Where did all the monarchs go?
by Carol Cullar, February 11, '08
October is Monarch Month all across South Texas. Starting around the 9th the skies are generally filled with the flutter of hundreds of thousands, if not millions of these colorful creatures in dedicated flight toward the south southwest, steering an uncharted course toward an unseen, but certainly not unsensed destination. Then again, not.
Take this past year for example. Monarchs zipped through Maverick County on the 18th-19th of October and by the 21st. they were gone. Perhaps as few as 50,000. Only the ragtag stragglers came through after that, begging the question, “Where did Maverick County’s usual plethora of monarchs go?” What happened to the 180,000 migrants we were expecting?
This past week, Francisco Luna Contreras, spokesperson for PROFEPA [Mexico’s environmental protection agency] announced that the overall area of December’s monarch population in the Transvolcanicos of Central Mexico was 1.5 hectares smaller than the area calculated at this time last year.
This interesting factoid presents quite a few questions for us: How does one measure 200 million butterflies, all densely packed and clinging to trees in rugged canyons above 9,000 feet elevation on the side of a cinder cone volcano—albeit, an extinct one? It wasn’t until the deadly snow of ’02-03 that scientists began to get a handle on that one. They counted the dead on the forest floor in a square meter and 6 inches deep and extrapolated the numbers based on the thickness of the dead zone and its area.
What we learned from that macabre task was that previous population estimates were low by several tens of thousands per tree, if not millions per forest. At that point researchers realized it was more accurate to give up on a head count (or proboscis count) and rely on the area into which the monarchs had condensed. In late December of each year the monarchs manage to shuffle about within their colonies and condense even further, so another measurement of their area will be taken late January/early February.
For the present, the total area of all the colonies within the Reserva de la Biosfera de la Mariposa Monarca, some 56,000 hectares (a hectare is 2.6 acres) of forest and high meadows in the states of México and Michoacán, is 4.6. They expect the January area to be 4.1, which is the typical consolidation percentage.
Last year’s figures were 5.6 with the early figures at 6.6. In the last fifteen years, only the years of 2000/2001 and 2004/2005 were smaller than this year. These were due to die offs in the Reserva caused by severe weather.
Fortunately, so far this season there have been no freezing rains or snows to reduce the population. The current survivors have only to weather the next four weeks before they began their trek back north to this part of Texas. [Between February 28th and March 15th all the monarchs will begin to mate and struggle northward looking for milkweed in South and Southwest Texas.]
Another question or two: Why was the migration through here in October so small and short and quick? What factors determine the migration route through the Central Flyway and the numbers arriving in the Reserva each fall?
Three nations are interested in actively seeking answers to these questions. It will be noted that Sr. Luna Contreras was quick to mention that Mexico had no responsibility for the numbers arriving at her doorstep, but was willing to concede that reduced numbers in the colony around Contepec might in fact be attributed to local logging in the region, although Mexico is working harder each year to protect the forests and huge Oyamel fir trees vital to the sheltering of the monarchs.
Luna Contreras eagerly pointed to factors in the monarchs principle breeding grounds in the North Central U. S. and southern Canada as possible determinants for the reduced numbers.
It’s a fascinating and complex topic. A circle without end. Here are the primary factors:
1)Monarchs arriving in the spring from Central Mexico need to be met with abundant milkweed and few predators in South Texas. These conditions are contingent on rainfall and man. High rainfall encourages the growth of milkweed, but also gives a boost to the fireant population. Fireants destroy 98% of the monarch breeding population in SW Texas. Pesticide applications to remove fireants also impact monarch larvae in negative ways. Humans are in charge of the roadside mowing schedules throughout the region. Herbicides in pastures to knock out shrubs and weeds also take their toll on the availability of milkweed. Recent research has determined that size of the successful Generation One monarchs in this region determines the entire U. S. population east of the Rockies for the coming year.
So how could humans improve those conditions? No-mow regions along roadsides from January through early May would promote increased monarch numbers, as would fireant eradication through non-pesticide means. Garden plantings of native milkweed, other than the Tropical or Mexican milkweed would be of tremendous benefit.
2)North of San Antonio and reaching to the Plains in Canada, the increased use of GMO crops like Bt corn and soybeans has seriously threatened the number of milkweeds available in the primary breeding grounds. Another factor in this region is occasional cool, wet summers, which slow the lifecycle and therefore reduce the number of monarchs hatched in a single season.
Thousands of individuals have joined the Monarch Waystation Program to install butterfly havens and native plant gardens in this region to counterbalance deleterious agribusiness practices in the Corn Belt.
3)And then there is the fall weather pattern during the migration. For example, this past summer of 2006 saw favorable weather conditions for a healthy summer population in the North Central states. Migration progressed nicely out of Canada, Minnesota, Michigan, Iowa in late September. BUT then the jet stream got involved, and a huge dome of high pressure stalled out over the Central U.S. Monarchs by the millions began to pile up in Eastern Kansas and Oklahoma. They typically wait for a norther to blow them south, conserving energy expenditures. Hundred of thousands built up in commercial sunflower fields, nectaring and fattening up for the long haul. They waited and waited. Finally, a front came through, blowing fiercely to the southeast.
Monarchs that would have arrived in Maverick County around October 9th to the 12th, instead were blown into East Texas and Louisiana. When they struck the Gulf, they were compelled to turn along the Coast and fly back toward the southwest. Monarchs from the Central Flyway were forced to join the more heavily diseased monarchs from the Eastern migration. Two things happened. One: They added many hundreds of miles to the distance they were forced to fly. Two: The number of days they perforce had to travel were greatly increased. At roughly 24 miles per day one can see that an additional 500 miles would create considerable delay.
Evidently, fat storage plus distance plus timeline is somehow locked into the genetic clock each migratory monarch is committed to. Research has shown that monarchs that fail to depart their latitude in a timely manner or are delayed by other factors can experience a shutdown of their timing apparatus and the migratory imperative switches off. Even extreme warm weather in South Texas in late October can trigger them out of their reproductive diapause and into reproductive mode. Once that happens, the impetus to migrate is lost.
Dr. Orly Taylor of the U. of Kansas Entomology Dept. has long claimed, “If it’s the Coast; then they’re toast.” Dr. Taylor’s fifteen year migratory study, based on tagging monarchs with a small wing sticker, has shown that Coastal monarchs do not arrive in Michoacán and the Reserva. It is possible, these monarchs are over-wintering in as yet undiscovered colonies or the smaller colonies on the flanks of Popocatepetl east of Mexico City. But they don’t make it to Michoacán.
So where did the few monarchs who blustered out of here on a rough wind October 20th come from? It’s possible they were the leading edge of the migration that had actually made it south of the developing high pressure before it stalled out to the north. But they still arrived a full ten days later than normal.
One thing is clear. In four weeks the whole, vast, complex engine will power up, fluttering out across the continent one monarch at a time, beginning the cycle of life for this fragile creature yet again.