Other September events
If black bears don't find them, ladybird beetles ("ladybugs") may gather by the hundreds of thousands on Green Mountain summit and Bear Peak. Surviving beetles will burrow into crevices in the rocks to spend the winter.
Giant evening-stars (Nuttallia decapetala) unfurl around dusk on sandstone outcrops along U.S. 36 just north of Boulder. Their saucer-sized, snowy white blossoms are pollinated by night-flying sphinx moths.
Aspen color peaks around the fourth week of the month.
Sage thrashers and pinyon jays pass through. Look for them in shrubby areas of Doudy Draw, south of Boulder and in the foothills around Lyons.
When many of us think of milkweed, we envision monarch butterflies dangling from rose-petaled blossoms or yellow- and black-striped monarch caterpillars munching on soft green leaves.
But milkweeds, though toxic to most species, host a potpourri of insects, including enchanting tomato-red milkweed beetles.
Copious amounts of rain — nearly 40 inches drenched south Boulder during the past 12 months — have contributed to a profusion of milkweeds and their associated beetle populations.
On a recent walk at Sawhill Ponds Wildlife Area, we counted more than 50 red milkweed beetles (Tetraopes tetraophthalmus) during 30 minutes, and we found ourselves stopping to admire nearly every one.
The scientific name Tetraopes derives from the Latin for "four eyes" and describes the remarkable placement of this longhorn beetle's antennae. The antennae bisect the eyes, creating the appearance of one set of eyes looking down and a second set looking up. Such an adaptation seems almost unnecessary, since the red background coloration of these black-spotted beetles warns potential predators of their toxicity.
Insects that feed on milkweeds accumulate concentrations of cardiac glycosides in their systems, and birds that eat a monarch or red milkweed beetle typically grow sick and vomit up their meal. Most insects that habitually feed on milkweed, from day-glow yellow oleander aphids to red and black milkweed bugs to orange and black monarchs, have evolved brilliant coloration to warn predators that they are distasteful.
With their near immunity to predation, red milkweed beetles seem almost fearless, and it's easy to approach to within a few inches of feeding or basking individuals. On cool mornings, we've even been able to coax one or two onto an index finger. The beetles' sharp black tarsi tickle the skin, and when we bend an ear we sometimes hear little squeaking or purring sounds they make by rubbing together friction-producing structures on the first two segments of the thorax. If they do finally fly off, it's in slow motion, like miniature helicopters.
Of course, feeding on a perennially toxic plant creates challenges for these beetles, along with monarchs, milkweed bugs and other insects that complete their entire life cycles on milkweeds. Red milkweed beetles may sequester some of the cardiac glycosides in regions of their bodies where the toxins pose little threat to the internal organs.
In addition, both red milkweed beetles and oleander aphids actively "farm" their host plants, chewing through major veins in the milkweed leaves to cut off the flow of toxic latex to areas where they intend to feed. Studies have shown that for some individuals, vein-severing can reduce latex consumption by more than 90 percent.
In early summer, female red milkweed beetles lay their eggs at the base of a milkweed stem, sometimes using their ovipositers to inject the eggs directly into the stem. Larvae feed on milkweed roots and remain underground or within the root throughout the fall and winter. They typically pupate in spring and emerge a month later as adults.
So assuming we receive enough moisture throughout the coming fall and winter to keep the milkweed roots from withering away, we should have even more of these gorgeous red beetles to admire next summer.