Dr. Lincoln Brower shares his concerns:

"The scientific community is very disturbed by the sharp decline in the numbers of overwintering monarch butterflies in Mexico. When I first visited the Sierra Chincua Sanctuary in January 1977, the colony was 1.50 hectares in area. Until recently, Chincua always had one or more large colonies. This year it dropped to 0.02 hectares, a 98% drop from the 1.50 ha in 1977. Similarly catastrophically low numbers this year occurred in all but the Rosario colony.

"I have proposed that the normal huge number of monarchs somehow leave a chemical fingerprint on the Oyamel trees that the next year's butterflies sense to locate the area the following November. If this is true, so little chemical may be left behind by the current tiny colonies that next year's butterflies may not be able to sense their way back to the marked trees.

"The evidence we have strongly points to one main cause of this decline: the current soybean and corn crops are genetically engineered to tolerate heavy doses of herbicide that kills all competing plants, including milkweeds. Growing virtually unlimited genetically modified plants doused with poisonous herbicides is starving the rest of nature's food chains, including that of our beloved monarch butterfly as well as pollinating insects in general."

• Journal: What do the pie charts reveal about monarch distribution?