction: It is thought that once in the proper habitat, they use olfaction to get them on to a possible host. (As the students point out, the antennae are responsible for the sense of smell--not a nose!)

Gustation: Once on the possible host, they use gustation to make sure it's the right host plant. In the case of Lepidoptera, spines on the forelegs are used to stab the leaf surface. The butterfly tastes whatever comes out of the leaf with sensory hairs that twist around one another from the base of the spines.

Now think about this from the viewpoint of the plant. (Plants don't like to be eaten! I don't think many vegetarians realize this!) In general, plants tend to grow in groups called "guilds". That is, certain plant species are often found grouped together. Scientists think that these guilds might serve to protect individual plants. Here's how: Together, the different kinds of plants produce a mixture of plant odors. The herbivore can't find the correct host because there are too many confusing odors. This would explain why other senses need to be used.

In the case of monarchs, stabbing forelegs and associated gustatory sensillae are needed. The female flits over the fields, and when she stops, she drums the surface with her forelegs. In the process, she tastes the plant for suitability. If suitable she will lay an egg on it.

I have found preferences in laying for certain hosts. The one I have been finding attractive to female monarchs is A. oenotheroides. They much prefer this to A. asperula which overlaps their range. But in areas with only A. asperula, they appear to do quite well. This is adult choice. I don't really know the basis of it. One of Lincoln Brower's students did some work on this. I do remember that it wasn't a simple function of cardinolide concentration.

Now Larval choice is a different matter. It's easy to set up a simple experiment. All you do is simply.....

Challenge Question #34
"How could you design an experiment to test what kind of plants larvae prefer?"


Hummingbirds First to Cross the Canadian Border!
On May 1, the first ruby-throated hummingbird was reported in Ontario, Canada. Sorry monarchs, the prize this year goes to this other hard-flying, flower-powered migratory species. Last year, the monarch won, with the first Canadian sighting on May 4 in Castilia, New Brunswick (44.73 N, -66.75W), beating the hummingbird by a single day. The first 1998 sighting was from Quebec on May 5. Let's see who will win in 2000...
How to Respond to Today's Challenge Question

1. Address an e-mail message to:
2. In the Subject Line of your message write: Challenge Question #34

The Next Monarch Migration Update Will Be Posted on May 11, 1999.

