Some Observations and Control of the Rhododendron Lace Wing Fly
In control of the lace wing fly (illustration of leaf damage American Rhododendron Society Bulletin vol. 3, no. 1, fig. 2) many gardeners may not have the necessary spray equipment to follow the spray program as prescribed. These notes do not underestimate the spray control, which is a perfect control, but rather as presenting an alternate control for anyone that has many plants or few.
I know of no instance where the effects of this insect have ever proved fatal to a plant, but the spotted foliage resulting from the feeding insect is unsightly and undesirable.
The lace wing fly (Stephanitis rhododendri) does its damage in very localized portions of the foliage of a plant, and seemingly is even less prone to use its wings than the strawberry weevil. This comparison between the two insects, does not place the lace wing fly in the same category as the weevil, for the latter besides notching the foliage of a plant will also feed on the roots and stems, and in cases girdle the stem entirely killing the afflicted plant.
One rarely ever sees the weevil for its nocturnal feeding habits preclude such a possibility though on occasions in the cool early fall of the year the weevil remains on the stems of plants for days without even bothering to hide. This is far from the case in the early spring, for then even the slightest movement towards it will cause this pest to drop to the earth, and I must state that I have never seen one use its wings in flight.
The lace wing fly is a stubborn, stupid acting insect, and when approached on the bottom of the leaf where it usually hides, it backs up very slowly much as a bull, and makes no effort to escape. If the leaf is turned upside down the lace wing flies will slowly retreat to the bottom side. The insect has a very fine set of wings of lacy design, but never uses them even when pushed with the finger. Whether this pest can actually fly or not has not been my fortune to observe, but the fact that on a single plant during the season, the lace wing fly will do its damage in certain portions of the plant certainly does not bespeak of insect's mobility.
With the advent of new insect killers after the war D. D. T. was the most maligned, mostly through misunderstanding, but has since been redeemed by the trial and error method and should henceforward be used to control such pests as the lace wing fly.
Three years ago a badly infested plant of R. ponticum some three feet in height was dusted with a 10% D. D. T. The lace wing fly disappeared as if by magic, but the period of waiting to see if the treatment killed the patient was still to be reckoned with. The plant grew well during the summer and showed no ill effects. The following spring the plant made a normal growth and the so called "long toxic effect" of the D. D. T. had seemingly done no damage.
During the month of July, the first lace wing flies appeared again but in very limited numbers, on this same plant. A light dusting with a hand duster during the early morning hours when the wind is negligible showed good effect, and this spring the lace wing fly was not apparent.