How Do Monarchs Know When to Leave?
Knowing When To Leave
Where Do They Go?
Finding Overwintering Sites
Monarch butterflies have a complicated
life cycle, in that monarchs emerging at different times of the year do different
things. Monarchs that emerge in the spring and summer months become reproductive
within a few days. Monarchs emerging in the fall are in reproductive diapause, which
is a state of suspended development of the reproductive organs. Even though these
butterflies look like summer adults, they won’t mate or lay eggs until the following
spring. Monarchs have to know when to fly south, and also when to begin the journey
Monarchs in the fall begin clustering together.
When the late summer and early fall monarchs emerge from their pupae, they are physically
and behaviorally different from those emerging in the summer. The shorter days,
cooler air, and milkweed senescence (aging) of late summer trigger changes. In the
northern part of their range, this occurs around the end of August, when monarchs
begin to emerge in reproductive diapause.
Diapause is controlled by the nervous system and by hormones. Environmental factors
signaling the onset of unfavorable conditions are involved in triggering this physiological
response. These factors include day length, temperature, and hostplant quality.
Decreasing daylength is one of the most important factors that cause monarchs to
emerge in reproductive diapause. In a series of experiments, Liz Goehring (U of
MN graduate student scientist) found that monarchs reared under constant short and
long daylengths were mostly reproductive, while those reared under decreasing daylength
were more likely to be in diapause. Therefore, she concluded that it is the change
in daylength that is an important cue, rather than absolute day length.
Fluctuating temperatures also contribute to the onset of diapause in monarchs, although
not as strongly as decreasing day length. Temperatures become cooler in the fall
in northern states, but they also begin to fluctuate more. It might still be quite
warm in the day, but nights are much cooler than they are in the summer. In the
same series of experiments mentioned above, Liz Goehring found that diapause was
twice as likely to occur in monarchs reared under a fluctuating temperature treatment
where night temperatures were lower (21°C / 70°F) than day temperatures
(27°C / 80°F) than those reared under a constant temperature (27°C /
Host Plant Quality
Young (top) versus old (bottom) milkweed plants
Another cue that monarchs might use is host plant quality. As cold weather approaches,
plants begin to senesce and their leaves become yellow and dry. Liz Goehring manipulated
the quality of potted tropical milkweed plants (Asclepias curassavica) grown
in a greenhouse so that some were young (good quality) and some were old (poor quality).
She found that monarchs reared on old plants were more likely to emerge in diapause
than ones reared on young plants. However, in another experiment in which she compared
cuttings of wild and greenhouse grown common milkweed (A. syriaca), hostplant
characteristics had no effect. This could have been because cuttings may not convey
plant quality cues as accurately as uncut plants. It is also possible that a hostplant
effect is expressed differently in A. currisavica and A. syriaca.
However, the first experiment suggests that host plant quality can be an important
cue in the onset of diapause in monarchs.
A Combination of Cues
These cues (decreasing day length, fluctuating temperature, and poor host plant
quality) act together to induce diapause in monarchs. However, diapause can occur
in monarchs exposed to only one cue. Making use of more than one cue to assess the
current and near future habitat suitability could be a more optimal strategy for
organisms in unpredictable environments.
Monarchs at the Mexican overwintering sites become very active as the winter ends
in late February.
North American monarchs spend the winter roosting in trees at sites in Mexico and
southern California. They cluster together, covering whole tree trunks and branches.
As the winter ends and the days grow longer, the monarchs become more active and
begin a 3-5 week period of intense mating activity. In Mexico, they begin to leave
their roosts during the middle of March, flying north and east looking for milkweed
plants on which to lay their eggs.
The timing of diapause completion seems to vary considerably across individuals
within an overwintering colony. Overwintering populations are comprised of monarchs
coming from a wide geographic area, subjected to a wide range of environmental conditions.
Consequently, these monarchs are not all the same age and haven’t experienced the
same environmental conditions. Interestingly, monarch diapause appears to last longer
in females than in males.
There are several factors that may influence the progression of diapause in monarchs
and trigger the development of the reproductive organs. The rate of diapause development
in insects is often driven by temperature. Other factors that may influence diapause
development include day length, moisture, food, mating, host plant availability,
and stimulation by body damage. Once diapause is complete, the insect may continue
to remain dormant until environmental conditions are suitable.
Availability of Milkweed
Monarchs overwinter in patches of forest, which typically contain few, if any, milkweed
plants. Optimally, monarchs should not begin development of their reproductive organs
unless they will soon have access to milkweed, as females cannot lay their eggs
on any other type of plant. Liz Goehring (U of MN scientist) conducted a series
of experiments on post-diapause reproductive development in monarchs. She found
that most females without access to milkweed lacked mature oocytes while the majority
with access to milkweed developed mature oocytes within 3-4 days. Therefore, access
to milkweed stimulated post-diapause reproductive development. However, all females
developed mature oocytes within 2 weeks of experiencing warm temperatures, indicating
that milkweed is not required for diapause completion.
Females must mate before they can lay fertile eggs, so females may be more likely
to complete diapause and become reproductively mature after they have mated. This
has been found to be the case in monarchs; postdiapause females produced mature
oocytes more rapidly if mated. However, it also is not required for oogenesis in
monarchs. Females can complete diapause and become reproductively mature before
they have mated.
Because decreasing day length is very important in signaling monarchs to enter diapause,
one might suspect that increasing day length might be important in signaling them
to complete diapause. However, Liz Goehring found no evidence to support this hypothesis.
In her series of experiments, increasing day length had no effect on monarch post-diapause
ovarian development, although it may be important in triggering other changes related
to diapause completion.
Mating monarchs. Note that the male (top) is much smaller and more faded than the
Body condition might be important in determining when monarchs complete diapause.
The longer a monarch remains in diapause, the more energy it uses. One might think
that monarchs in poor condition would complete diapause earlier, when they begin
to deplete their energy reserves. Several researchers have found this to be true
of males. They have found that males mating early in the season (who have completed
diapause early) are thinner and more tattered than males still roosting early in
the season (who are still in diapause). However, Liz Goehring found evidence to
support the opposite hypothesis in female monarchs. She collected hundreds of females
at an overwintering site in Mexico, and found that larger females were more likely
to have initiated ovarian development, suggesting that larger females complete diapause
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