Box elders, which are among Iowa’s common trees, are nearly all more or less infested with the box elder aphid, or box elder plant louse. Often these tiny insects are so thick as to become a great nuisance. In southwestern Iowa, around Shenandoah, this insect became very obnoxious in 1908. Again in 1910 it was very common and box elder trees over the entire state had very light foliage, due to these plant lice.
The presence of the box elder aphid is made known by certain “ear marks.” These are the checking of the growth of the leaves and the presence of innumerable fine drops of moisture upon and beneath the infested trees. In May, when the box elder aphid multiplies very rapidly, or early June, a person walking or sitting beneath an infested tree at this time often is made aware of the insects by drops of moisture striking the face. This moist substance is a sweet fluid, the so-called "honey-dew," which is secreted by the aphids. Its secretion frequently is so excessive that pavements beneath infested trees become fairly damp with it.
Webster, R. L.
"The box elder aphid,"
Bulletin: Vol. 15
, Article 1.
Available at: https://lib.dr.iastate.edu/bulletin/vol15/iss173/1