Many readers of The Wildlife Volunteer have expressed interest in the long term status of the Monarch butterfly. In recent years this beautiful insect has been declining in numbers throughout Eastern North America and has become a concern of many wildlife conservationists. Because Michigan is close to the northern terminus of the butterfly’s breeding grounds all our citizens should take a special interest in this animal.
The numbers are in for flies on their wintering
habitat of oyamel fir trees in Mexico.
Scientists census the Monarch by measuring the amount of land the
population occupies during the critical wintering period. This is made possible by the animals
themselves, because they “pack” as close as is necessary to keep warm during
winter nights. Measuring the acreage
they occupy provides a relative population estimate, or index. The winter population census began in 1994
and the long term average acreage of monarchs is 17.27 acres (6.99
hectares). There are millions of
individuals in that packed area so absolute numbers are impossible to
The measured area the Monarch occupied in the
winter of 2011-2012 was 7.14 acres (2.89 hectares), a decline of 28% from last
year’s numbers, and 59% decline from the eighteen year average. The sad news for Michiganders is – expect a
30% decline in butterfly numbers this summer.
It would be easy to blame the vagaries of climate
for the Monarch’s decline - Exhibit A – 86o in Lansing on March
21. But there appears to be so much more
going on here. Researchers in Texas and
Kansas point to an inverse correlation between the number of acres planted to
herbicide tolerant row crops on the Monarch’s flight path and the fly’s
numbers. Crops are now being bred to
tolerate spraying of Round-up and other herbicides, so that fields can be
sprayed to kill weeds after the crop is growing. The later spraying is very effective to
control weeds, even milkweed, in and around crop fields. Without milkweed there will be no Monarch
butterfly, the animal can’t exist without it to lay eggs on.
Herbicide tolerant corn and soybeans were
introduced in 1996, the year Monarch numbers peaked at 51.82 acres in
Mexico. The butterflies have been
declining as more herbicide tolerant row crops have been planted. By 2004 genetically modified corn and
soybeans accounted for 51% of planted acreage.
That figure reached 81% of all planted row crop acreage by 2010. This trend seems very ominous for the
Monarch, unless management strategies are adopted to accommodate the insect in
the agricultural landscape.
One possible step forward is to encourage
additional research on alternatives to widespread herbicide use. Organic farmers are experimenting with
sandblasting of corn fields early in the growing cycle. Sandblasting takes ground-up corn cobs
(available at most farms) and sprays them under pressure at the undesirable
weeds. Early research found that weeds
can be controlled in corn fields if multiple treatments are done at the
one-leaf, 3-leaf, and five leaf stage of the corn plant.
Other sandblasting materials are being considered
such as nut shells and even fertilizer, or crushed limestone. These alternatives may help with soil
fertility or structure, which are important in agriculture.
If sandblasting weeds replaced even some herbicide
spraying in agricultural crops on the Monarch’s migration route it may be very
beneficial for the butterflies.
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