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 determine.
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.