By Diane Waltman, Colorado State University Cooperative Extension agent, Horticulture
"It's sap," say the neighbors, as you deplete your supply of windshield wiper fluid while removing the sticky stuff from your car windows.
What the neighbors call "sap" probably is the honeydew from aphids feeding in the trees above your car.
The elm leaf aphid is widely distributed throughout the state, particularly on American elms. High populations can cause premature leaf yellowing and drop. The linden aphid and western dusky-winged oak aphid also are common on their associated hosts.
If you have one aphid, you can have thousands -- actually, millions. That's because they reproduce at a startling rate. In fact, every expectant adult aphid is at the same time an expectant grandmother! Mathematically, from one aphid in nine generations, we could have 600 billion aphids. We don't have to contend with that many, however, because predators and parasites are busy keeping the aphid population somewhat under control.
The lady bird beetle is one of the best known predators and both the adult and larva will consume large numbers of aphids. The lacewing is another naturally occurring predator of aphids in this area. Lacewing larvae are voracious feeders and can consume as many as 60 aphids in one hour. A third predator is the syrphid fly (right). The adult fly is harmless and will feed on nectar, but the larva are typical fly maggots with a big appetite for aphids. Aphids will cause ash tree leaves to curl but the larva can crawl into these leaves, whereas lady beetles cannot.
In the spring, with warm temperatures, little aphids hatch from overwintering eggs. These wingless female nymphs then head to the closest, tender shoots or leaves. After a few pokes with their syringe-like stylet, the young aphid strikes gold -- sap! They'll stay as long as the sap supply remains.
Still wingless and after several molts, (the shedding of their outer skeleton which allows for growth), their daughters are born. No eggs or males are involved. These virgin females clone themselves generation after generation, each daughter being a genetic photocopy of the mother. Each female aphid is born with offspring ready, themselves, to be born.
Though they are clones, aphids can change after final molting, and some of them develop wings. This new generation of aphids then will produce others with wings. Once the sap begins to wane at their designated site, or if the population becomes too great, the winged aphids fly off in search of a new food source. Then it's down to business again on the new host, vacuuming sap and converting it to more clones.
This unique reproduction keeps things moving along with no time wasted on mating. Usually males are produced only late in the summer when mating must take place so eggs can be laid for overwinter survival. Otherwise it is a total female society of aphids throughout the year.
And that's the thick of it. That's why, when you sprayed a few days ago and thought you got rid of those fat, sticky, disgusting bodies on your willow, aspen, linden, lupine, poppy or rose -- you didn't. Instead you zapped just a smidgen of a generation.
By the way, that sticky stuff is aphid excrement, so plan to use that wiper fluid for a good long time.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010