Monarch Butterfly Migration Update: May 9, 2002
Today's Report Includes:
News From the Migration Trail
- Once again this week, the most significant news is how few migration sightings have been reported. We hit an
all time low, with only 2 new sightings during the past week. (The others listed above were REPORTED after the
May 2nd update but OCCURRED earlier.)
- If past years' patterns are an accurate predictor, we expect the number of sightings to jump next week. The
number of sightings typically doubles between the 1st and 2nd weeks of May.
- Last May 2nd, the first monarch had already been reported in Ontario, Canada.
- Several "ghost monarchs," apparently from Mexico, were seen during the last week. In North Carolina,
Elizabeth Hunter saw a "VERY faded female" lay 40 eggs in three hours in her back yard. "Her wings
were dark and light brown rather than black and orange, and where the forewing didn't overlap another wing, the
wing was translucent."
Monarch Life Cycle and Mortality
Discussion of Challenge Question #26
After witnessing the single female monarch in Arkansas lay hundreds of eggs, we wondered why the world isn't overcome
with adult monarch butterflies. We asked, "At what stage of their life cycle (egg, larva, chrysalis, adult)
do you think monarch mortality is the highest?"
A New Jersey father/son team drew from their own observations to answer this question:
"My dad and I went searching for larva in New Jersey last summer. We found lots of eggs, many 1st instar,
fewer second instar and so on and the least 5th instar. Of the 5th instar, 10-15% had parasites and did not hatch.
Overall, we think that spiders, praying mantisand other predator insects get the most of the cats in the 2-4th
instar when they are big enough for a good meal."
Monarchs are under tremendous pressure from predators. The effect is being measured by participants in Dr. Karen
Oberhauser's Monarch Larva Monitoring Project (MLMP).
"We don't really know what stage monarchs are most vulnerable to predation," says Karen. "Certainly
more predators can eat smaller organisms, and our MLMP data show that most mortality occurs sometime during the
egg or 1st instar stages. Work by Malcolm, Zalucki and Brower has also shown that many 1st instar larvae die as
they take their first bite of milkweed - the latex gums up their mandibles."
Here are initial impressions:
- Of all eggs laid, as few as 5-10% survive to the 5th instar stage. (In other words, as many as 90-95% die before
reaching the 5th instar.)
- Mortality after the 5th instar stage, like that caused by the tachinid flies, MLMP data don't catch. However,
more and more of the volunteers are collecting late instar larvae to measure parasitism rates. The results are
quitevariable - samples vary from 50% parasitism in some cases, and in other cases the rates are under 10%.
Monitoring Monarch Populations: How You Can Help
Predators play a significant role in keeping the monarch population in balance--but nobody really knows their impact
on overall monarch abundance. This summer, you can help study these questions. One of MLMP's goals is to determine
what factors affect mortality rates at all stages, and from a variety of causes.
Predators and Parasites on the Prowl
Sucking the life from their victims, devouring the eggs, eating the young--you don't
have to go to Africa to see it. You can explore the predator/prey interactions that monarchs face in your own backyard.
You'll be amazed at what you see. Here are pictures of monarch predators and parasites, plus links to more information.
(See female tachinid fly laying her eggs on the skin of larva.)
Exploring Milkweed Ecology
Ecology is the study of interactions between living things and their environment. As you know, milkweed plays a
central role in the lives of monarch butterflies. Here are 8 different animals that also interact with milkweed,
in one way or another. This lesson challenges you to match each organism with the behavior described.
The Next Monarch Migration Update Will Be Posted on May 16, 2002
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