Flight of the Fall Monarch

Ever wonder why a monarch butterfly typically lives its entire life in only two to six weeks, while the fourth or fifth generation typically lives for up to eight or nine months? Perhaps this timely discussion with Orley “Chip” R. Taylor Jr., in part, will provide the answer.

Sun Angle at Solar Noon (SASN)

Last December we published a paper which pointed out the timing and pace of the monarch migration was associated with the pace of the declining angle of the sun angle at solar noon (SASN). The migration appears to start at the northern latitudes when the SASN drops below 57 degrees at that latitude. The first monarch migrants generally arrive in the vicinity of the overwintering sites of Mexico in the last days of October, when the SASN again drops below 57 degrees. Further, the mating picks up at the overwintering sites (temperatures permitting) after February 12 when the SASN increases beyond 57 degrees. The first wave of monarchs reaching Texas in the spring is also aligned (weather permitting) with an increase in SASN greater than 57 degrees. There is also a “migration window,” a range of SASN values (57-46 degrees) associated with successful migration. Over 90% of the monarchs reaching Mexico were tagged within this range of sun angles.

The SASN values generally DECREASE in the fall in the migration window (57-46 degrees) when the monarchs do not reproduce. In the spring, the monarchs INCREASE their reproduction generally based on the height of the sun at solar noon of 57 degrees for any and all latitudes on the June 21. SASN values decrease slowly after June 21, but do not drop below 57 degrees at the most northerly points until late in the first week of August. In both cases, the increases and decreases are slow, incremental yet erratic.

While the above describes an overall pattern, the migrations are not in lock step with the SASN since they can be slowed by weather conditions as were the migrations of 2004 and 2019 and a few others. What we can say is, to date, there is no evidence of migrations moving faster than expected based on the pace of the SASN.

What determines the breeding cycle?

If we assume temperatures are within nominal ranges for fall migration, the SASN is just above 57 degrees, and a female lays eggs in a patch of milkweed. But not all the larvae* which hatch and develop normally, pupate* and eclose,* not all of these individuals migrate. Some do migrate, and some go on to breed.

The breeders do not emerge in reproductive diapause* – their maturation is not on hold. It just takes some number of days for the breeders to reproduce mature. Those who not go into maturity, go on to be a new season’s breeders.

In monarchs, breeding season individuals are sexually mature four to five days after they emerge as adults. The generation that migrates is not sexually mature until after the overwintering period

Mid-Summer Migration of the Monarchs

The adults that emerge from eggs laid from July 15 to August 20 at the NORTHERN latitudes (49-51N, e.g. Winnipeg and vicinity) are likely to migrate.

At more southerly latitudes, the mid-summer migration will be underway when the butterflies move south reaching latitudes south of 40N for the first week of August, progressively moving south through Oklahoma late in the second and third weeks, and Texas late in the third and fourth weeks.

Look for late-season breeding at latitudes south of 40N in the mid portion of the country. This year precipitation southward from Kansas through Texas has been abundant and should refresh dormant milkweeds in much of this region, allowing better opportunities for late-season production than during most years.

This Year’s Forecast for Mexico

The earliest record I have seen of monarchs moving into Mexico is October 3-4 near Del Rio, Texas.

The SASN drops below 57 degrees at Del Rio on October 2, signaling the earliest date that monarchs should reach that latitude under the most favorable conditions. The same applies to Acuna, Mexico. The site is just across the river from the Del Rio where they do the Mexican observations.

Today’s migration is advancing at a rate we haven’t seen since the 90s, and that is a good thing, since on-time migrations are associated with higher survival during the migration. This is a report from Journey North.

 

Notes:

The stages in the life cycle of insects undergoing complete metamorphosis: egg, larvae, pupa and imago/adult.

Egg: The egg is the first stage in the life cycle of most insects. Eggs can be laid singly, in clusters or in specialist structures called oothecae. Insect eggs are very small and often susceptible to drying out (dessication) so the female insect often selects the site to lay her eggs on very carefully. (Amateur Entomologist’s Society)

Larve: But about 75% of insects undergo a complete metamorphosis beginning with a larval stage. In this stage, the insect feeds and grows, usually molting several times. …The eruciform larvae look like caterpillars and in most cases, are caterpillars. The body is cylindrical with a well-developed head capsule and very short antennae. Eruciform larvae have both thoracic (true) legs and abdominal prolegs. (Thought Co)

Molting in monarchs is called instar. Caterpillars go through five stages of growth. Each stage is called an “instar.” As a caterpillar grows, it “molts” five times before it becomes a chrysalis. Each time it molts the caterpillar progresses to the next instar (1st instar, 2nd instar, 3rd instar, 4th instar and 5th instar). (JourneyNorth)

Pupate: Although the pupa might appear to be an inactive stage, inside the pupa the larval body of the insect is being broken down and the body of the adult insect is then formed. During pupation the insect is often completely sessile (incapable of movement) although in some species limited movement is possible. The insect is quite vulnerable at this time and so many choose to pupate out of sight from predators (such as underground) or create a camouflaged cocoon around their pupae. (Amateur Entomologist’s Society)

Imago/Adult: The adult, sexually mature, stage of the insect is known as the imago. (Wonderful Butterflies)

Diapause:  Diapause is a period of suspended or arrested development during an insect’s life cycle. Insect diapause is usually triggered by environmental cues, like changes in daylight, temperature, or food availability. Diapause may occur in any life cycle stage—embryonic, larval, pupal, or adult—depending on the insect species. (ThoughtCo)

Eclose: The emergence of an adult insect from a pupa or a larvae (or nymph) from an egg is called eclosion. Adults of diurnal insects (such as butterflies and dragonflies) often emerge at dawn whereas nocturnal insects (such as many mosquitoes) eclose at night. (Amateur Entomologists Society)