The rhododendron gall midge (or tip midge), Clinodiplosis rhododendri (Felt), is a native pest of Rhododendron catawbieuse Michaux.,R. maximum and their many hybrids during the May to October growing season. The larval feeding damage from this fly can cause discoloration and distorted foliage. This damage can appear as in-rolled, twisted leaves that have turned a yellow color. This can be seen on young leaves that have separated from the bud scale. Leaves attacked in the bud stage may die if the injury is severe. Care must be taken to distinguish this pest damage from similarly manifested damage caused by leafhopper injury, chemical injury, leaf-roller activity and aphid feeding.
This insect overwinters in the soil as a prepupa, with complete pupal development taking place in the spring. The adults are very small (less than 1/16") and may be difficult to detect. Eggs are laid on the undersurfaces or rolled edges of the leaves as soon as they are free from the bud but before the leaves have fully separated from each other. Within three days, the eggs hatch and the larvae, which are small white maggots start to eat leaf tissue within the moist, protected area of the inner surface of the leaf roll. The larvae mature in about seven days, drop to the ground and pupate in the top one inch of soil inside a flimsy, silken cocoon.
Work done in Connecticut showed that most midge populations will complete three generations, but depending on growing conditions, this could reach four to five generations per year.
Early season damage, if present, should be visible in early June. It appears that infestation of the first growth flush is considerably less (20% infestation) than the second and third growth flushes (95%infestation combined). This is an important fact to take into consideration in order to protect the crop. Early detection and treatment will greatly reduce the pest population and, thus, the damage in later growth periods.
Past recommendations were based on timing applications of insecticides with the bud break stage of rhododendron development. Though this strategy was effective, it did have some drawbacks. Since workers are busy with the spring shipping season, it is more difficult to divert labor to a large spraying assignment. Also, exposure to insecticides would increase for the workers since contact with the plants is high at that time of the year.
An alternative strategy is to apply an insecticide to the soil in late fall or early spring to reduce the number of overwintering prepupae. The timing of this treatment provides a large window of control at the same time it reduces human contact with the insecticide. This strategy can also be used during the growing season, but the spray window is shortened because of the reduced time period between growth flushes. An assortment of chemicals can be sued for the soil application, such as chlorpyrifos (Dursban), diazinon and isazofos (Triumph). One benefit to using Dursban or diazinon is that their chemical structure binds to organic material in the soil. This keeps the chemical concentrated in the target site for optimum control and prevents environmental contamination due to runoff.
At this time, there is still little information available on nonchemical alternatives for control. A cultural practice that may have some value as a control strategy is to disturb the soil around the plants so that the pupae are exposed to harsher environmental conditions. This may not be practical due to the time involved. Other cultural practices would be to maintain healthy plants so that they could survive any damage caused by an infestation or to prune newly-infested foliage that contain larvae.
References: Baker, J.R. (ed.). 1980. Insects and Related Pests of Shrubs: Some important, Common and Potential Pests In the Southeastern United States. North Carolina Agricultural Extension Service.
Brand, M.H. 1989. The Rhododendron Tip Midge--A Relatively Unknown Pest Gains Momentum. The Connecticut Nursery Newsletter. 1(1):6-8.
Hanula, J.L. 1991. Seasonal Abundance and Control of the Rhododendron Gall Midge, Clinodiplosis rhododendri(Felt), in Container Grown Rhododendron catawbiense Michaux. J. Environ. Hort. 9(2):68-71.
Johnson, W.T. and H.H. Lyon. 1988. Pages 470-471. In: Insects that Feed on Trees and Shrubs, 2nd. Ed. Cornell University Press, Ithaca.
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