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 back north.

Fall Migration

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.

Day Length

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.

Temperature

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 (21⁰C / 70⁰F) than day temperatures (27⁰C / 80⁰F) than those reared under a constant temperature (27⁰C / 80⁰F).

Host Plant Quality

Young (left) versus old (right) 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.

Spring Migration

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.

Mating

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.

Day Length

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.

Body Condition

Mating monarchs.  Note that the male (top) is much smaller and more faded than the female (bottom).

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 earlier.

On To:  Where do the monarchs go?