By now many gardeners have noticed moderate- sized dark speckles on the foliage of many landscape and garden plants (mint, cone flower, black- eyed Susan, to name but a few). On most of these plants the damage in on older leaves. That is, new foliage at the tops of the plants appears unaffected.
What happened? Much of this old damage was done by the nymphs of the fourlined plant bug, an uncommon pest of garden and landscape plants in Iowa. The nymphs puncture plant leaves to suck the sap from the tissue. The points of attack (punctures) develop characteristic damage symptoms; clusters of black or translucent spots, each spot surrounded by a halo of different color.
Fourlined plant bugs are not picky eaters. They will feed on sap from over 250 different kinds of plants. The adults are present now in many gardens and are easily recognized. They are 1/2 inch long and have 4 shiny black stripes separated by three bright yellow stripes. The nymphs that did the damage in May were 1/16 to 3/8 inch long. Tiny nymphs were red while larger nymphs develop the striped coloration of the adults.
It is too late for effective treatment in most areas. Nymphs feed from late April to mid-June at which time they change into adults. The adults are present the remainder of the summer, though they only feed until approximately mid-July. There is only one generation per summer.
Late sprays, that is, after the damage becomes apparent and after the nymphs are grown, are of no benefit. If only adults are observed or if damage is found and no insects are present, do not spray. Watering, pruning, fertilizing and other cultural practices should be done to promote plant health and recovery.
This article originally appeared in the June 5, 1998 issue, p. 67.