There is nothing like an outing on a glorious autumn afternoon to lift the spirits amidst Mother Nature’s brilliant display of fall colors. While deciduous trees like maples, tupelos, and sweet gums revel in their harlequin colors, stately pines provide a deep green backdrop for their showy cousins. While jogging beneath a row of white pines, I was surprised by swarms of awesome stingers, yellow jackets, bald faced hornets, European hornets, and paper wasps amassing on a sidewalk and patrolling branches overhanging the sidewalk. Was this some aberrant flash mob of the fierce hymenoptera? A closer look downward revealed dark wet patches of pavement beneath the pines. A closer look upward at the pine revealed thousands of white pine aphids lining the branches, and as I stood beneath the tree a sticky shower of honeydew rained down on me.
A closer look at a sidewalk soirée reveals hungry yellow jackets tasting the sidewalk with busy antennae and lapping up sweet honeydew with their long tongues.
Autumn is a time of high activity in many species of aphids as trees transport nutrients in their vascular tissues. Carbohydrates move from photosynthetic factories in leaves and needles through vascular plumbing called phloem, to nourish roots and other living tissues in the tree. Many sucking insects, including aphids, evolved sucking mouthparts to access the nutrients carried in phloem. Both adult aphids and their immature stage, called nymphs, insert stout beaks into the plant’s phloem. The beak encloses a tube connected to a miniature pump in the aphid’s head. The pump passes food along to the digestive system of the aphid where nutrients are extracted and used for growth and development. However, phloem sap is relatively poor in nitrogen but high in carbohydrates. To obtain enough nitrogen for growth and development, large volumes of plant sap must be imbibed. Excess residue of this extraction process is excreted by the aphid as sweet sticky liquid called honeydew. Honeydew coats branches and needles and provides a substrate for the growth of a non-pathogenic fungus called sooty mold that blackens plant parts on which it falls. Honeydew also accumulates on sidewalks and vehicles, creating a wet sticky mess.
A steady shower of honeydew rains from the rear ends of thousands of white pine aphids as they suck sweet sap from branches of an unfortunate pine while yellow jackets zoom about.
Amidst my honeydew shower, I pondered how populations of white pine aphids exploded seemingly overnight. I had passed beneath these very same trees just a few weeks ago and did not notice hordes of aphids. Here’s the secret. Many species of aphids engage in a type of asexual reproduction known as parthenogenesis, meaning that the population contains only females and no males. No wasting time on guys. That’s right, just grandma, mom, and the granddaughters giving birth to more daughters by the hundreds. When it comes to parthenogenetic aphids, there’s a sucker born every minute.
Another fact of life contributing to explosions of aphids is their ability to eschew the usual insect-like business of laying eggs. At certain times of the year, many species of aphids dispense with the egg stage and, like humans, give live birth to their babes. This blessed event takes only a few minutes but appears to be fraught with significant drama. Birthing aphids do lots of posturing and pushing. Fortunately, aphids have sucking mouthparts, so the loud vocalizations that accompany human births are conspicuously absent, hence the silence of the aphids. To further accelerate the process of filling the world with their kind, female aphids carry embryos of their grandchildren within their bodies even before they are born. This greatly compresses the generation time for aphids and is part of the reason aphid populations rapidly grow from a few to thousands.
At 32 times normal speed, this live birth of an aphid takes less than half a minute. Birth in aphids is breech. And, is that a midwife or simply a bystander who came to watch?
Burgeoning populations of white pine aphids are about to come to a screeching halt with the approach of winter and cold weather. To survive this inimical season, parthenogenesis will be replaced with the more traditional mode of sexual reproduction. Female aphids will give birth to winged sons and daughters that will mate with others of their kind. Live birth, the hallmark of aphids in the growing season, will be replaced by the production of eggs by inseminated females. In a few weeks, pine needles will be festooned with rows of beautiful ebony eggs capable of surviving winter’s chill. White pine aphids are quite host specific and if you have a chance to pass by a white pine in the next week or so, look up and look down for white pine aphids, their honeydew, and associates they attract.
“Managing Insects and Mites on Woody Plants: An IPM approach” by J. A. Davidson and M. J. Raupp, and “Insects that Feed on Trees and Shrubs” by W. T. Johnson and H. H. Lyon, were used as references for this episode.